SANRAL Geometric Design Guide

Chapter

Contents Glossary

1

Introduction

2

Design Philosophy

3

Design Controls

4

Design Elements

5

Alignment Design

6

Intersections

7

Interchanges

8

Roadside Safety

9

RRR

10

Grade Separations

11

Toll Plazas Bibliography Covers

Glossary A

summation for all vehicles. Also referred to as

Acceleration lane. An auxiliary lane used by an

space mean speed whereas time mean speed is

entering vehicle to accelerate before entering

simply the average of all recorded speeds.

the travelled way.

Axis of rotation. The line about which the pavement is rotated to superelevate the roadway.

Access control. The condition whereby the road

This line normally maintains the highway profile

agency either partially or fully controls the right of abutting landowners to direct access to and

B

from a public highway or road.

Barrier sight distance. The limiting sight distance below which overtaking is legally prohibit-

Access interchange. An interchange providing

ed.

access to a freeway from the adjacent non-freeway road network.

Boulevard. The area separating sidewalks from the through lanes.

Arterial. Highway designed to move relatively large volumes of traffic at high speeds over long

Bridge. A structure erected with a deck for car-

distances. Typically, arterials offer little or no

rying traffic over or under an obstruction and

access to abutting properties.

with a clear span of six metres or more. Where the clear span is less than six metres, reference

Auxiliary lane. Short lane located immediately

is to a culvert.

adjacent to the basic or through lane to accommodate some or other special circumstance

Broken-back curve. Two curves in the same

such as a turning movement to right or to left,

direction with a tangent shorter than 500 metres

acceleration to or deceleration from the speeds

long connecting them.

cles reduced to crawl speeds on a steep

C

upgrade.

Camber. The slope from a high point (typically

Average Daily Traffic (ADT).

at the centre line of the highway) across the

The number of

lanes of a highway. Negative camber refers to

vehicles per day passing a point on the highway

a central low point, usually with a view to

during a defined period. If this period extends

drainage of a small urban street or alley.

from 1 January to 31 December, reference is to Annual Average Daily Traffic (AADT).

Capacity.

The maximum number of vehicles

Average running speed. The distance summa-

that can pass a point on a highway or in a des-

tion for all vehicles divided by the running time

ignated lane in one hour without the density

i Glossary

Geometric Design Guide

prevailing on the travelled way or heavy vehi-

being so great as to cause unreasonable delay

Collector-Distributor road. A road used at an

or restrict the driver’s freedom to manoeuvre

interchange to remove weaving from the

under prevailing roadway and traffic conditions.

through lanes and to reduce the number of entrances to and exits from the through lanes.

Carriageway. Roadway forming part of a divided highway and intended for movement in one

Compound curve. A combination of two or more

direction only – hence dual carriageway as an

curves in the same direction without intervening

alternative name for divided highway.

tangents between them.

Catchwater drain. Located above a cut face to

Criterion. A yardstick according to which some

ensure that storm water does not flow down the

or other quality of the road can be measured.

cut face causing erosion and deposition of silt

Guideline values are specific numerical values

on the roadway.

of the criterion. For example, delay is a criterion of congestion.

Channel grading.

Where side channels are

designed to gradients that differ from those of

Critical length of grade. The maximum length of

the road centreline, typically on either side of the

a specific upgrade on which a loaded truck can

highest points on crest curves and the lowest

operate without an unreasonable reduction in

points on sag curves where the centreline gradi-

speed. Very often, a speed reduction of 15 km/h

ent is less than 0,5 per cent.

or more is considered “unreasonable”. Cross fall. See camber. In the case of cross

Channelisation. The use of pavement markings

fall, the high point is at the roadway edge.

or islands to direct traffic through an intersection

Cross-over crownline. The line across which an

Clearance profile. Describes the space that is

instantaneous change of camber takes place.

exclusively reserved for provision of the road or

In the case of a normally cambered road, the

highway. It defines the minimum height of the

centreline is a special case of the cross-over


soffit of any structure passing over the road and

crownline.

the closest approach of any lateral obstacle to

located anywhere on the road surface and need

the cross-section.

Cloverleaf interchange.

The cross-over crownline can be

not even be parallel to the road centreline. An interchange with

Crosswalk. A demarcated area or lane desig-

loop ramps in all quadrants to accommodate

nated for the use of pedestrians across a road

right turns and outer connectors for the left

or street.

turns. Crown runoff.

(Also referred to as tangent

Collector. A road characterised by a roughly

runout) The rotation of the outer lane of a two-

even distribution of its access and mobility func-

lane road from zero cross fall to normal camber

tions.

(NC). ii Glossary

Culvert. A structure, usually for conveying water

Design hour. The hour in which the condition

under a roadway but can also be used as a

being designed for, typically the anticipated flow,

pedestrian or stock crossing, with a clear span

is expected to occur. This is often the thirtieth

of less than six metres.

highest hour of flow in the design year.

Cut. Section of highway or road below natural

Design speed. The speed selected as the basis

ground level. Sometimes referred to in other

for establishing appropriate geometric elements

documents as a cutting or excavation.

for a section of road.

Cycle lane. A portion of the roadway which has

Design vehicle.

been designated by road markings, striping and

A compilation of the 85th percentile values of the

signing as being exclusively for the use of

various parameters of the vehicle type being

cyclists.

designed for, e.g. length, width, wheelbase, overhang, height, ground clearance, etc.

Cycle path. Also known as a bike way. A path physically separated from motorised traffic by

Design year. The last year of the design life of

an open space or barrier and located either

the road or any other facility, often taken as

within the road reserve or an independent

twenty years although, for costly structures such

reserve.

as major bridges, a longer period is usually adopted.

D Decision sight distance. Sometimes referred to

Directional distribution (split). The percentages

as anticipatory sight distance, allows for circum-

of the total flow moving in opposing directions,

stances where complex decisions are required

e.g. 50:50, 70:30, with the direction of interest

or unusual manoeuvres have to be carried out.

being quoted first.

As such, it is significantly longer than Stopping Divided highway. A highway with separate car-

Sight Distance.

riageways for traffic moving in opposite direc-

given length of road.

Usually averaged over Driveway. A road providing access from a pub-

time and expressed as vehicles per kilometre.

lic road to a street or road usually located on an abutting property.

Depressed median. A median lower in elevation than the travelled way and so designed to carry

E

portion of the storm water falling on the road.

Eighty-fifth percentile speed. The speed below Design domain. The range of values of a design

which 85 per cent of the vehicles travel on a

criterion that are applicable to a given design,

given road or highway.

e.g. lane widths of more than 3,3 metres. iii Glossary


tions.

Density. The number of vehicles occupying a

F

Gradient. The slope of the grade between two

Footway. The rural equivalent of the urban side-

adjacent Vertical Points of Intersection (VPI),

walk.

typically expressed in percentage form as the vertical rise or fall in metres/100 metres. In the

Freeway. Highest level of arterial characterised

direction of increasing stake value, upgrades

by full control of access and high design

are taken as positive and downgrades as nega-

speeds.

tive.

Frontage road. A road adjacent and parallel to

Guideline.

but separated from the highway for service to

approximate threshold, which should be met if

abutting properties and for control of access.

considered practical.

Sometimes also referred to as a service road.

value whereas a standard is a prescriptive value

A design value establishing an It is a recommended

allowing for no exceptions.

G

H

Gap. The elapsed time between the back of

High occupancy vehicle ( HOV) lane. A lane

one vehicle passing a point on the road or high-

designated for the exclusive use of buses and

way and the nose of the following vehicle pass-

other vehicles carrying more than two passen-

ing the same point. A lag is the unexpired por-

gers.

tion of a gap, i.e. the elapsed time between the arrival of a vehicle on the minor leg of an inter-

High-speed. Typically where speeds of 80 km/h

section and the nose of the next vehicle on the

or faster are being considered.

major road crossing the path of the entering vehicle.

Horizontal sight distance. The sight distance determined by lateral obstructions alongside the

Gore area. The paved triangular area between

road and measured at the centre of the inside

the through lanes and the exit or entrance

lane.

ramps at interchanges plus the graded areas


immediately beyond the nose (off-ramp) or

I

merging end (on-ramp). Grade line.

Interchange. A system of interconnecting roads (referred to as ramps) in conjunction with one or

The line describing the vertical

more grade separations providing for the move-

alignment of the road or highway.

ment of traffic between two or more roadways Grade. The straight portion of the grade line

which are at different levels at their crossing

between two successive vertical curves.

point.

Grade separation. A crossing of two highways

Intersection sight distance. The sight distance

or roads, or a road and a railway, at different

required within the quadrants of an intersection

levels.

to safely allow turning and crossing movements. iv Glossary

J, K

verse natural slopes are severe and changes in

Kerb. Concrete, often precast, element adja-

elevation abrupt. Many trucks operate at crawl

cent to the travelled way and used for drainage

speeds over substantial distances.

control, delineation of the pavement edge or protection of the edge of surfacing.

N

Usually

Normal crown (NC). The typical cross-section

applied only in urban areas.

on a tangent section of a two-lane road or fourlane undivided road.

Kerb ramp. The treatment at intersections for gradually lowering the elevation of sidewalks to

O

the elevation of the street surface.

Overpass. A grade separation where a minor highway passes over the major highway.

K-value. The distance over which a one per cent change in gradient takes place.

Outer separator.

Similar to the median but

L

located between the travelled way of the major

Level of Service (LOS). A qualitative concept,

road and the travelled way of parallel lanes

from LOS A to LOS F, which characterises

serving a local function if these lanes are con-

acceptable degrees of congestion as perceived

tained within the reserve of the major road. If

by drivers. Capacity is defined as being at LOS E.

they fall outside this reserve, reference is to a frontage road.

Low speed. Typically where speeds of 70 km/h

P Partial Cloverleaf (Par-Clo) Interchange.

An

M

interchange with loop ramps in one, two or three

Median. The portion of a divided highway sep-

(but usually only two) quadrants. A Par-Clo A

arating the two travelled ways for traffic in oppo-

Interchange has the loops in advance of the

site directions. The median thus includes the

structure and Par-Clo B Interchange has the

inner shoulders.

loops beyond the structure.

A Par-Clo AB

Interchange has its loops on the same side of the crossing road.

Median opening. An at-grade opening in the median to allow vehicles to cross from a road-

Passenger car equivalents (units) (PCE or

way to the adjacent roadway on a divided road.

PCU). A measure of the impedance offered by The public facility at

a vehicle to the passenger cars in the traffic

which passengers change from one mode of

stream. Usually quoted as the number of pas-

transport to another, e.g. rail to bus, passenger

senger cars required to offer a similar level of

car to rail.

impedance to the other cars in the stream.

Modal transfer station.

Passing sight distance. The total length visibiliMountainous terrain.

ty, measured from an eye height of 1,05 metres

Longitudinal and transv

Glossary


or slower are being considered.

to an object height of 1,3 metres, necessary for

R

a passenger car to overtake a slower moving

Ramp. A one-way, often single-lane, road pro-

vehicle. It is measured from the point at which

viding a link between two roads that cross each

the initial acceleration commences to the point

other at different levels.

where the overtaking vehicle is once again back in its own lane.

Relative gradient. The slope of the edge of the travelled way relative to the gradeline.

PC (Point of curvature). Beginning of horizontal curve, often referred to as the BC.

Reverse Camber (RC). A superelevated section of roadway sloped across the entire travelled

PI (Point of intersection). Point of intersection of

way at a rate equal to the normal camber.

two tangents. Reverse curve. A combination of two curves in PRC (Point of reverse curvature). Point where

opposite directions with a short intervening tan-

a curve in one direction is immediately followed

gent

by a curve in the opposite direction. Typically applied only to kerb lines.

Road safety audit. A structured and multidisciplinary process leading to a report on the crash

PT (Point of tangency). End of horizontal curve,

potential and safety performance of a length of

often referred to as EC.

road or highway, which report may or may not include suggested remedial measures.

PVC (Point of vertical curvature) The point at which a grade ends and the vertical curve

Roadside. A general term denoting the area

begins, often also referred to as BVC.

beyond the shoulder breakpoints.

PVI (Point of vertical intersection). The point

Road bed.

where the extension of two grades intersect.

shoulder breakpoints.

The extent of the road between


The initials are sometimes reversed to VPI. Road prism. The lateral extent of the earthworks.

PVT (Point of vertical tangency). The point at which the vertical curve ends and the grade

Road reserve. Also referred to as Right-of-way.

begins. Also referred to as EVC.

The strip of land acquired by the road authority

Q

for provision of a road or highway.

Quarter link. An interchange with at-grade intersections on both highways or roads and two

Roadway. The lanes and shoulders excluding

ramps (which could be a two-lane two-way road)

the allowance (typically 0,5 metres) for rounding

located in one quadrant. Because of its appear-

of the shoulders.

ance, also known as a Jug Handle Interchange. vi Glossary

Rolling terrain. The natural slopes consistently

Single point urban interchange.

rise above and fall below the highway grade

interchange where all the legs of the inter-

with, occasionally, steep slopes presenting

change meet at a common point on the crossing

some restrictions on highway alignment.

road.

In

A diamond

general, rolling terrain generates steeper gradients, causing truck speeds to be lower than

Speed profile. The graphical representation of

those of passenger cars.

the 85th percentile speed achieved along the length of the highway segment by the design

Rural road or highway. Characterised by low-

vehicle.

volume high-speed flows over extended distances. Usually without significant daily peaking

Standard. A design value that may not be trans-

but could display heavy seasonal peak flows.

gressed, e.g. an irreducible minimum or an absolute maximum. In the sense of geometric

S

design, not to be construed as an indicator of

Shoulder. Usable area immediately adjacent to

quality, i.e. an ideal to be strived for.

the travelled way provided for emergency stopping, recovery of errant vehicles and lateral sup-

Stopping sight distance. The sum of the dis-

port of the roadway structure.

tance

travelled

during

a

driver’s

perception/reaction time and the distance travShoulder breakpoint. The hypothetical point at

elled thereafter while braking to a stop.

which the slope of the shoulder intersects the line of the fill slope. Sometimes referred to as

Superelevation. The amount of cross-slope pro-

the hinge point.

vided on a curve to help counterbalance, in combination with side friction, the centrifugal

Side friction (f). The resistance to centrifugal

force acting on a vehicle traversing the curve.

force keeping a vehicle in a circular path. The Superelevation runoff.

sents a threshold of driver discomfort and not

superelevation development)

the point of an impending skid. Sidewalk.

(Also referred to as The process of

rotating the outside lane from zero crossfall to reverse camber (RC), thereafter rotating both

The portion of the cross-section

lanes to the full superelevation selected for the

reserved for the use of pedestrians.

curve.

Sight triangle. The area in the quadrants of an

Systems interchange. Interchange connecting

intersection that must be kept clear to ensure

two freeways, i.e. a node in the freeway system.

adequate sight distance between the opposing legs of the intersection.

T

Simple curve. A curve of constant radius with-

Tangent.

out entering or exiting transitions.

The straight portion of a highway

between two horizontal curves. vii Glossary


designated maximum side friction (fmax) repre-

V

Tangent runoff. See crown runoff

Value engineering. A management technique in Traffic composition. The percentage of vehicles

which intensive study of a project seeks to

other than passenger cars in the traffic stream,

achieve the best functional balance between

e.g. 10 per cent trucks, 5 per cent articulated

cost, reliability and performance.

vehicles (semi-trailers) etc. Verge. The area between the edge of the road Transition curve. A spiral located between a

prism and the reserve boundary

tangent and a circular curve.

W Travelled way. The lanes of the cross-section.

Warrant. A guideline value indicating whether or

The travelled way excludes the shoulders.

not a facility should be provided. For example, a warrant for signalisation of an intersection

Trumpet interchange.

A three-legged inter-

would include the traffic volumes that should be

change containing a loop ramp and a direction-

exceeded before signalisation is considered as

al ramp, creating between them the appearance

a traffic control option. Note that, once the war-

of the bell of a trumpet.

ranting threshold has been met, this is an indication that the design treatment should be con-

Turning roadway. Channelised turn lane at an

sidered and evaluated and not that the design

at-grade intersection.

treatment is automatically required.

Turning template. A graphic representation of a

X, Y, Z

design vehicle’s turning path for various angles

Yellow line break point. A point where a sharp

of turn. If the template includes the paths of the

change of direction of the yellow edge line

outer front and inner rear points of the vehicle,

demarcating the travelled way edge takes place.

reference is to the swept path of the vehicle.

Usually employed to highlight the presence of the start of a taper from the through lane at an


U

interchange.

Underpass. A grade separation where the subject highway passes under an intersecting highway. Urban road or highway. Characterised by high traffic volumes moving at relatively low speeds and pronounced peak or tidal flows. Usually within an urban area but may also be a link traversing an unbuilt up area between two adjacent urban areas, hence displaying urban operational characteristics. viii Glossary

TABLE OF CONTENTS 1.

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1

Chapter 1 INTRODUCTION The geometric design of a highway or of one of

demands from different sections of the commu-

its many elements is only one step in a multifac-

nity as they endeavour to design safe and oper-

eted process from concept to construction.

ationally efficient roads.

However, the constraints, which the physical

A major objective of any road design guide is to

elements ultimately place on the function and

ensure that designs achieve value for money

form of a highway, pervade every step in the

without any significant deleterious effect on

process. Knowledge of the parameters, which

safety.

govern planning and design together with their

The design philosophy, systems and

techniques developed elsewhere in this docu-

practical application, is thus essential. These

ment have been based on the Design Speed

guidelines seek to meet that need.

approach and related geometric parameters which will result in a much greater flexibility to

The emphasis previously of Geometric Design

achieve economic design in varied and some-

Manuals was on design standards for new conwork is, however, substantially complete and

In line with this, the standards in this guideline

new road works are largely limited to urban

will address a spectrum of road types, varying

developments. This Manual thus deals not only

from multi-lane freeways carrying traffic vol-

with new works but also pays attention to reha-

umes of over 100 000 vehicles per day, to single

bilitation, reconstruction and upgrading projects.

carriageway roads carrying volumes of the order

A feature of these projects is that the designer’s

of 500 vehicles per day. In respect of this latter

freedom of choice is often restricted by develop-

class of road design, recommendations have

ments surrounding the road to be rehabilitated.

been considerably extended to allow greater

In consequence, adherence to rigidly applied

flexibility in design, with particular emphasis on

standards is not possible, in addition to the fact

the co-ordination of design elements to improve

that blind adherence has never been construed

safety and overtaking conditions.

as a thinking designer’s approach to the problem at hand.

The guidelines distinguish between roads in rural areas and those in urban areas and also

These geometric design guidelines are intended

caters for situations where National Roads tra-

for use on National Roads – or on any other

verse the CBDs of smaller municipalities.

roads falling within the domain of the S A National Roads Agency Limited. For this rea-

Overall, the greater flexibility in design intro-

son, the guidelines address a wide range of

duced in these guidelines will enable more eco-

functional uses and requirements. They will also

nomic designs, reducing both the construction

need to cater for a multiplicity of users, and

costs and the impact of new roads and road

designers will be faced with competing

improvements on the environment. 1-1

Chapter 1: Design philosophy and techniques


times difficult circumstances.

struction. The South African primary road net-


DESIGN PHILOSOPHY AND TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 2.1

BACKGROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

2.2

FUNDAMENTAL PRINCIPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

2.3

DESIGN PHILOSOPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3

2.4

DESIGN TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4 2.4.1 Flexibility In highway design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4 2.4.2 Interactive Highway System Design Model (IHSDM) . . . . . . . . . . . . . . . . . . . . . . . 2-6 2.4.3 The "design domain" concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7 2.4.4 Road safety audits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10 2.4.5 Economic analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11 2.4.6 Value engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12

TABLE OF FIGURES Figure 2.1 :The design domain concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8 Figure 2.2 :Example of design domain application - shoulder width. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9

Chapter 2 DESIGN PHILOSOPHY AND TECHNIQUES 2.1

BACKGROUND

the focus to move towards the whole-life economy of the network.

The Department of Transport completed the "Moving South Africa" project in 1999. One of

The network with the shortest overall length

the findings was that, in order to reduce con-

compatible with linking all origins and destina-

gestion, a shift from private to public transport

tions would theoretically have the lowest cost. It

would be required. As many people are captive

could also represent a saving in maintenance

to public transport, it is also necessary to create

cost provided that the attempt to reduce the net-

an environment supportive of public transport.

work length did not adversely impact on the vertical alignment, resulting in very steep gradients

This will require encouraging:

carry a maintenance and construction penalty.

Settlement densities supportive of public

Assuming that maintenance is practical, it is

transport;

•

Network layouts with geometric design

possible that the network, short though it may

standards suitable for bus and taxi

be, forces circuitous travel paths, which would

routes including:

nullify any savings on construction and maintenance. It follows that the shape of the network

-

Safe stopping sight distance;

-

Reduced gradients;

-

Minimum horizontal curvature;

often incorrectly construed as the selection, siz-

-

Intersection layouts that are simple

ing and grouping of a set of components to cre-

to negotiate;

ate a road network, must therefore contain a

-

User friendly bus stops;

-

Terminals and modal transfer sta-

is as important as its overall length in optimising the life-cycle cost. Geometric Design, which is

strong element of Geometric Planning.

Geometric Planning includes careful selection of

tions; and -

the cross-section. The road width and shape has a significant impact on the cost of construc-

High occupancy vehicle lanes.

tion but economizing on the cross-section by Optimising resources requires the building of

reducing the number and width of lanes could

networks with the lowest possible whole-life

have a crippling effect on traffic flow and a con-

costs. This has always been a goal but, histori-

sequential increases in road user costs.

cally, the emphasis tended to be on minimum

such, classification of the various links in the

construction costs and, more recently, on mini-

road network and estimating their traffic vol-

mum combined construction and maintenance

umes is essential for planning a truly economi-

costs. A subsequent shift in emphasis caused

cal road network.

2-1 Chapter 2: Design philosophy and techniques

As


•

or a poorly drained road, both of which could

Another historic emphasis was on design for

mulation of these laws states that "The change

mobility and accessibility. Design was specifi-

of motion is proportional to the motive force

cally for passenger cars with some attention

impressed; and is made in the direction of the

being paid to the requirements of other vehicles,

right line in which that force is impressed" and

particularly at intersections. However, geomet-

also that

ric designers must now recognize that the road

opposed an equal reaction: or, the mutual

network, particularly in dense settlements,

actions of two bodies upon each other are

serves other functions in addition to mobility and

always equal, and directed to contrary parts".

accessibility.

Community needs, including

Professor Newton clearly understood the impli-

social interaction, relaxation and commerce, are

cations of these laws for he goes on to say "The

becoming ever more important. In urban areas

power and use of machines consists only in this,

there is a trend towards mixed land usage. A

that by diminishing the velocity we may aug-

consequence of this change is that trip lengths

ment the force, and the contrary."

"To every action there is always

are shorter and modes of transport other than By applying the laws of motion, together with

passenger cars and buses become a practical

judicious experimentation, we are able to gain a

option. Walking and cycling can be expected to

reasonable understanding of the interaction

become more pervasive in the urban environ-

between the vehicle and the roadway, as they

ment. The design process will have to make

are essentially deterministic. In essence, this

provision for these mobility options as part of the

understanding describes what a vehicle moving

total package available to the traveller.

along a road can do and not necessarily what the driver wishes to do. Therefore, to properly

As there is a need to consider:

describe a highway operating system these

• •


•

network reconstruction and rehabilita-

laws must be integrated with the human factor,

tion;

which includes the perceptions, reactions, toler-

the findings of the Moving South Africa

ances and failures of a wide spectrum of indi-

project;

viduals under continuously changing circum-

the whole-life economy of the road net-

stances.

work;

•

Design manuals tend to focus on vehicle

the broader functionality of the road net-

dynamics, with all the frailties of the human

work; and

•

component of the system being summed up in a

the possibility of an increase in non-

single reaction time. The randomness of human

motorised transport;

behaviour is disregarded. Crash investigations

it follows that the focus of geometric planning

often reveal, however, that it is not always the

and design has to change.

road or the vehicle but rather the human com-

2.2

ponent of the system that fails under stress.

FUNDAMENTAL PRINCIPLES

The laws of motion govern the interaction of the

A vehicle moving along a roadway is a highly

vehicle and the roadway. Isaac Newton's for-

complex system with an infinite range of possi2-2


bilities and outcomes. There are numerous crit-

that many drivers manage to make the same

ical elements, each with its own probability of

mistake at the same point along the road. While

failure. When these are factored together, the

it is necessary to reconsider the role of the

sheer number of elements ensures that the

Newtonian models on which geometric stan-

probability of failure of the system as a whole is

dards are based, human factors require careful

very high indeed. We measure these failures as

evaluation.

crashes.

2.3

DESIGN PHILOSOPHY

According to Hauer, roads designed to pub-

Commonly advocated design philosophies tend

lished standards are neither safe nor unsafe and

towards the simplistic and are inclined to ignore

the linkage between standards and safety is

the issues discussed in Section 2.1. In search of

largely unpremeditated. He illustrates his con-

safety they place inordinate reliance on models

tention by reference to the vector diagram that

derived exclusively from Newtonian dynamics.

describes the forces operating on a vehicle tra-

Current philosophy is, in short, based on the

versing a superelevated curve.

This is

assumption that any design that accords with

Newtonian dynamics and, if it offered a proper

established geometric design policies is safe

explanation of the situation, curves should theo-

and that those that do not are unsafe. This is

retically have no accidents at all or, at worst,

taken for granted by designers and often is

should have exactly the same accident rate as

accepted by the courts when making decisions

the tangents that precede and follow them.

on questions of liability.

Furthermore, vehicles leaving the road should be equally distributed between the inside and

Despite many decades of research the complex

the outside of the curve. The reality of the situ-

relationship between vehicle, roadway, driver;

ation is that the accident rate on curves is high-

and operational safety is not always well under-

er than on tangents and most vehicles leaving

stood.

the road do so on the outside of the curve.

investigated the relationships between accident

Although numerous researchers have

rates and specific geometric design elements, Clearly, the vector diagram is not a complete or

the results were often not sufficiently definitive

sufficient exposition of the problem. For exam-

of this research, which, in examining the rela-

after they have passed its starting point and are

tionship between accidents and individual

thus obliged to follow a path with a smaller

design elements, fails to consider the interactive

radius than that provided by the designer. If the

effects of other parameters, which could lead to

designed curve is at minimum radius, the sub-

bias and mask important relationships.

minimum path actually being followed could A panic

From this rather unhappy state of affairs we can

reaction under these circumstances could

only conclude that a new design philosophy is

cause the vehicle to swerve out of control.

warranted.

have unanticipated consequences.

While reference is made to human error as the

A design philosophy should encompass two lev-

prime cause for most crashes, it is noteworthy

els. In the first instance, the focus should be on 2-3



for practical use. This is due to the narrow focus

ple, drivers sometimes steer into a curve only

Geometric Planning, which has seldom, if ever,

malised expressions of particular objectives and

been discussed in Geometric Design Manuals.

include:

Geometric Planning explicitly addresses the matters discussed in Section 2.1. In a sense, it is these issues that dictate how user-friendly the ultimate design will be to both the road user and the community. Detailed Design is about operational safety,

• • • • • •

Flexibility in highway design;

2.4.1

Flexibility In highway design

Interactive highway design; Design domain concept; Safety audits; Economic analysis; and Value engineering.

which is the second level of geometric design. This is the level on which Manuals typically focus and the effectiveness and the safety of road elements enjoy equal attention. It is pro-

A review of the standards and warrants in this

posed that, in the new philosophy, safety should

manual will quickly reveal that it allows some

be the prime consideration. Sacrificing safety in

degree of design flexibility. The degree to which

the interests of efficiency and economy is not an

this flexibility is employed in the design process

acceptable practice.

is in fact, nothing more than the application of the art and science of engineering.


A more holistic philosophy should thus be founded on the concept of reducing the proba-

In an attempt to formalise the process and to

bility of failure to the lowest possible level and,

guide the designer towards appropriate choices,

furthermore, should seek to minimise the con-

the United States Department of Transportation

sequences of those failures that do occur. To

published a report in 1997 entitled "Flexibility in

achieve this goal, designs must begin with a

Highway Design". It consists of three main sec-

clear understanding of purpose and functionali-

tions: an introduction to the highway design

ty. From this foundation comes the selection of

process, general guidelines referring to the

appropriate design elements followed by their

major elements of highway design, and exam-

integration into the landform and its current and

ples of six design projects presented as case

future use. The hallmark of professionalism in

studies. The concepts described are now more

road design is the ability to foresee and optimize

commonly referred to as "context sensitive

the conflicting objectives that are inherent in any

design".

project. The most important concept to keep in mind

2.4

throughout the highway design process is that

DESIGN TECHNIQUES

every project is unique. The setting and charTo arrive at an acceptable design there is no

acter of an area, the values of the surrounding

substitute for experience and study. There is,

community, the needs of the highway users and

however, a range of useful tools and techniques

the associated physical challenges and opportu-

at the designer's disposal.

nities are unique factors that highway designers

These are for2-4


For each

later in the process. Public input can also help

potential project, designers are faced with the

to assess the characteristics of the area and to

task of balancing the need for improvement of

determine what physical features are most val-

the highway with the need to safely integrate the

ued by the community and, thus, have the great-

design into the surrounding natural and human

est potential for impact. Awareness of these val-

environments.

ued characteristics at an early state will help

must consider with each project.

designers to avoid changing them during the To accomplish this, highway designers must

project, reducing the need for mitigation and the

exercise flexibility.

likelihood of controversy.

There are a number of

options available to aid in achieving a balanced road design and to resolve design issues.

After working with the community to define the

Among these are the following:

basic project need and to assess the physical character of the area, public involvement is nec-

Use the flexibility available within the

essary to obtain input on design alternatives.

design standards;

• •

Working with the affected community to solve

Recognise that design exceptions may be required where environmental impact

design challenges as they arise is far more

consequences are great;

effective than bringing the public into the

Be prepared to re-evaluate decisions

process only after major design decisions have

made earlier in the project planning and

been made. The public needs to be involved at

environmental impact assessment

all points in the project where there are the

phase;

•

greatest opportunities for changes to be made

Lower the design speed where appropri-

in the design.

ate;

• •

Maintain the road's existing horizontal and vertical geometry and cross section

One of the major and continuing sources of con-

where possible;

flict between highway agencies and the commu-

Consider developing alternative design

nities they serve relates to the topic of function-

standards, especially for scenic or his-

al classification. In particular, the need to iden-

toric roads; and

•

tify the "correct" functional classification for a

Recognise the safety and operational

particular section of highway, and a regular re-

impacts of various design features and

examination of functional classification as

modifications.

changes in adjacent land use take place, would In addition to exercising flexibility, a successful

resolve many potential design conflicts before

highway design process should include the pub-

they take place.

lic. To be effective, the public view should be canvassed at the outset, even before the need

There are a number of other fundamental

for the project has been defined. If the primary

design controls that must be balanced against

purpose and need for the improvement has not

one another. These include:

been agreed on, it would be extremely difficult to

• •

reach consensus on alternative design solutions

The design speed of the facility; The design-year peak-hour level of



•

The Design Consistency Module

service on the facility;

• • •

The physical characteristics of the design vehicle;

This module evaluates the operating-speed

The performance characteristics of the

consistency of two-lane highways. The evalua-

design vehicle;

tion is performed using a speed-profile model

The capabilities of the typical driver on

that estimates 85th percentile speeds on each

the facility (i.e., local residents using

element along an alignment. The module gen-

low-speed neighbourhood streets

erates two consistency-rating measures:

versus long distance travellers on inter-

•

urban freeways); and

•

The existing and future traffic demands

percentile speeds and the design speed

likely to be placed on the facility.

of the roadway, and

• 2.4.2

The difference between estimated 85th

The reduction in 85th percentile speed

Interactive Highway System

between each approach tangent-curve

Design Model (IHSDM)

pair.

A suite of computer modules within the CAD

The module will consist of a speed-profile model

environment is currently under development by

and consistency rating measures that have

the U.S. Federal Highway Administration. When

been validated and are applicable to most two-

completed, designers will have a powerful tool

lane, free flowing

with which to assess the safety effects of their

States.

highways in the United

geometric design decisions.

The Driver/Vehicle Module As currently planned, IHSDM will be applicable

This will consist of a Driver Performance Model

to two lane highways. It is composed of six

linked to a Vehicle Dynamics Model. Driver per-

modules.

formance is influenced by cues from the road-


way/vehicle system (i.e., drivers modify their

The Crash Prediction Module

behaviour based on feedback from the vehicle

This module will estimate crash potential for a

and the roadway). Vehicle performance is, in

design alternative, including all roadway seg-

turn, affected by driver behaviour/performance.

ments and intersections.

Estimates will be

The Driver Performance Model will estimate a

quantitative and will include the number of

driver's speed and path along a two-lane high-

crashes for a given roadway segment or inter-

way in the absence of other traffic. These esti-

section as well as the percentages of fatal and

mates will be input to the Vehicle Dynamics

severe crashes.

Model, which will estimate measurements including lateral acceleration, friction demand,

The module will allow the user to compare the

and rolling moment.

number of crashes over a given time period for different design alternatives or to perform sensi-

The Driver/Vehicle Module will produce the fol-

tivity analyses on a single alternative.

lowing measures of effectiveness and, where 2-6


appropriate, threshold or reference values for

ance with policy will be identified, and an expla-

comparison purposes:

nation of the policy violated will be provided. In

•

response to this information, the user may cor-

Lateral acceleration in comparison with

rect any deficiencies, analyse the design further

discomfort, skid, and rollover threshold

using other IHSDM modules, and/or prepare a

values;

• • •

Friction demand in comparison with the

request for design exception. A summary of the

skid threshold;

policy review will be provided, including a listing

Rolling moment in comparison with the

of all design elements that do not comply with

rollover threshold;

policy. The categories of design elements to be

Estimated vehicle speed in comparison

verified include: horizontal alignment, vertical

with threshold speeds for discomfort,

alignment, cross section, intersections, sight

skidding, and rollover; and

•

distance, and access control/management.

Vehicle path (lateral placement) relative to the lane lines.

The Policy Review Module will notify designers

The Intersection Diagnostic Review

of any design elements that deviate from mini-

Module

ma/maxima set by the AASHTO Green Book,

This module will be used to evaluate the geo-

the "Roadside Design Guide," and the "Guide

metric design of at-grade intersections on two-

for the Development of Bicycle Facilities." The

lane highways and to identify possible safety

Module will also have the capability of reviewing

treatments. The Intersection Diagnostic Review

designs relative to alternative, user-specified

Module will incorporate qualitative guidance

design policies, such as State Department of

from the American Association of State Highway

Transportation design guidelines.

on the geometric design of highways and

The Traffic Analysis Module

streets" (generally referred to as the Green

This module will link highway geometry data

Book) and other design policies, design guide-

with a traffic simulation model to provide infor-

lines based on past research and design guide-

mation on speed, travel time, delay, passing

lines based on expert opinion.

The primary

rates, percentage following in platoons, traffic

focus is to identify combinations of geometric

conflicts and other surrogate safety measure-

design elements that suggest potential design

ments. TWOPAS, a traffic simulation model for

deficiencies, even when each element consid-

two-lane highways, will form the basis for this

ered individually could be regarded as being

module.

within good design practice.

2.4.3

The "design domain" concept

The Policy Review Module

The design domain concept recognizes that

This module is intended for use in all stages of

there is a range of values, which could be adopt-

highway planning and design, including design

ed for a particular design parameter within

review, for both new and reconstruction proj-

absolute upper and lower limits. Values adopted

ects. Design elements that are not in compli-

for a particular design parameter within the 2-7



and Transportation Officials document "A Policy

design domain would achieve an acceptable

for assessment of safety and operational.

though varying, level of performance in average

These improvements, as well as initiatives in the

conditions in terms of safety, operation, and

assessing and auditing of scheme layouts, have

economic and environmental consequences.

considerably improved the design process.

Figure 2.1 illustrates the concept.

It is now practical to estimate the changes in the level of service, cost and safety when the design

While values within the lower region of the

is changed within the design domain. Where

design domain for a particular parameter are

data are not available, guidance is available to Practical upper limit

Cost or benefit

Absolute upper limit

Absolute lower limit

Practical lower limit

Design domain

Range of values

Guideline


Figure 2.1 : The design domain concept generally less safe and less operationally effi-

the designer in the literature on the sensitivity of

cient, they are normally less costly than those in

safety to changes in the parameter under con-

the upper region. In the upper region of the

sideration within the design domain.

domain, resulting designs are generally "safer"

evaluations are however limited in comparison

and more efficient in operation, but may cost

to the evaluation of operational adequacy or

more to construct. In fact, the design domain

construction costs.

These

sets the limit within which parameters should be selected for consideration within the value engi-

The benefits of the design domain concept are:

neering concept.

•

It is directly related to the true nature of the road design function and process,

During recent years there have been many

since it places emphasis on developing

advances in road design and in the procedures

appropriate and cost-effective designs,


•

rather than on those which simply meet

Application of the concept of a design domain in

"standards";

practice presents practical challenges. In some

It directly reflects the continuous nature

cases, the concept of a design domain with

of the relationship between service, cost

upper and lower bounds, and a continuous

and safety and changes in the values of design dimensions.

range of values in between, may not be practi-

It thus reinforces

cal or desirable. Lane widths provide a good

the need to consider the impacts of

•

trade-offs throughout the domain and

example of such a case. In these instances, it

not just when a "standards" threshold

may only be necessary to consider a series of

has been crossed, and;

discrete values for the dimension in question. In

It provides an implicit link to the concept

other instances, there may be no upper limit to

of "Factor of Safety" - a concept that

a design domain other than what is practical or

isused in other civil engineering design

economic. In these cases, the upper boundary

processes where risk and safety are

of the design domain generally reflects typical

important.

upper level values found in practice, or the gen-

The illustration in Figure 2.2 is an example of

eral threshold of cost-effective design.

how different costs and benefits may vary within the design domain for a specific parameter - in

The designer must respect controls and con-

this case shoulder width. The application of this

straints to a greater or lesser degree, depending

concept to all design parameters will lead to an

on their nature and significance.

optimal project design.

designer is faced with the dilemma of being



Figure 2.2: Example of design domain application - Shoulder width.

Often, the

unable to choose design dimensions or criteria

considered in the design process. If a design

that will satisfy all controls and constraints, and

involves compromise, it may be more appropri-

a compromise must be reached.

ate to vary several elements by a small amount

These are

engineering decisions that call for experience,

than to alter one element excessively.

insight and a good appreciation of community

important that a design be balanced.

It is

values.

2.4.4

Some design criteria such as vertical clearance at structures are inviolate. Others are less rigid

As the term implies, road safety auditing is a

and some are little more than suggestions.

structured process that brings specialised and

Some of those chosen are for safety reasons,

explicit safety knowledge to bear on a highway

some for service or capacity, while others are based on comfort or aesthetic values.

Road safety audits

project so that it can be quantitatively consid-

The

ered. It is a formal examination of a future or

choice of design criteria is very important in the

existing project in which an independent, quali-

design process and it is essential for the design-

fied examination team reports on the accident

er to have a good understanding of their origin

potential and safety performance of the project.

and background. A design carefully prepared by a designer who has a good understanding, not

The benefits of road safety audits include:

only of the criteria, but also of their background

•

and foundation, and who has judiciously applied

A reduction in the likelihood of accidents on the road network;

•

the community values, will probably create the

A reduction in the severity of accidents on the road network;

desired level of service, safety and economy.

•

An increased awareness of safe design practices among traffic engineers and

For many elements, a range of dimensions is

road designers;

given and the designer has the responsibility of

•

choosing the appropriate value for a particular

A reduction in expenditure on remedial measures; and

application. A designer with economy upper-

•

most in mind may be tempted to apply the mini-

A reduction in the life-cycle cost of a road.


mum value, reasoning that so long as the value is within an accepted range, the design is "sat-

Australian and New Zealand experience has

isfactory". This may or may not be the case.

shown that road safety audits do not add more than four per cent to the cost of a road project.

The designer might find it appropriate to reduce

It is, however, necessary to equate this cost to

values of design criteria, which is not necessar-

the potential benefits of the road safety audit,

ily a poor decision. However, the consequences

e.g.:

need to be thoroughly understood, particularly

•

as they impacts on safety and also on the costs

A saving in time and cost by changing project details at the planning and

and benefits. Ameliorating measures, such as

design stage rather than by changing or

the use of traffic control devices, may need to be

removing a road element once installed; 2-10


• •

A reduction in the likelihood of accidents

By spending more money on construction, other

and therefore in accident costs; and

costs may be reduced (e.g. travel time or crash-

A reduction in the cost of litigation.

es). However, additional expenditure must create increases in benefits or reductions in other costs.

The objectives of a road safety audit are;

Economic analyses can evaluate the

trade-offs between costs and benefits.

•

To identify and report on the accident potential and safety of a road project;

•

The analysis when applied to a road can be

To ensure that road elements with an

highly complex, depending on the scope of the

accident potential are removed; or

•

project.

That the risk of crashes is reduced.

Many formal or informal evaluations

may have been carried out and decisions made, before the geometric designer gets involved. In

Road safety can be audited at any of the follow-

extreme cases, the designer may be so con-

ing six stages, however, the sooner the better: Stage 1 Road safety audit:

strained by decisions already made, that there

Preliminary

is little or no opportunity to judge many of the

design stage Stage 2 Road safety audit:

potential costs and benefits. It is, however, the

Draft design

designer's task to incorporate those judgements

stage Stage 3 Road safety audit:

into planning and design wherever that freedom

Detailed

exists. The designer should also identify situa-

design stage Stage 4 Road safety audit:

tions where policy decisions may unreasonably

Preconstruct-

constrain a satisfactory design. When present-

ion stage Stage 5 Road safety audit:

ed effectively, arguments made by designers

Pre-opening

may affect the timing and scope of projects and

stage

2.4.5

also influence changes to existing policy.

Existing facility The geometric designer determines the horizon-

Economic analysis

tal and vertical alignment and cross section at

Economic analyses form an intrinsic part of any

every point on the road.

In addition, special

civil engineering project where the "value for

planning is required at every location where

money" concept is important.

roadways intersect, to accommodate diverging, converging and conflicting traffic movements. In

Roads are essential for mobility of people and

selecting design dimensions and layouts, the

goods. The benefits of mobility are attained at a

designer can directly affect some of the benefits,

cost. Roads cost money to build and maintain;

costs and impacts of the road, as well as allow

they consume space and affect the environ-

for future expansion.

ment; road travel consumes time, creates noise and pollution, and brings about crashes, etc. All

The hallmark of professionalism in road design

these are the costs of mobility.

is the ability to optimise and foresee the reper-



Stage 6 Road safety audit:

cussions of design decisions on the benefits,

ment technique based on an intensive, system-

costs and impacts of the road.

atic and, especially, creative study of the project to seek the best functional balance between its cost, reliability and performance.

For most, if not all, road projects, the designer will have some scope for value judgements, although this will vary from place to place and

In a road design context, this means that a value

from project to project, governed by policy deci-

engineering exercise should be more than

sions already made. Factors that the designer

merely a way of minimizing construction costs,

may be able to influence include:

but that equal and explicit attention should also be given to the important aspects of safety,

• • • • • • •

Mobility;

operational performance and quality.

Environmental impacts;

value engineering can, and sometimes does,

Safety;

In fact,

result in increased construction costs to reduce

Capital costs;

the life-cycle costs.

Aesthetics; Maintenance costs and

More and more authorities are using the con-

Vehicle operating costs.

cept of value engineering to a more cost-effecIn influencing these factors, the designer will be

tive design. If properly applied, this approach is

guided by jurisdictional policy decisions, such as

a valuable input to the design process where

the relative importance of maintenance cost ver-

functional balances are evaluated explicitly and

sus capital cost or of fuel consumption and air

quantitatively for the full range of life cycle costs

pollution against capital cost.

and benefits and re-evaluated in response to proposed changes in design, construction

2.4.6

sequences and practices. Only in this way can

Value engineering

the true benefits of the value engineering process be realised.

Road design is generally carried out in an envi-


ronment where a limited budget needs to be stretched as far as possible. For this reason

Engineers acting independently of the design

designers are placed under considerable pres-

team often do value engineering. However, the

sure to minimize costs.

concept is applicable at all times to all projects and, to do a complete job, this design team

While economy and fiscal efficiency is a key

should embody value engineering in its design

goal of all designs and should continue to be so,

process. If this is done, the independent value

it is essential that changes in design should be

engineering process will become less neces-

analysed explicitly, evaluating safety in the

sary.

same manner as other criteria, such as construction and maintenance costs, and environmental and operational impacts. One method is "value engineering" which is a proven manage2-12 Chapter 2: Design philosophy and techniques

TABLE OF CONTENTS 3

DESIGN CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1 3.1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

3.2

HUMAN FACTORS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

3.3

3.4

3.5

3.6

3.7

3.8

3.2.1

Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

3.2.2

Other road users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

SPEED. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5 3.3.1

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

3.3.2

Speed classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

3.3.3

Design speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

3.3.4

Operating speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

3.3.5

Application of design speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

DESIGN VEHICLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10 3.4.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

3.4.2

Vehicle classifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

3.4.3

Vehicle characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

3.4.4

Selecting a design vehicle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13

SIGHT DISTANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14 3.5.1

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

3.5.2

Deceleration rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

3.5.3

Object height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

3.5.4

Stopping sight distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15

3.5.5

Effect of gradient on stopping sight distance . . . . . . . . . . . . . . . . . . . . . . . . . 3-16

3.5.6

Variation of stopping sight distance for trucks . . . . . . . . . . . . . . . . . . . . . . . . 3-16

3.5.7

Passing sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18

3.5.8

Decision sight distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-19

3.5.9

Headlight sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20

3.5.10

Barrier sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-21

3.5.11

Obstructions to sight distance on horizontal curves . . . . . . . . . . . . . . . . . . . . 3-21

ENVIRONMENTAL FACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22 3.6.1

Land use and landscape integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23

3.6.2

Aesthetics of design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23

3.6.3

Noise abatement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-24

3.6.4

Air pollution by vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-24

3.6.5

Weather and geomorphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-25

TRAFFIC CHARACTERISTICS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26 3.7.1

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26

3.7.2

Traffic volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26

3.7.3

Directional distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-27

3.7.4

Traffic composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-27

3.7.5

Traffic growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28

3.7.6

Capacity and design volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28

ROAD CLASSIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-30 3.8.1

Classification criteria for South African roads . . . . . . . . . . . . . . . . . . . . . . . . 3-30

3.8.2

Functional classification concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-31

3.8.3

Administrative classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-32

3.8.4

Design type classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-32

LIST OF TABLES Table 3.1: Typical design speeds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8 Table 3.2: Dimensions of design vehicles (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12 Table 3.3: Minimum turning radii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13 Table 3.4: Object height design domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15 Table 3.5: Recommended stopping sight distances for design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16 Table 3.6: Passing sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18 Table 3.7: Decision sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20 Table 3.8: Equivalent passenger car units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28 Table 3.9: Road functional classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-31

LIST OF FIGURES Figure 3.1: Five Axle Vehicles and Multi Vehicle Combinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12 Figure 3.2: Stopping distance corrected for gradient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-17 Figure 3.3: Horizontal restrictions to stopping sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22 Figure 3.4: Relationship of functional road classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-33

Chapter 3 DESIGN CONTROLS 3.1

INTRODUCTION

and behavioural characteristics is thus a vital input into the design task.

The design of a road is that of a three-dimensional structure which should ideally be safe, efficient, functional and economical for traffic

Road users do not all behave in the same way

operations, and which should also be aestheti-

and designs should cater for substantial differ-

cally pleasing in its finished form. However, the

ences in the range of human characteristics and

designer uses dimensions and related criteria

a wide range of responses. However, if the per-

within a design context that recognizes a series

ceptual clues are clear and consistent, the task

of design controls constraining what can be

of adaptation is made easier and the response

achieved. These limitations are imposed by the

of drivers will be more appropriate and uniform.

characteristics of vehicle and driver perform-

For roadway design this translates into some

ance as well as by environmental factors. The

useful principles, viz:

•

designer should, therefore, relate the physical

A roadway should confirm what drivers expect based on previous experience;

elements of the road to the requirements of the

and

driver and vehicle so that consistency in the dri-

•

ver's expectations is achieved and, at the same

Drivers should be presented with clear clues about what is expected of them

time, ensures that environmental and other constraints are accommodated.

Driver Workload and Expectations The driver workload comprises

•

Good road design is the art of combining and

Navigation:

trip planning and route following;

balancing the various controls in a perceptive

•

fashion and is not merely an exercise in three-

Guidance:

following the road and maintaining a safe path in

dimensional geometry. In this chapter, the con-

response to traffic condi-

straints and controls on the design process are

tions; and Control:

steering and speed control

3.2

HUMAN FACTORS These tasks require the driver to receive and

3.2.1

Drivers

process inputs, consider the outcome of alter-

An appreciation of driver performance as part of

native actions, decide on the most appropriate,

the road traffic system is essential for effective

execute the action and observe its effects

road design and operation. When a design is

through the reception and processing of new

incompatible with human capabilities (both of

information. There are numerous problems

the driver and any other road user) the opportu-

inherent in this sequence of tasks, arising from

nities for errors and accidents increase.

both the capabilities of the human driver, and

Knowledge of human performance, capabilities

the interfaces between the human and other 3-1

Chapter 3: Design Controls


•

discussed.

components of the road traffic system (the road

Continuation expectancy. This is the expecta-

and the vehicle). These include inadequate or

tion that the events of the immediate past will

insufficient input available for the task at hand

continue. It results, for example, in small head-

(e.g. during night time driving, as a result of poor

ways, as drivers expect that the leading vehicle

sight distance, or because of complex intersec-

will not suddenly change speed. One particu-

tion layouts). When they become overloaded,

larly

drivers shed part of the input to deal with that

expectance is that of subliminal delineation, e.g.

judged to be more important. Most importantly,

a line of poles or trees or lights at night which

drivers are imperfect decision-makers and may

suggests to the driver that the road continues

make errors, including in the selection of what

straight ahead when, in fact, it veers left or right.

input to shed.

These indications are subtle, but should always

perverse

aspect

of

continuation

be looked out for during design.

The designer must provide all the information the driver needs to make a correct decision

Event expectancy. This is the expectation that

timeously, simultaneously ensuring that the

events that have not happened will not happen.

information is provided at a tempo that does not

It results, for example, in disregard for "at grade"

exceed the driver's ability to absorb it. In the

railway crossings and perhaps for minor inter-

words of the American Association of State

sections as well, because drivers expect that no

Highway and Transportation Officials: (AASH-

hazard will present itself where none has been

TO)

seen before.

A response to this situation is

‘A common characteristic of many high-acci-

more positive control, such as an active warning

dent locations is that they place large or

device at railway crossings that requires that the

unusual demands on the information-pro-

driver respond to the device and not to the pres-

cessing capabilities of drivers.

ence of a hazard.

Inefficient

operation and accidents usually occur where the chance for information-handling errors is

Temporal expectancy. This is the expectation

high. At locations where the design is defi-

that, where events are cyclic (e.g. traffic sig-


cient, the possibility of error and inappropriate

nals), the longer a given state prevails, the

driver performance increases.'

greater is the likelihood that change will occur.

Prior experience develops into a set of

This, of course, is a perfectly reasonable expec-

expectancies that allows for anticipation and for-

tation, but it can result in inconsistent respons-

ward planning, and these enable the driver to

es. For example, some drivers may accelerate

respond to common situations in predictable

towards a green signal, because it is increas-

and successful ways. If these expectancies are

ingly likely that it will change, whereas others

violated, problems are likely to occur, either as a

may decelerate. A response to this is to ensure,

result of a wrong decision or of an inordinately

to the extent possible, that there is consistency

long reaction time. There are three types of

throughout the road traffic system to encourage

driver expectancy:

predictable and consistent driver behaviour.

3-2 Chapter 3: Design Controls

The combined effect of these expectancies is

brakes. Recognition that complex decisions are

that:

time-consuming leads to the axiom in highway

•

drivers tend to anticipate upcoming situ-

design that drivers should be confronted with

ations and events that are common to

only one decision at a time, with that decision

the road they are travelling;

•

being binary, e.g. "Yes" or "No" rather than com-

the more predictable the roadway fea-

plex, e.g. multiple choice. Anything up to 10

ture, the less likely will be the chance for

seconds of reaction time may be appropriate in

errors;

•

complex situations.

drivers experience problems when they are surprised;

•

Design Response

in the absence of evidence to the contrary, drivers assume that they will only have to react to standard situations;

•

Designers should strive to satisfy the following

the roadway and its environment upstream

criteria:

create an expectation of

•

downstream conditions; drivers experi-

and unexpected, unusual or inconsistent

ence problems in transition areas and

design or operational situations avoided

locations with inconsistent design or

or minimized.

operation, and

•

Driver's expectations are recognized,

•

expectancies are associated with all lev-

Predictable behaviour is encouraged through familiarity and habit (e.g. there

els of driving performance and all

should be a limited range of intersection

aspects of the driving situation and

and interchange design formats, each

include expectancies relative to speed,

appropriate to a given situation, and

path, direction, the roadway, the envi-

similar designs should be used in similar

ronment, geometric design, traffic oper-

situations).

ations and traffic control devices.

•

Consistency of design and driver behaviour is maintained from element to ele-

Driver Reaction

ment (e.g. avoid significant changes in

It takes time to process information. After a per-

design and operating speeds along a

son's eyes detect and recognize a given situa-

roadway). The information that is provided should decrease the driver's uncertainty, not

reaction occurs. Reaction time is appreciable

increase it (e.g. avoid presenting sever-

and differs between persons. It also varies for

al alternatives to the driver at the same

the same individual, being increased by fatigue,

time).

drinking, or other causes. The AASHTO brake

•

reaction time for stopping has been set at 2,5 s

Clear sight lines and adequate sight dis tances are provided to allow time for

to recognize all these factors. This value has

decision-making and, wherever possi-

been adopted in South Africa.

ble, margins are allowed for error and recovery.

Often drivers face situations much more complex than those requiring a simple response

With the major response to drivers' require-

such as steering adjustments or applying the

ments being related to consistency of design, it 3-3



•

tion, a period of time elapses before muscular

is worthwhile considering what constitutes con-

between -0,04 and 0,01 results in a fair design.

sistency. Consistency has three elements that

A value of less than -0,04 is not acceptable. A

are the criteria offered for the evaluation of a

negative value for the difference between side

road design:

friction assumed for design and the side friction Design consistency - which cor-

demanded means that drivers are demanding

responds to relating the design speed to actual

more side friction than is assumed to be avail-

driving behaviour which is expressed by the

able - a potentially dangerous situation.

Criterion I

85th percentile speed of passenger cars under

3.2.2

free-flow conditions; Criterion II

Other road users

Operating speed consistency

which seeks uniformity of 85th percentile

Pedestrians

speeds through successive elements of the

The interaction of pedestrians and vehicles

road and

should be carefully considered in road design,

Criterion III

Consistency in driving dynam-

principally because 50 per cent of all road fatal-

ics - which relates side friction assumed with

ities are pedestrians.

respect to the design speed to that demanded at Pedestrian actions are less predictable than

the 85th percentile speed.

those of motorists. Pedestrians tend to select In the case of Criterion 1, if the difference

paths that are the shortest distance between

between design speed and 85th percentile

two points. They also have a basic resistance to

speed on an element such as a horizontal curve

changes in gradient or elevation when crossing

is less than 10 km/h, the design can be consid-

roadways and tend to avoid using underpasses

ered good. A difference of between 10 km/h and

or overpasses that are not convenient.

20 km/h results in a tolerable design and differences greater than 20 km/h are not acceptable.

Walking speeds vary from a 15th percentile


speed of 1,2 m/s to an 85th percentile of 1,8 In the case of Criterion 2, the focus is on differ-

m/s, with an average of 1,4 m/s. The 15th per-

ences in operating speed in moving from one

centile speed is recommended for design pur-

element, e.g. a tangent, to another, e.g. the fol-

poses.

lowing curve. A difference in operating speed between them of less than 10 km/h is consid-

Pedestrians' age is an important factor that may

ered to be good design and a difference of

explain behaviour that leads to collisions. It is

between

tolerable.

recommended that older pedestrians be accom-

Differences greater than 20 km/h result in what

modated by using simple designs that minimize

is considered to be poor design.

crossing widths and assume lower walking

10

and

20

km/h

is

speeds.

Where complex elements such as

For the third Criterion the side friction assumed

channelisation and separate turning lanes are

for the design should exceed the side friction

featured, the designer should assess alterna-

demanded by 0,01 or more.

tives that will assist older pedestrians.

A difference 3-4


•

Pedestrian safety is enhanced by the provision of:

•

er behaviour;

• •

median refuge islands of sufficient width at wide intersections, and

•

Driver capability, driver culture and driv-

lighting at locations that demand multi

Vehicle operating capabilities; The physical characteristics of the road and its surroundings;

• • •

ple information gathering and process ing.

Weather; Presence of other vehicles, and Speed limitations (posted speed limits).

Cyclists Bicycle use is increasing and should be consid-

Speeds vary according to the impression of con-

ered in the road design process. Improvements

straint imparted to the driver as a result of these

such as:

• •

factors.

paved shoulders; wider outside traffic lanes (4,2 m mini-

The objective of the designer is to satisfy the

mum) if no shoulders exist;

• • •

bicycle-safe drainage grates;

road users' demands for service in a safe and

adjusting manhole covers to the grade,

economical way. This means that the facility

and

should accommodate nearly all reasonable

maintaining a smooth, clean riding sur-

demands (speed) with appropriate adequacy

face

(safety and capacity) but should not fail com-

can considerably enhance the safety of a street

pletely under severe load, i.e. the extremely

or highway and provide for bicycle traffic:

high speeds maintained by a small percentage

At certain locations it may be appropriate to sup-

of drivers.

plement the existing road system by providing

designed to operate at a speed that satisfies

specifically designated cycle paths. The design

most, but not necessarily all, drivers.

Roads should, therefore, be

elements of cycle paths are discussed in Various studies have shown that the 85th per-

Chapter 4.

centile speed generally exceeds the posted

SPEED

3.3.1

General

speed limit by a margin of at least 10 km/hr when weather and traffic conditions are favourable. For this reason, design speed is typically equated to the 85th percentile speed.

Drivers, on the whole, are concerned with minimising their travel times, and speed is one of the most important factors governing the selec-

The relationship between road design and

tion of alternate routes to gain time savings.

speed is interactive. While the designer shapes

The attractiveness of a specific road or route is

the elements of the road by the anticipated

generally judged by its convenience in travel

speed at which they will be used, taking into

time, which is directly related to travel speed.

account the inherent economic trade-offs between construction and environmental costs

Various factors influence the speed of vehicles

of alternative alignments (vertical and horizon-

on a particular road. These include:

tal) to match desired travel speed, the speed at 3-5 Chapter 3: Design Controls


3.3

which they will be used depends to a large

rather than for geometric design consid-

extent on the chosen design features.

erations and is aimed at encouraging drivers to travel at appropriate speeds

3.3.2

for all prevailing conditions.

Speed classification

The term "speed" is often used very loosely

3.3.3

Design speed

when describing the rate of movement of road traffic. Road design recognizes various defini-

The most important factor in geometric design is

tions or classifications of speed, all of which are

the design speed. This was previously defined

interrelated. The sub-divisions are:

•

as the highest continuous speed at which indi-

Desired Speed - the speed at which a

vidual vehicles can travel with safety on the road

driver wishes to travel, determined by a

when weather conditions are favourable, traffic

combination of motivation and comfort.

•

Design Speed - the speed selected as a

volumes are low and the design features of the

safe basis to establish appropriate geo-

road are the governing condition for safety. The

metric design elements for a particular

current definition is simply states that the design

section of road and which should be a

speed is the speed selected as the basis for

logical one with respect to topography,

establishing appropriate geometric elements for

anticipated operating speed, the adja-

a section of road. These elements include hori-

cent land use and the functional classifi-

zontal and vertical alignment, superelevation

cation of the road.

•

and sight distance.

Operating Speed - observed speeds

Other elements such as

during free flow conditions. For an indi-

lane width, shoulder width and clearance from

vidual driver, operating speed is gener-

obstacles are indirectly related to design speed.

ally lower than desired speed since

•

operating conditions are not usually

The chosen design speed should be a logical

ideal.

one consistent with the road function as per-

Running Speed - the average speed

ceived by the driver and also one that takes into

maintained over a given route while a

account the type of road, the anticipated operat-

vehicle is in motion. The running time is

ing speed, and the terrain that the road travers-


the length of the road section divided by the time required for the vehicle to trav-

es. Where a difficult condition is obvious, driv-

el through the section. Thus, in deter-

ers are more apt to accept a lower speed than

mining the running speed, the times en

where there is no apparent reason for it.

route when the vehicle is at rest are not taken into account in the calculations.

Other relevant factors include traffic characteris-

Running speeds are generally used in

tics, land costs, speed capabilities of vehicles,

road planning and capacity and service

aesthetics, economics and social or political

level analyses. The difference between

impacts. A highway of higher functional classifi-

running speed and design speed is

•

strongly affected by traffic volumes.

cation may justify a higher design speed than a

Posted Speed - is a speed limitation set

less important facility in similar topography, par-

for reasons of safe traffic operations

ticularly where the savings in vehicle operation 3-6


and other operating costs are sufficient to offset

always possible that the signpost may be

the increased costs of right-of-way and con-

obscured, illegible, removed or even simply not

struction. A low design speed, should not be

perceived by the driver. Isolated design speed

assumed where the topography is such that

changes are, therefore, to be avoided.

drivers are likely to travel at high speeds. The need for a multilane cross-section suggests When carefully selected, these factors should

that traffic volumes are high. A design speed of

result in a design speed which is acceptable to

at least 120 km/h should be used if the topogra-

all but a very few drivers.

Above minimum

phy permits. Major roads, even if two-lane two-

design values should be used where feasible

way roads, should also be designed to this

though consistency is essential.

speed if possible. Rolling terrain may, however, necessitate a reduction to 100 km/h in the

When a substantial length of road is being

design speed and, in the case of mountainous

designed, it is desirable to adopt a constant

terrain, it may even be necessary to reduce the

design speed to maintain consistency. Changes

design speed to 80 km/h.

in terrain and other physical controls may, however, dictate a change in design speed on cer-

Secondary and tertiary roads may have lower

tain sections. Each section, however, should be

design speeds than those advocated for the pri-

relatively long, compatible with the general ter-

mary road network. However, where traffic is

rain or development through which the road

likely to move at relatively high speeds on these

passes.

roads, higher design speeds should be select-

The justification for introducing a

ed.

reduced design speed should be obvious to the driver. A case in point is where a road leaves relatively level terrain and starts traversing hilly

There is still debate as to whether speeds

or mountainous terrain. Moreover, the introduc-

greater than 120 km/h should be used for

tion of a lower or higher design speed should

design purposes on freeways.

not be effected abruptly but over sufficient dis-

speeds not only safeguard against early obso-

tance to encourage drivers to change speed

lescence of the highway, but also provide an

gradually.

increased margin of safety for those driving at high speeds. That there is some validity in this

Where design speeds exceed 90 km/h the vari-

statement is reflected by the fact that the design

ation between successive speeds should be lim-

speed of high-type roads is now at least 120

ited to 10 km/h and, below 80 km/h, this varia-

km/h as compared with 56 km/h in 1927, a

tion should be limited to 20 km/h. Where it is

change brought about by the continuing

necessary to change the design speed, the new

increase in vehicle performance.

design speed should apply to an extended section of road. Even if properly signposted, isolat-

The choice of a design speed for a dual car-

ed design speed variations are hazardous as

riageway is much less influenced by construc-

they do not match driver expectations and it is

tion cost than that for other rural roads. In prac3-7



Higher design

tice, lower design speeds are often accepted on

120 km/h for unhindered vehicles on a four-lane

single carriageway roads in order to keep con-

divided roadway. Use of a design speed of 130

struction costs within certain limits.

km/h should therefore satisfy driver demands in

There is

danger in this philosophy since, although drivers

most areas.

will obviously accept lower speeds in what are clearly difficult locations, repeated studies have

The selected design speed should be logical

shown that they do not adjust their speeds to the

and in harmony with the topography and the

importance of the facility. Instead they endeav-

functional classification of a road. Careful con-

our to operate at speeds consistent with the traf-

sideration should also be given to its relation-

fic on the facility and its physical limitations.

ship to other defined speeds. While no hard relationships have been established, choice of

Ideally, then, design speed should be chosen to

design speed can simultaneously accommodate

reflect the 85th percentile desired speed that is

and influence desired, operating, running and

likely to materialize. This is often achievable for

posted speeds.

roads for which the primary function is mobility and where severe physical constraints do not exist.

Limited studies in South Africa have

Table 3.1 provides an indication of typical


shown that the 85th percentile speed exceeds

design speeds for different classes of roads.


3.3.4

Operating speed

factors, the driver's initial response is to react to the anticipated situation rather than to the actu-

Operating speed is measured under free flow

al situation. In most instances, the two are sim-

conditions. The term "spot speed" is sometimes

ilar enough not to create conflicts. If the initial

used to denote operating speed. For an individ-

response is incorrect, operation and safety may

ual driver, operating speed is generally lower

be severely affected.

than desired speed since operating conditions are not usually ideal. When reference is made

Some agencies conduct speed surveys to deter-

to the operating speed of all vehicles in the traf-

mine operating speeds at various points along a

fic stream, this is taken as being the 85th per-

section of roadway. The results can be com-

centile of all observed speeds.

pared with the design speed, and may lead to a policy change in the selection of design speeds.

Operating speed has a variety of uses. It is gen-

3.3.5

erally used as a measure of level of service at

Application of design speed

uninterrupted flows. It can also be used to monitor the effect of flow constrictions, such as inter-

Consistency of design is fundamental to good

sections or bridges. Since operating speeds at

driver performance, based on satisfying the dri-

ideal sections of road are indicative of speeds

ver's expectations. Design consistency exists

desired by motorists, they can be used to guide

when the geometric features of a continuous

the selection of design speed on improved or

section of road are consistent with the opera-

new facilities.

tional characteristics as perceived by the driver. The traditional approach to achieving design

When the design speed is less than the desired

consistency has been through the application of

speed, drivers should be warned to modify their

the design speed process. Once selected, the

speed, as studies have shown that crash rates

design speed is used to determine values for

increase as the operating speed of a particular

the geometric design elements from appropriate

vehicle deviates from the mean operating speed

design domains. However, application of this procedure alone

The typical driver can recognize or sense a log-

does not guarantee design consistency. There

ical operating speed for a given roadway based

are several limitations of the design speed con-

on knowledge of the system, posted speed lim-

cept that should be considered during design:

its, appraisal of the ruggedness of the terrain,

1.

traffic volumes and the extent, density and size

date specified design speed does not necessar-

of development. Studies have shown that char-

ily ensure a consistent alignment design.

acteristics, such as the number of access

Design speed is significant only when physical

points, nearby commercial development, road

road characteristics limit the speed of travel.

width and number of lanes, have a significant

Thus, a road can be designed with a constant

influence on vehicle speeds. Based on these

design speed, yet have considerable variation in

Selection of dimensions to accommo-



of the other vehicles on the roadway.

speeds achievable and therefore to a driver

where the design speeds are less than 100

appear to have a wide variation in character. For

km/h at horizontal curves on rural two-lane high-

example, the radii of curves within a section

ways.

should be consistent, not merely greater than

5.

the minimum value.

may have quite different levels of perceived haz-

2.

For horizontal alignments, design speed

ard. Entering a horizontal curve too fast will

applies only to curves, not to the connecting tan-

almost certainly result in loss of control, so driv-

gents. Design speed has no practical meaning

ers adjust their speed accordingly.

on tangents. As a result, the operating speed on

the possibility of a curtailed sight distance con-

a tangent, especially a long one, can often sig-

cealing a hazard is considered as a remote

nificantly exceed the design speed of the road

occurrence. Unfortunately drivers do not gener-

as a whole.

ally adjust their speed to compensate for sight

3.

distance restrictions.

The design speed concept does not

In addition, different alignment elements

However,

ensure sufficient coordination among individual It

To help overcome these weaknesses in the use

controls only the minimum value of the maxi-

of design speed to design individual geometric

mum speeds for the individual features along an

elements, speed profiles are used. A speed pro-

alignment. For example, a road with an 80 km/h

file is a graphical depiction (which can be mod-

design speed may have only one curve with a

elled) showing how the 85th percentile operat-

design speed of 80 km/h and all other features

ing speed varies along a length of road. This

with design speeds of 120 km/h or greater. As

profile helps to identify undesirably large differ-

a result, operating speeds approaching the crit-

entials in the 85th percentile operating speed

ical curve are likely to exceed the 80 km/h

between successive geometric elements, e.g. a

design speed. Such an alignment would comply

curve following a tangent.

geometric features to ensure consistency.

with an 80 km/h design speed, but it would vio-

When an existing road is being improved, actu-

late a driver's expectancy and result in undesir-

al operating speeds can be measured to create

able alignment.


4.

a speed profile, but interpretation of the profile

Vehicle operating speed is not neces-

can be difficult, depending on the complexity of

sarily synonymous with design speed. Drivers

geometric and other features that may cause

normally adjust speed according to their desired

drivers to change speed. For a new road, pre-

speed, posted speed, traffic volumes and per-

diction of operating speeds is needed to create

ceived alignment hazards. The perception of

a speed profile model.

hazard presented by the alignment may vary along a road designed with a constant design

3.4

DESIGN VEHICLES

3.4.1

Introduction

speed. The speed adopted by a driver tends to vary accordingly and may exceed the design speeds. A report on studies in Australia and the US concluded that 85th percentile operating

The physical characteristics of vehicles and the

speeds consistently exceed design speeds

proportions of the various sizes of vehicles 3-10


using a road are positive controls in design and

with full trailers.

Buses include single unit

define several geometric design elements,

buses, articulated buses and intercity buses. In

including intersections, on- and off-street park-

establishing the design dimensions for the vari-

ing, site access configurations and specialized

ous vehicle classes, this guide focuses on vehi-

applications such as trucking facilities. It is nec-

cles in regular operation only.

essary to identify all vehicle types using the facility, establish general class groupings and

Vehicles defined in the Road Traffic Act include:

select hypothetical representative design vehi-

•

Passenger cars and minibuses (kombis);

cles, within each design class. The dimensions

• • •

used to define design vehicles are not averages or maxima, nor are they legal limiting dimensions. They are, in fact, typically the 85th per-

Standard single unit buses; Articulated buses ("Bus Train"); Two axle trucks, with and without trailers;

centile or 15th percentile value of any given

• •

dimension. The design vehicles are therefore hypothetical vehicles, selected to represent a

Three and four axle vehicles; Three, four and five axle articulated trucks;

particular vehicle class.

• •

Five and six axle articulated trucks, and

3.4.3

Vehicle characteristics

Multi vehicle combinations.

The dimensions in the previous Geometric Design Manual were based on typical design vehicles in South Africa pertinent to 1965. The range of vehicle types and their operating char-

The dimensions adopted for the various design

acteristics have changed significantly since

vehicles are given in Table 3.2.

then. The vehicle size regulations have also undergone substantial revisions which have

The WB15 vehicle has an overall length of 17 m,

generally resulted in larger trucks on the roads

whereas the regulations allow for a semi-trailer

as well as in an increased use of recreational

to have an overall length of 18,5 m. The multi-

utility vehicles.

ple vehicle combination, being a semi-trailer

3.4.2

ration, can have a maximum overall length of 22

Vehicle classifications

m. Examples of these vehicles are illustrated in Figure 3.1.

Three general classes of vehicles have been selected for this design guide: passenger cars, The passenger car class

f these vehicles are expected to use a route with

includes compacts and subcompacts, recre-

any frequency, the designer will have to careful-

ational utility vehicles and all light vehicles and

ly plan the layout of the intersections to ensure

light delivery trucks (vans and pickups). The

that they can be accommodated. As described

truck class includes single-unit trucks, truck

below, accommodation does not necessarily

tractor-semi trailer combinations, and trucks or

require lane widths sufficient to complete a turn-

truck tractors with semi trailers in combination

ing movement within the lane.

trucks and buses.


A degree of


plus trailer and typically in an Interlink configu-

* Distance between SU rear wheels and trailer front wheels


Figure 3.1: Five Axle Vehicles and Multi Vehicle Combinations

encroachment on adjacent lanes is permissible

In terms of regulation 355 (a) of the Road Traffic

depending on the frequency of occurrence.

Act, all vehicles must be able to describe a minimum turning radius not exceeding 13,1m.

Turning Radii

Vehicle height

In constricted situations where the templates

Regulation 354 in the Road Traffic Act limits the

are not appropriate, the capabilities of the

overall height of a double decker bus to 4,6

design vehicle become critical. Minimum turn-

metres, and that of any other vehicle to 4,3

ing radii for the outer side of the vehicle are

metres. The 15th percentile height of a passen-

given in Table 3.3. It is stressed that these radii

ger car has been established to be 1,3 m. This

are appropriate only to crawl speeds.

has been selected for design purposes as the


passenger car is also an object that has to be

for the purpose of measuring critical turning

seen by the driver in the cases of passing and

dimensions.

intersection sight distance.

Commercially available templates and computer software define the turning envelope of vehicles

Driver Eye Height

in forward motion and also support plotting of

The passenger car is taken as the critical vehi-

the turning envelope of reversing non-articulat-

cle for driver eye height and a figure of 1,05

ed vehicles. Prediction of the reversing behav-

metres is recommended. For buses and single

iour of articulated vehicles is, however, very

unit vehicles a typical value is 1,8 metres and for

complex, mainly because this behaviour is

semi-trailer combinations the height of the eye

inherently unstable, and additional turning con-

can vary between 1,9 metres and 2,4 metres.

trols come into play.

Vehicle Turning Paths

3.4.4

The designer should allow for the swept path of The

swept path is established by the outer trace of

In general, buses and heavy vehicles should be

the front overhang and the path of the inner rear

used as the design vehicle for cross section ele-

wheel. This turn assumes that the outer front

ments, with the car as the design vehicle for the

wheel follows the circular arc defining the mini-

horizontal and vertical alignment.

mum turning radius as determined by the vehi-

major intersections along arterial roads or with-

cle steering mechanism.

in commercial areas, it is common practice to

For most

accommodate the semi trailer. The occasional It is assumed that the turning movements critical

larger vehicle may encroach on adjacent lanes

to the design of roadway facilities are done at

while turning but not on the sidewalk.

low speeds.

At these speeds, the turning

behaviour of vehicles is mainly determined by

Many authorities designate and signpost specif-

their physical characteristics. The effects of fric-

ic truck routes. The intersections of two truck

tion and dynamics can safely be ignored. It is

routes or intersections where trucks must turn to

also assumed that groups of evenly spaced

remain on a truck route should be designed to

axles mounted on a rigid bogie act in the turn as

accommodate the largest semi-trailer combina-

a single axle placed at the centre of the group

tion expected to be prevalent in the turning traf-



the selected design vehicle as it turns.

Selecting a design vehicle

fic stream. Where local residential roads inter-

It is also necessary to consider the terrain or

sect truck routes or arterials, the intersections

obstructions on the inside of horizontal curves

should be specifically designed not to accom-

when evaluating adequate sight distance.

modate trucks easily, in order to discourage

3.5.2

them from travelling through the residential

Deceleration rates

area. Although research in North America has shown On major haulage routes, large tractor-trailer

that drivers can choose (or apply) a deceleration

combination trucks are prevalent and these

of greater than 5 m/sec2, there is a large degree

routes should be designed to accommodate

of variability in driver and vehicle capabilities

them. Raised channelising islands are typically

and the 90th percentile deceleration is of the

omitted in recognition of low pedestrian volumes

order of 3,4 m/sec2.

and other constraints such as right of way and

Transportation Engineers' Traffic Engineering

construction costs.

The absence of raised

Handbook states that decelerations of up to 3,0

islands also allows more manoeuvring area for

m/sec2 are reasonably comfortable for passen-

large trucks.

ger car occupants. This deceleration rate has

The Institute of

been adopted for these guidelines.

3.5

SIGHT DISTANCE 3.5.3

3.5.1

General The object height to be used in calculation of

A critical feature of safe road geometry is ade-

stopping sight distance is often a compromise

quate sight distance. As an irreducible mini-

between the length of the resultant sight dis-

mum, drivers must be able to see objects in the

tance and the cost of construction. Stopping is

road with sufficient time to allow them to

generally in response to another vehicle or large

manoeuvre around them or to stop. Other forms

hazard in the roadway. To recognize a vehicle

of sight distance are pertinent. They are:

•

as a hazard at night, a line of sight to its head-

Passing sight distance, which is

lights or taillights would be necessary. Larger


required for substantial portions of the

•

•

length of two-lane roads;

objects would be visible sooner and provide

Intersection sight distance, to allow a

longer stopping distances. To perceive a very

driver on the minor road to evaluate

small hazard, for example, a surface obstruc-

whether it is safe to cross or enter the

tion, a zero object height may be necessary.

opposing stream of traffic;

However, at the required stopping sight dis-

Decision sight distance where, for

tances for high speeds, small pavement varia-

example, a driver must be able to see

tions and small objects (especially at night)

and respond to road markings;

• •

Object height

would not be easily visible. Thus, most drivers

Headlight sight distance, typically applied to sag vertical curves, and

travelling at high speeds would have difficulty in

Centre line barrier sight distance.

stopping before reaching a small obstruction.


3.5.4

A driver will usually attempt to take evasive

Stopping sight distance

action rather than to stop for small objects on The minimum sight distance on a roadway

the roadway. Although not recommended as a

should be sufficient to enable a vehicle travelling

design parameter, the time available to manoeu-

at the design speed on a wet pavement to stop

vre is a useful measure when examining varia-

before reaching a stationary object in its path.

tions of geometry in restricted situations or reconstruction projects. In this case, the appro-

Stopping sight distance is the sum of two dis-

priate object is the pavement surface.

tances:

•

The designer should adopt an object height

the distance traversed by the vehicle from the instant the driver sights an

based on the probability of a particular object

obstruction to the instant the brakes are

occurring on the roadway, as shown in Table 3.4

applied, and

below. For stopping sight distance, a conserva-

•

tive tail light height of 0,60 m is recommended. If fallen trees or rocks are a real risk, an object

the distance required to stop the vehicle from the instant the brakes are applied

text, research has established that the probability of a collision involving an object of a height of

These are referred to as brake reaction distance

0,15 m or less is infinitesimally small. For pass-

and stopping distance, respectively.

ing sight distance, an object height of 1,30 m will

These two components, using a reaction time of

allow the driver to discern the top of an oncom-

2,5 seconds and a deceleration rate of 3,0 m/s2,

ing car. A zero object height is recommended

result in the relationship

where road washouts are a serious risk. It is

s =

v (0,694 + 0,013v)

s =

stopping sight distance, m

v =

initial speed, km/h

also recommended for pavement markings in

where:

situations such as at intersections or interchanges, where these provide essential guidance. 3-15 Chapter 3: Design Controls


height of 0,15 m is recommended. In this con-

Stopping sight distances calculated using this

ously stated.

The brake reaction time is

equation are given in Table 3.5, rounded up for

assumed to be the same as for level conditions.

design purposes. Also shown in the table for

The stopping sight distance for design speeds

general interest are the values of stopping sight

from 30 to 130 km/h as corrected for gradient is

distance adopted in the 2000 AASHTO Policy

illustrated in Figure 3.2.

on the Geometric Design of Highways and The sight distance at any point on the highway

Streets, the "Green Book 2000".


is generally different in each direction, particuIn the measurement of stopping sight distance,

larly on straight roads in rolling terrain. As a

the driver's eye height is taken as being at 1,05

general rule, the sight distance available on

m and the object height is as defined in Table

downgrades is longer than on upgrades, more

3.4.

or less automatically providing the necessary corrections for grade. This is because down

3.5.5

Effect of gradient on stopping

grades are normally followed by sag vertical

sight distance

curves, with the following grade also being visible to the driver

When a highway is on a gradient, the equation

3.5.6

for stopping sight distance becomes

Variation of stopping sight dis-

tance for trucks in which G is the percentage gradient divided by

The recommended minimum stopping sight dis-

100, with upgrades being positive and down-

tance model directly reflects the operation of

grades negative and the other terms as previ-

passenger cars and trucks with antilock braking 3-16


maximum efficiency under load;

systems. Trucks with conventional braking sys-

• •

tems require longer stopping distances from a given speed than do passenger cars. However, AASHTO suggests that the truck driver is able to

Uneven load between axles; Propensity of truck drivers not to obey posted speed limits;

•

see the vertical features of the obstruction from substantially further because of the higher driv-

Inefficient brakes of articulated trucks, and

•

er eye height. In addition, posted speed limits

Effect of curvature. where some of the

for trucks in South Africa are considerably lower

friction available at the road/tyre inter-

than for passenger vehicles. Separate stopping

face is used to hold the vehicle in a cir-

sight distances for trucks and passenger cars

cular path.

are, therefore, not generally used in highway To balance between the costs and benefits in

design.

designing for trucks, truck stopping sight disThere is, however, evidence to suggest that the

tances should be checked at potentially haz-

sight distance advantages provided by the high-

ardous locations. In general, the deceleration

er driver eye level in trucks do not always com-

rate for trucks is 1,5 m/s2. The driver's eye

pensate for their inferior braking. Some reasons

height is taken as being at 1,8 m and the object

for the longer truck braking distances include:

height is as defined in Table 3.4.

Poor braking characteristics of empty trucks. The problem relates to the sus

The designer should also consider measures

pension and tyres that are designed for

such as additional signs to improve road safety

Figure 3.2: Stopping distance corrected for gradient



•

if stopping sight distance is found to be inade-

It should be pointed out that there are a variety

quate for trucks and it is not possible to improve

of models defined for the overtaking manoeu-

the geometric design. However, it is empha-

vre.

sized that provision of signage is not a substi-

required to enable an overtaking driver to com-

tute for appropriate design practices.

plete or abort a manoeuvre already com-

3.5.7

menced, with safety. In addition to this distance,

Passing sight distance

The distances usually given are those

the Austroads approach introduces a distance On a two-lane rural road, the passing manoeu-

that is needed for the driver to identify a length

vre is one of the most significant yet complex

of road as a potential overtaking zone. This

and important driving tasks. The process is rel-

"establishment" distance is considerably longer

atively difficult to quantify, primarily because of

than the overtaking manoeuvre distance.


the many stages involved, the relative speed of vehicles and the lengthy section of road needed

Table 3.6 shows the minimum overtaking sight

to complete the manoeuvre.

Road safety,

distances generally used for various design

capacity and service levels are all affected by

speeds. Passing manoeuvres involving trucks,

the passing ability of faster vehicles. This abili-

particularly in South Africa, require longer dis-

ty is influenced by a variety of factors, including

tances than those indicated. Designers must

traffic volumes, speed differentials, road geom-

take this into account for roads where significant

etry and human factors. The minimum sight dis-

percentages of heavy vehicles are expected in

tance required by a vehicle to overtake safely on

the traffic stream.

two-lane single carriageway roads is the distance which will enable the overtaking driver to

As mentioned above, the designer should seek

pass a slower vehicle without causing an

opportunities to introduce passing lanes on two-

oncoming vehicle to slow below the design speed.

lane roads, particularly where the terrain limits 3-18


sight distance. A report on a review and evalu-

Limiting sight distances to those provided for

ation of research studies concluded that passing

stopping may also preclude drivers from per-

and climbing lane installations reduce collision

forming evasive manoeuvres, which are often

rates by 25 per cent compared to untreated two-

less hazardous and otherwise preferable to

lane sections.

They provide safer passing

stopping. Even with an appropriate complement

opportunities for drivers who are uncomfortable

of standard traffic control devices, stopping sight

in using the opposing traffic lane and for those

distances may not provide sufficient visibility for

who become frustrated when few passing

drivers to corroborate advance warning and to

opportunities exist, owing to terrain or traffic

perform the necessary manoeuvres. It is evi-

density.

dent that there are many locations such as exits from freeways, or where lane shifts or weaving

Sections with adequate passing sight distance

manoeuvres are performed where it would be

should be provided as frequently as possible.

prudent to provide longer sight distances. In

The appropriate frequency is related to operat-

these circumstances, decision sight distance

ing speed, traffic volumes and composition, ter-

provides the greater length that drivers need. If

rain and construction cost. As a general rule, if

the driver can see what is unfolding far enough

passing sight distance cannot be economically

ahead, he or she should be able to handle

provided at least once every 2 km, passing

almost any situation.

lanes should be considered. The 2+1 crosssection currently in vogue in Europe has some

Decision sight distance, sometimes termed

merit.

anticipatory sight distance, is the distance

This three-lane cross-section has two

required for a driver to:

lanes in one direction and a single lane in the

•

opposing direction. At about two to three kilo-

cult-to-perceive information source or

metre intervals, the second lane is allocated to

hazard in a roadway environment that

movement in the opposite direction. A minimum

may be visually cluttered;

shoulder width is required as discussed in

•

Chapter 4.

recognize the hazard or its potential threat;

Decision sight distance

select an appropriate speed and path; and

•

initiate and complete the required safety manoeuvre safely and efficiently.

Stopping sight distances are usually sufficient to allow reasonably competent and alert drivers to

Because decision sight distance gives drivers

stop under ordinary circumstances. However,

additional margin for error and affords them suf-

these distances are often inadequate when:

• • •

Drivers must make complex decisions;

ficient length to manoeuvre their vehicles at the

Information is difficult to perceive, or

same or reduced speed rather than to just stop,

Unexpected or unusual manoeuvres are

it is substantially longer than stopping sight dis-

required.

tance.



• 3.5.8

detect an unexpected or otherwise diffi-

Drivers need decision sight distances whenever

traffic control devices for advance warning.

there is likelihood for error in either information

Although a sight distance is offered for the right

reception, decision-making, or control actions.

side exit, the designer should bear in mind that

Critical locations where these kinds of errors are

exiting from the right is in total conflict with driv-

likely to occur, and where it is desirable to pro-

er expectancy and is highly undesirable. The

vide decision sight distance include:

only reason for providing this value is to allow

•

Approaches to interchanges and inter

for the remote eventuality that a right side exit

sections;

has to be employed.

•

Changes in cross-section such as at toll plazas and lane drops;

• •

In measuring decision sight distances, the 1 050

Design speed reductions, and

mm eye height and 0 mm object height have

Areas of concentrated demand where

been adopted.

there is apt to be "visual noise", e.g where sources of information, such as

3.5.9

roadway elements, opposing traffic, traf-

Headlight sight distance

fic control devices, advertising signs and construction zones, compete for attention.

Headlight sight distance is typically used in


establishing the rate of change of grade for sag The minimum decision sight distances that

vertical curves. At speeds above 80 km/h, only

should be provided for specific situations are

large, light coloured objects can be perceived at

shown in Table 3.7. If it is not feasible to provide

the generally accepted stopping sight distances.

these distances because of horizontal or vertical

A five-fold light increase is necessary for a 15

curvature or if relocation is not possible, special

km/h increase in speed and a 50 per cent reduc-

attention should be given to the use of suitable

tion in object size.


For night driving on highways without lighting,

ensure that two opposing vehicles travelling in

the length of visible roadway is that which is

the same lane should be able to come to a stop

directly illuminated by the headlights of the vehi-

before impact. A logical basis for the determi-

cle. This length is typically shorter than the min-

nation of the barrier sight distance is that it

imum sight distance.

should at least equal twice the stopping distance. Values given in the South Africa Road

When headlights are operated on low beam, the

Traffic

reduced candlepower at the source and the

approach.

Signs

Manual

approximate

this

downward projection angle significantly restrict the visible length of roadway surface.

Barrier sight distance is measured to an object height of 1,3 metre from an eye height of 1,05

For crest vertical curves, the area beyond the

m.

headlight beam point of tangency with the road-

The object height is the height of an

approaching passenger car.

way surface is shadowed and receives only indirect illumination. Also, a general limit of 120 to

Hidden dip alignments are poor design practice,

150 metres sight distance is all that can be safe-

but are found on many rural roads. They typi-

ly assumed for visibility of an unilluminated

cally mislead drivers into believing that there is

This corre-

more sight distance available than actually

sponds to a satisfactory stopping sight distance

exists. In checking vertical alignment, designers

for 80 to 90 km/h or a decision time of about 5

should pay attention to areas where this defi-

seconds at 100 km/h.

ciency exists, and ensure that drivers are made aware of any such inadequacies.

Since the headlight mounting height (typically about 600 mm) is lower than the driver eye

3.5.11 Obstructions to sight distance

height (1 050 mm for design), sight distance is

on horizontal curves

controlled by the height of the vehicle headlights and a one degree upward divergence of the light

Physical features, such as a concrete barrier

beam from the longitudinal axis of the vehicle.

wall, a bridge pier, a tree, foliage, or the back

Any object within the shadow zone must be high

slope of a cutting, can affect available sight dis-

enough to extend into the headlight beam to be

tance. Accordingly, designs need to be checked

directly illuminated.

in both the horizontal and vertical planes for

3.5.10 Barrier sight distance

obstructions.

Barrier sight distance is not a geometric design

Minimum radii of horizontal curvature are deter-

factor, but is rather an operational guide to the

mined by application of vehicle dynamics and

driver to promote safety on two-lane roads.

not through sight distance controls. It is, therefore, possible that the selected radius may not

Barrier sight distance is the limit below which

be adequate to ensure the safe stopping sight

overtaking is legally prohibited, in order to

distance requirements. 3-21


If the obstructions to


object on a bitumen surfacing.

.

sight distance are immovable, realignment

L

=

lane width (m)

may be necessary.

s

=

stopping sight distance for specific gradient

The problem is illustrated in Figure 3.3. The dri-

and design speed (m)

ver's eye is assumed to be at the centre of the

from Figure 3.1.

nearside lane. The chord AB is the sight line and the curve ACB is the stopping sight dis-

Given the nature of the relationship, a trial-and-

tance. A zero gradient is assumed. It follows

error approach to the solution is required.

that selection of a radius for a given distance of

Figure 3.3: Horizontal restrictions to stopping sight distance obstruction from the inner lane centre line will

3.6

ENVIRONMENTAL FACTORS


constitute an under-design if the inner lane is on a downgrade.

A road is a key element in the modern environment with wide ranging implications. Planning

A radius to satisfy stopping sight distance crite-

for effective integration is therefore essential. In

ria can be calculated from the following formula;

South Africa, the National Environmental Management Act of 1998 lays down prescriptions for the provision and operation of infrastructure, including roads. The designer should

where C

R

=

=

distance from centre of

be aware of the constraints imposed by this law.

inside lane to obstruc-

For example, constraints may include avoiding a

tion (m)

particular watercourse or wetland area, or

radius of curve (m)

accommodating prescribed mitigatory meas3-22


ures such as screening berms or sound fences.

ing landform best by avoiding disruption

In carrying out their mandate to plan and design

of major topographical features;

•

road systems, road designers should consider

ing landform to good effect and which

on the one hand, making facilities aesthetically

minimizes the scale of earthworks;

pleasing and being "good neighbours" in the

•

community and, on the other, providing safe and

To design profiles which reflect existing natural slopes;

•

efficient transportation links to users.

3.6.1

To find an alignment that uses the exist-

To retain the least road footprint, by the return of land to its former use;

Land use and landscape integra

•

tion

To use existing landform to minimize noise and visual intrusion: for example, placing a road in a cutting or behind rising

With regard to environmental factors, the objec-

ground to protect settlements;

•

tive of route selection should be to choose a

To develop new landforms, including

route that has both the minimum effect on land-

mounds and false cuttings, to screen the

form and requires the smallest number of large

road from settlements, and

•

earthworks. Integration with the existing land-

To achieve a balance between horizontal and vertical alignment.

form can best be achieved by grading out cuttings and embankments to slopes that reflect the surrounding topography. This in turn may

3.6.2

Aesthetics of design

affect adjacent sites of conservation or heritage interest and, in such cases, a balance needs to

Design aesthetics and attention to landform are

be struck. A major consideration is that non-

very closely related topics. Aesthetic improve-

renewable resources, such as wetlands, should

ments can often be achieved without incurring

be avoided wherever possible.

additional

costs,

provided

the

designer

approaches the subject in a sensitive manner.

Designs should aim to achieve the best possible

In fact, alignments that are visually pleasing are

use of excavated materials, thus minimizing the

usually less hazardous than other alignments.

need for off-site spoil or borrow pits. If off-site

On any roadway, creating pleasing appearance

the same good design principles as those used

is a worthwhile objective. Scenic values can be

on site, achieved by liaison with the appropriate

considered along with safety, utility, economy,

planning authority. Earthworks can only be integrated successfully if the new landform and its

and all the other factors considered in planning

soil structure allow effective strategic rehabilita-

and design. This is particularly true of the many

tion. Restoration to agricultural use can be a

portions of the National Road system situated in

particularly effective strategy.

areas of natural beauty.

The location of the

road, its alignment and profile, the cross section design, and other features should be in harmo-

Design objectives should be:

•

To choose the route least damaging to

ny with the setting. Economy consistent with

the landscape and which respects exist-

traffic needs is of paramount importance, 3-23



works are necessary, they should be subject to

although a reasonable additional expenditure

either by absorbing the noise or deflecting it

can be justified to enhance the beauty of the

upwards.

highway.

include, for example, depressing and some-

Strategies addressing noise levels

times covering the roadway or by installing This topic is addressed in detail in Chapter 5.

3.6.3

sound barriers of earth or masonry. However, these may also trap air pollutants.

Noise abatement

Special sound barriers may be justified at cer-

Noise is defined as an unwanted sound, a sub-

tain locations, particularly along ground level or

jective result of sounds that intrude on or inter-

elevated roads through noise-sensitive areas.

fere with activities such as conversation, think-

Concrete, wood, metal, or masonry walls are

ing, reading or sleeping. Motor vehicle noise is

very effective in deflecting noise. One of the

generated by the functioning of equipment with-

more aesthetically pleasing barriers is the earth

in the vehicle, by its aerodynamics, by the action

berm that has been graded to achieve a natural

of tyres on the roadway and, in metropolitan

form that blends with the surrounding topogra-

areas, by short-duration sounds such as braking

phy. The feasibility of berm construction should

squeal, exhaust backfires, hooters and sirens.

be planned as part of the overall grading plan for

The decrease in sound intensity with distance

the roadway. There will be instances where an

from the source is influenced by several factors.

effective earth berm can be constructed within

Measurements taken near roads show that dou-

the normal right-of-way or with a minimal addi-

bling the distance results in a lowering of 3 dBA

tional right-of-way purchase. If the right-of-way

over clean, level ground and 4,8 dBA over lush

is insufficient to accommodate a three metre

growth.

high berm, a lower berm can be constructed in combination with a wall or screen to achieve the desired height.

Some sustained (ambient) noise is always present.

In a quiet residential neighbourhood at Shrubs, trees or ground covers are not very effi-

night, it is in the 32 to 43 dBA range; the urban

cient in shielding sound because of their perme-


residential daytime limits are about 41 to 53

ability to the flow of air. However, almost all

dBA. In industrial areas the range is 48 to 66

buffer plantings offer some noise reduction, and

dBA, while, in downtown commercial locations

exceptionally wide and dense plantings may

with heavy traffic, it is 62 to 73 dBA. Increases

result in substantial reductions in noise levels.

up to 9 dBA above ambient noise levels bring

Even where the noise reduction is not consid-

only sporadic protests. Protests become wide-

ered significant, the aesthetic effects of the

spread with increases in the 9 to 16 dBA range.

plantings will produce a positive influence.

At increases greater than 16 dBA, there may be community action.

3.6.4

A design objective is to keep noise at or below

The highway air-pollution problem has two

acceptable levels and this can be achieved

dimensions: the area-wide effects of primarily

Air pollution by vehicles


reactive pollutants; and the high concentrations

If at all possible, major routes should not tra-

of largely non-reactive pollutants at points or

verse such areas but should rather be located

corridors along or near roads. The motor vehi-

on the higher ground surrounding inversion-

cle is a primary contributor to both forms,

prone valleys, with relatively low-volume road

accounting for an estimated 70 per cent of the

links serving developments in the valley areas.

CO, 50 per cent of the HC, and 30 per cent of

Attention should also be paid to prevailing wind

the NOx.

directions so that routes bypassing local communities are located downwind of these settle-

Area-wide conditions are exacerbated when

ments.

temperature inversions trap pollutants near the ground surface and there is little or no wind, so

3.6.5

that concentrations of pollutants increase. For some individuals, eyes burn and breathing is dif-

Weather and geomorphology

Land shape, on a broad scale, as well as pre-

ficult. It is alleged that lives can be shortened

vailing weather conditions, which could influ-

and some deaths have actually resulted from

ence the design, are factors over which the

these exposures. Also, certain kinds of vegeta-

designer does not have any control. Certain

tion are killed, stunted, or the foliage burned.

areas of the country are prone to misty condi-

The quantity of air pollutants can be reduced by

tions and others subject to high rainfall. Both

judicious design. Exhaust emissions are high

are factors that have to be taken into account in

while vehicle engines are operating at above-

design.

average levels of output, for example while accelerating from a stationary position or when

Mist and rain both cause reduced visibility.

climbing a steep hill. Smooth traffic flow at con-

Where these are a regular occurrence, they

stant speeds, such as in "green wave" condi-

tend to lie in belts, sometimes fairly narrow,

tions on a signalised route, reduces exhaust

across the landscape.

emissions in addition to leading to a reduction in

acquire local knowledge about the quirks of the

noise levels. By way of contrast, speed humps,

weather patterns and seek ways to reduce their

which are popular as speed-reducing devices in

effect.

residential areas, have the dual penalty of increased pollution and increased noise levels.

Where it is not possible to avoid a mist belt, the

In rural areas, vertical alignments should be

designer should pay particular attention to the

designed with a minimum of "false rises".

concept of the "forgiving highway", by providing flat side slopes and avoiding alignments where

In addition to being able to modify the quantity

short radius curves follow each other in quick

of pollutants in the atmosphere, the designer

succession.

can influence the extent to which emissions

short radius horizontal curves are particularly to

impact on local communities.

Temperature

be avoided. A real effort should also be put into

inversions that trap polluted air are typically

avoiding high fills. In conditions of heavy mist,

associated with closed or bowl-shaped valleys.

vehicles will tend to move very slowly but, even 3-25


Steep downgrades followed by


Designers should

at speeds significantly below the design speed

If hourly flows are ordered from highest to low-

of the road, the restricted visibility will lead to

est, it is customary, in rural areas, to design for

high levels of stress. Drivers are more likely to

the thirtieth highest hourly flow, i.e that flow

make incorrect decisions when under stress and

which is exceeded in only 29 hours of the year.

designers should thus do everything possible to

This is because rural roads have very high sea-

keep stress levels within manageable limits.

sonal peaks and it is not economical to have a road congestion-free every hour throughout the

3.7

year. In urban areas, seasonal peaks are less

TRAFFIC CHARACTERISTICS

pronounced and the 100th highest hourly flow is

3.7.1

considered a realistic flow level for design pur-

General

poses. Factual information on expected traffic volumes is an essential input to design. This indicates

To predict hourly flows, it is necessary to know

the need for improvements and directly affects

the ADT and the peaking factor, ß. The param-

the geometric features and design.

eter, ß, is a descriptor of the traffic flow on a given road and depends on factors such as the

Traffic flows vary both seasonally and during the

percentage and incidence of holiday traffic, the

day. The designer should be familiar with the

relative sizes of the daily peaks, etc. The peak-

extent of these fluctuations to enable him or her

ing factor can fluctuate between -0,1 and -0,4. A

to assess the flow patterns. The directional dis-

value of -0,1 indicates minimal seasonal peak-

tribution of the traffic and the manner in which its

ing. This value of ß should be used in urban

composition varies are also important parame-

designs. A value of -0,4 suggests very high

ters. A thorough understanding of the manner in

seasonal peaks and would normally be applied

which all of these behave is a basic requirement

to roads such as the N3. As a general rule, a

of any realistic design.

value of -0,2 could be used as being a typical value. Equation 3.1 below can be used to esti-

3.7.2

Traffic volumes

mate flows between the highest and 1030th


highest hour. Although not a particularly good Traffic flow is measured by the number of vehi-

model, flows beyond the 1030th hour can be

cles passing a particular station during a given

estimated by using a straight line relationship

period of time. Typically, the flow of interest is

from the 1030th flow to zero veh/hr at the

the Average Daily Traffic (ADT). Flows may also

8760th or last hour of the year.

be reported per hour, such as the "hourly

QN

=

0,072ADT(N/1030)ß

observed traffic volume" or the "thirtieth-highest

where QN

=

two-directional flow in

hour" or "hundredth-hour", which are commonly

N-th hour of year (veh/h)

used for design purposes. Very short duration

ADT

=

average daily traffic (veh/day)

flows, such as for a five-minute period, are typically applied to studies of signalised intersec-

N

=

hour of year

tions.

ß

=

peaking factor.


It is interesting to note that the peak hour factor,

towards the central business district (with rela-

K, quoted in the Highway Capacity Manual is

tively low outward-bound flows), whereas the

often assumed for design purposes to be 0,15.

afternoon peak is in the reverse direction. It is

Reference is commonly made to the 30th high-

important to realize that the design flow is actu-

est hour of the year as being the design hour.

ally a composite and not a single value. A road

Applying a value of -0,2 to ß, and assuming N to

must be able to accommodate the major flow in

be 30, QN according to Equation 3.1 is 0,146 x

both directions.

ADT for the thirtieth highest hour. The actual distribution to be used for design purDesigners need to estimate future traffic flows

poses should be measured in the field. If an

for a road section. It is recommended that a

existing road is to be reconstructed, the field

design period of 20 years be used for forward

studies can be carried out on it beforehand. For

planning. The 30th or 100th highest flow used

new facilities, measurements should be made

in the design is that occurring in the design year,

on adjacent roads from which it is expected the

typically twenty years hence. Staged construc-

traffic will be diverted and modelling techniques

tion or widening of roads over this period can be

applied.

a feature of an economical design. Directional distribution is relatively stable and The capacity of rural road sections is influenced

does not change materially from year to year.

by the following key characteristics:

Relationships established from current traffic

•

Road configuration - e.g. two-lane two-

movements are normally also applicable to

way, multi-lane divided or undivided;

future movements.

Operating speed; Terrain;

3.7.4

Lane and shoulder width; Traffic composition, and

Vehicles of different sizes and mass have differ-

Gradients.

ent operating characteristics.

Trucks have a

higher mass/power ratio and occupy more road-

In the case of two-lane two- way roads, the fol-

way space than passenger cars. Consequently,

lowing additional factors are important:

• •

Traffic composition

they constitute a greater impedance to traffic

Directional distribution of traffic flow; and Passing opportunities - sight distance,

flow than passenger vehicles, with the overall

overtaking lanes, climbing lanes or slow vehicle

effect that one truck is equivalent to several pas-

turnout lanes.

senger cars. For design purposes, the percentage of truck traffic during the peak hours has to

3.7.3

Directional distribution

be estimated.

Directional distribution of traffic is an indication

For design of a particular highway, data on the

of the tidal flow during the day. In urban areas,

composition of traffic should be determined from

the morning peak traffic is typically inbound

traffic studies. 3-27


Truck traffic is normally


• • • • •

expressed as a percentage of total traffic during

It is difficult to define the life of a "road" because

the design hour in the case of a two-lane road;

major segments may have different lengths of

and as a percentage of total traffic in the pre-

physical life. Each segment is subject to varia-

dominant direction of travel in the case of a

tions in estimated life expectancy because of

multi-lane road.

influences not readily subject to analysis such as obsolescence and unexpected changes in

It is not practical to design for a heterogeneous

land use, resulting in changes in traffic volumes,

traffic stream and, for this reason, trucks are

pattern and load. Regardless of the anticipated

converted to equivalent Passenger Car Units

physical life of the various elements of the road,

(PCUs). The number of PCU's associated with

it is customary to use a single value as the

a single truck is a measure of the impedance

"design life". In essence, the road is expected

that it offers to the passenger cars in the traffic

to provide an acceptable level of service for this

stream. This topic is exhaustively addressed in

period. Whether or not any of its various com-

the Highway Capacity Manual and is not dis-

ponents have a longer physical life expectancy

cussed further here.

than this design life is irrelevant. For example,


the alignment and, in some instances, the surPassenger car unit equivalents have, in general,

facing of roads built during Roman times are still

been derived from observations as illustrated in

in use today without there being any reference

Table 3.8. The values offered serve only as a

to a design life of 2 000 years.

rough indication and the designer should refer

A typical value of design life is twenty years. In

to the Highway Capacity Manual for detailed

the case of major and correspondingly expen-

calculation of PCU's appropriate to the different

sive structures a design life of fifty years may be

environments and circumstances.

assumed. This should not, however, be confused with the concept of a bridge being able to

3.7.5

Traffic growth

withstand the worst flood in fifty years.

3.7.6

The design of new roads or of improvements to existing roads should be based on the future

Capacity and design volumes

The term capacity is used here to define the ability of a road to accommodate traffic under

traffic expected to use the facilities. 3-28


given circumstances. Factors to be taken into account are the physical features of the road itself and the prevailing traffic conditions

Physical features having considerable influence are the type of intersection, i.e. whether plain, channelised, roundabout or signalised, the number of intersecting traffic lanes and the adequa-

Prevailing road conditions

cy of speed-change lanes.

Capacity figures for uninterrupted flow on highways have to be modified if certain minimum

Unlike the physical features of the highway,

physical design features are not adhered to.

which are literally fixed in position and have def-

Poor physical features that tend to cause a

inite measurable effects on traffic flows, the pre-

reduction in capacity are:

•

Narrow traffic lanes.

vailing traffic conditions are not fixed but vary

Lane widths of

from hour to hour throughout the day. Hence,

3.65 m are accepted as being the mini-

•

mum necessary for heavy volumes of

the flows at any particular time are a function of

mixed traffic, i.e. before capacity of the

the speeds of vehicles, the composition of the

lane is reduced.

traffic streams and the manner in which the

Inadequate shoulders. The narrowness,

vehicles interact with each other, as well as of

or lack of, shoulders alongside a road

the physical features of the roadway itself.

cause vehicles to travel closer to the centre of the carriageway, thereby increasing the medial traffic friction. In

Capacity

addition, vehicles making emergency

The term "capacity" was introduced in the USA

stops must, of necessity, park on the

in the Highway Capacity Manual, in which it is

carriageway. This causes a substantial

defined as "the maximum number of vehicles

reduction in the effective width of the

that can pass a given point on a roadway or in a

road, thereby reducing capacity.

designated lane during one hour without the

Side obstructions. Vertical obstructions

traffic density being so great as to cause unrea-

such as poles, bridge abutments, retain

sonable delay, hazard, or restriction to the driv-

ing walls or parked cars that are located

•

within about 1,5 m of the edge of the

ers' freedom to manoeuvre under the prevailing

carriageway contribute towards a reduc-

roadway and traffic conditions". This definition

tion in the effective width of the outside

gives a reasonable method of approach but, in

traffic lane.

practice, it is necessary to choose one or more

Imperfect horizontal or vertical curva-

arbitrary criteria of what constitutes restriction of

ture. Long and/or steep hills and sharp

traffic movement, or "congestion".

bends result in restricted sight distance. As drivers then have reduced opportuni-

The Highway Capacity Manual procedure must

ties to pass, the capacity of the facility

however be used for specific road capacity

will be reduced.

designs. In addition to the above, the capacities of certain For typical South African conditions and to bal-

rural roads and the great majority of urban roads

ance financial, safety and operational consider-

are controlled by the layouts of intersections. 3-29



•

ations it is recommended that the capacity of a

Classification of roads by design types based on

two-lane rural road be taken on average as

the major geometric features (e.g. freeways) is

being between 10 000 and 12 000 vehicles per

the most helpful one for road location and

day while, for freeways, consideration could be

design purposes. Classification by route num-

given to changing from a four-lane to a six-lane

bering is the most helpful for road traffic opera-

freeway when the traffic flow is of the order of 35

tional purposes, whilst administrative classifica-

000 to 40 000 vehicles per day.

tion is used to denote the level of government responsible for, and the method of, financing

3.8

road facilities.

ROAD CLASSIFICATION

Functional classification, the

grouping of roads by the character of service The classification of roads into different opera-

they provide, was developed for transportation

tional systems, functional classes or geometric

planning purposes.

types is necessary for communication between engineers, administrators and the general pub-

3.8.1

lic. Classification is the tool by which a complex

African roads

Classification criteria for South

network of roads can be subdivided into groups having similar characteristics. A single classifi-

Although numerous classification criteria are

cation system, satisfactory for all purposes,

used on road networks world-wide, in South

would be advantageous but has not been found

Africa there are basically three criteria used to

to be practicable. Moreover, in any classifica-

classify road types. These are:

tion system the division between classes is

• • •

often arbitrary and, consequently, opinions differ on the best definition of any class. There are


various schemes for classifying roads and the

Functional classification; Administrative classification, and Design Type classification, based on traffic usage.

class definitions generally vary depending on

This document uses the third type for design

the purpose of classification.

purposes.

The principal purposes of road classification are

As a result of growing awareness of the interde-

to:-

pendency of the various modes of transport as

•

Establish logical integrated systems

well as the creation of Metropolitan Transport

that, because of their particular service,

Authorities within South Africa's major metropol-

should be administered by the same

itan areas there is considerable overlap

jurisdiction;

• •

between the functional and administrative clas-

Relate geometric design control and other design standards to the roads in

sification criteria.

each class, and

areas, the general public is more dependent on,

Establish a basis for developing long-

and understands, a route numbering or func-

range programmes, improvement priori

tional classification than on an administrative

ties and financial plans.

classification of roads within the area.


Within these metropolitan

Although these guidelines are based on a

Another and less comprehensive form of func-

design type classification, the three different

tional classification was developed for the pur-

approaches mentioned above are briefly

poses of road signing as shown in the South

described in order to provide a picture of the

African Road Traffic Signs Manual. As stated in

road system hierarchy in South Africa.

SARTSM, "There are definite limits to the number of ways

3.8.2

Functional classification concept

in which GUIDANCE signs and specifically DIRECTION signs can be made to indicate with

For transportation planning purposes, road are

sufficient immediate recognition potential, the

most effectively classified by function. The func-

different classes into which the road network is

tional classification system adopted for the

divided for signing purposes."

South African road network is illustrated in Table 3.9. This was used for the South African Rural

Classification for signing thus differentiates

Road Needs Study carried out during the early

mainly between numbered and unnumbered

1980s.

routes and, in respect of numbered routes, also draws a distinction between freeways and other



roads.

Roads have two functions: to provide mobility

arate levels of government each have a roads

and to provide land access. However from a

function mandated to them. Despite this sepa-

design standpoint, these functions are incom-

ration of authority for various classes or roads, it

patible. For mobility, high or continued speeds

is essential to bear in mind that roads act as a

are desirable and variable or low speeds unde-

total system or network and that the subdivision

sirable; for land access, low speeds are desir-

of roads into administrative classes bears no

able and high speeds undesirable. For exam-

relation to the functional type of a road under the

ple, freeways provide a high degree of mobility,

control of a specific authority. The administra-

with access provided only at spaced inter-

tive classification approach thus divides the

changes to preserve the high-speed, high-vol-

South African road network into:

• • •

ume characteristics of the facility. The opposite is true for local, low-speed roads that primarily provide local access. The general relationship of functionally classified systems in serving

Local Authority roads.

bears no relation to the design type of a road

3.4.

under the control of a specific authority. Thus, until the turn of the 21st century, there were sec-

Given a functional classification, design criteria

tions of National Roads that were unsurfaced,

can be applied to encourage the use of the road as intended.

with very low traffic volumes, the most heavily

Design features that can convey

trafficked roads in South Africa, also up to the

the level of functional classification to the driver

turn of the century, often being the responsibili-

include width of roadway, continuity of align-

ty of a local authority.

ment, spacing of intersections, frequency of

Furthermore certain

"routes" in the country comprise both National

access points, building setbacks, alignment and

and Provincial roads, and could even include

grade standards, and traffic controls.


Provincial, and

This administrative classification of roads also

mobility and land access is illustrated in Figure

3.8.3

National,

local authority roads.

Administratively, a

National Road, which is denoted as such by a

Administrative classification

legal proclamation, in general comprises those roads that form the principal avenues of com-

Legislation and, in some instances, the

munication between major regions of the coun-

Constitution, assigns to certain levels of govern-

try, and/or between major population conurba-

ment the responsibilities for providing, regulat-

tions and/or between major regions of South

ing and operating roads and streets for public

Africa and other countries.

use. This concept and the principles of law that support it were developed in Great Britain, and

3.8.4

even earlier by the Romans. Within the limits of

Design type classification

its constitutional powers, a particular road

The most widely accepted design type criteria

authority may delegate its authority for roads to

are those developed by AASHTO which classify

bodies such as a Roads Board or a Toll Road

a road system into:

Concessionaire. In South Africa, the three sep-

•

Freeways;


• •

Arterial roads other than freeways, and

Design designations of these specific National

Collector roads.

Roads are as follows;

For each classification, specific design stan-

Class I

Primary Roads

dards and criteria and access and other policies

Class IA

Primary Freeways in rural areas

have been developed and are applied.

•

Illustrative threshold ADT (with more

For use in the present document the following

than 12% heavy vehicles) = 15 000

service or design classifications are proposed,

veh/d.

related to design traffic volumes given in ADT terms.

•

Each classification groups roads with

Average travel distances on links indi-

similar functions. The factors that influence the

cated by at least 45 per cent of the trips

classification of a roadway to a certain group

being more than 2 hours in duration;

•

include;

• • •

Trip purpose (for the majority of users);

purposes, and

•

Trip length; Size and type of population centre

Minimum design speed 130 km/h.

Class IB

served;

• •

May be provided for network continuity

Primary Freeways in metropolitan areas

Traffic characteristics, and

•

Network and system requirements

Illustrative threshold ADT = 20 000 veh/d



Figure 3.4: Relationship of functional road classes

•

Average travel distance on links indicat ed by majority of trips being less than 2 hours in duration

•

Form integral element of Metropolitan Road Network

•

Generally extension of Rural Freeways (Class IA roads)

•

Minimum design speed 130 km/h

Class II

Primary Arterials, 4 lane single carriageway roads

Class IIA

•

Primary Rural Arterials

Generally provided when 2 lane single carriageway road reaches capacity and freeway not financially affordable

•

Ilustrative threshold ADT : 8 000 - 10 000 veh/d, with bottom end of scale applicable where percentage of truck traffic exceed 15 per cent

•

Minimum design speed 120 km/h

Class IIB

•

Design in context in which it operates.

Class III

•

Primary Metropolitan Arterial Secondary Rural Arterial

Provided to address inter-regional travel demands, or providing access to tourist or National resource areas

•

Provided to address inter-regional travel demands, or providing access to tourist or National resource areas

•

Provided to address inter-regional trav-


el demands, or providing access to tourist or National resource areas

• • • • • • • • • •

Two lane, single carriageway roads Class IIIA Design ADT greater than 4 000 veh/d Minimum design speed 120 km/h Class IIIB Design ADT 500 - 4 000 veh/d Minimum design speed 110 km/h Class IIIC Design ADT less than 500 veh/d Minimum design speed 100 km/h.


TABLE OF CONTENTS 4

ROAD DESIGN ELEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4.1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

4.2

HORIZONTAL ALIGNMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4.2.1 General Controls for horizontal alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4.2.2 Tangents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3 4.2.3 Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6 4.2.4 Superelevation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 4.2.5 Transition curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17 4.2.6 Lane widening. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18

4.3

VERTICAL ALIGNMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-21 4.3.1 General controls for vertical alignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-21 4.3.2 Grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-22 4.3.3 Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-26

4.4

CROSS-SECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29 4.4.1 General controls for cross-sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-30 4.4.2 Basic Lanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-32 4.4.3 Auxiliary lanes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-33 4.4.4 Kerbing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-39 4.4.5 Shoulders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-40 4.4.6 Medians . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-43 4.4.7 Outer separators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-45 4.4.8 Boulevards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-45 4.4.9 Bus stops and taxi lay-byes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-46 4.4.10 Sidewalks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-48 4.4.11 Cycle paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-49 4.4.12 Slopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-51 4.4.13 Verges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-52 4.4.14 Clearance profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53 4.4.15 Provision for utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53 4.4.16 Drainage elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-55

LIST OF TABLES Table 4.1: Minimum radii for various values of emax (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 Table 4.2: Design domain for emax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9 Table 4.3: Values of superelevation for above min radii of curvature (%): emax = 4 % . . . . . . . . . . . . . . . 4-11 Table 4.4: Values of superelevation for above min radii of curvature (%): emax = 6 % . . . . . . . . . . . . . . . 4-12 Table 4.5: Values of superelevation for above min radii of curvature (%): emax = 8 % . . . . . . . . . . . . . . . 4-13 Table 4.6: Values of superelevation for above minradii of curvature (%): emax = 10 %. . . . . . . . . . . . . . . 4-13 Table 4.7: Maximum relative gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14 Table 4.8: Lane adjustment factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15 Table 4.9: Maximum radii for use in spiral transition curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18 Table 4.10: Lengths of grade for 15 km/h speed reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-24 Table 4.11: Maximum gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-24 Table 4.12: Minimum values of k for crest curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27 Table 4.13: Minimum k-values for barrier sight distance on crest curves . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27 Table 4.14: Minimum k-values for sag curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29 Table 4.15: Warrant for climbing lanes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-35 Table 4.16: Shoulder widths for undivided rural roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-42 Table 4.17: Warrants for pedestrian footways in rural areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-49 Table 4.18: Cycle lane widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-51 Table 4.19: Typical widths of roadside elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53 Table 4.20: Scour velocities for various materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-57

LIST OF FIGURES Figure 4.1: Dynamics of a vehicle on a curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7 Figure 4.2: Methods of distributing of e and f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10 Figure 4.3: Attainment of superelevation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15 Figure 4.4: Superelevation runoff on reverse curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16 Figure 4.5: Superelevation runoff on broken-back curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16 Figure 4.6: Typical turning path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-19 Figure 4.7: Truck speeds on grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-23 Figure 4.8: Sight distance on crest curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-25 Figure 4.9: Sight distance on a sag curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-28 Figure 4.10: Cross-section elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-31 Figure 4.11: Verge area indicating location of boulevard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-46 Figure 4.12: Typical layout of a bus stop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-47 Figure 4.13: Bicycle envelope and clearances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-50 Figure 4.14: Collision rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-54 Figure 4.15: Prediction of utility pole crashes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-54 Figure 4.16: Typical drain profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-58

Chapter 4 ROAD DESIGN ELEMENTS 4.1

INTRODUCTION

4.2

HORIZONTAL ALIGNMENT

Regardless of the philosophy brought to bear on

The horizontal alignment comprises three ele-

the design of a road or the classification of the

ments: tangents, circular curves and the transi-

road in the network, the final design comprises

tions between tangents and curves.

a grouping and sizing of different elements. A vertical curve is an element. The development

Tangents (sometimes referred to as "straights")

of super elevation is an element. The cross-

have the properties of bearing (direction or

section is heavily disaggregated, comprising a

heading) and length. Circular curves also have

large group of elements. Geometric design thus

two properties; radius and deflection (or devia-

comprises:

tion) angle, these two properties directly leading

•

The selection of elements to be incorpo-

to a third property of interest, namely curve

rated in the design;

length. The basic properties of transition curves

The sizing of the selected elements, and

are shape and length. Design of the horizontal

Linking the elements into a three-dimen-

alignment includes selection of the values asso-

sional sequence.

ciated with each of these six properties.

The selected, sized and linked elements in

4.2.1

combination represent the final design of a road

General Controls for horizontal alignment

which, when built, must constitute a network link which will satisfactorily match the criteria of

The horizontal alignment should be as direction-

safe, convenient and affordable transportation

al as possible and consistent with the topogra-

with minimum side effects, simultaneously

phy. However, it is equally important in terms of

addressing needs other than those pertaining

context sensitive design to preserve developed

directly to the movement of people or freight.

properties and areas of value to the community.

A road network comprises links and nodes. The

Winding alignments composed of short curves

intersections and interchanges are the nodes

and tangents should be avoided if at all possible

and the roads connecting them the links.

because they tend to cause erratic operation

Intersections and the elements that constitute

and a high consequent crash rate.

them are discussed in Chapter 6, with interchanges being dealt with in Chapter 7.

Most design manuals recommend that minimum radii should be avoided wherever possible, sug-

In this chapter, the various elements involved in

gesting that flat curves (i.e. high values of

link design are discussed.

radius) should be used, retaining the minimum

4-1 Chapter 4: Road Design Elements


• •

for the most critical conditions. The concept of

1 000 metres. If a curve radius is such that the

consistency of design, on the other hand, sug-

curve either has a normal camber or a crossfall

gests that the difference between design speed

of 2 per cent, the limitation on curve length falls

and operating speed should ideally be held to a

away and the curve can be dealt with as though

maximum of 10 km/h, with a 20 km/h difference

it were a tangent.

still representing tolerable design. This could be construed as a recommendation that minimum

Broken-back (also referred to as "flat back")

curvature represents the ideal, which is wholly

curves are a combination of two curves in the

at variance with the historic approach to selec-

same direction with an intervening short tan-

tion of curve radius. What is actually intended

gent.

however is that the designer should seek to

grounds of aesthetics and safety. Not only are

employ the highest possible value of design

broken-back curves unsightly but drivers do not

speed for any given circumstance.

always recognize the short intervening tangent

These should be avoided on the twin

and select a path corresponding to the radius of For small deflection angles, curves should be

the first curve approached, hence leaving the

sufficiently long to avoid the appearance of a

road on the inside and part way along the tan-

kink.

A widely adopted guideline is that, on

gent. On roads in areas with restrictive topog-

minor roads, curves should have a minimum

raphy, such as mountain passes, the designer

5O

may have no option but to accept the use of bro-

and that this length should be increased by 30

ken-back curves but should, nevertheless, be

length of 150 metres for a deflection angle of metres for every

1O

decrease in deflection

aware of their undesirability. Where the inter-

angle. On major roads and freeways, the mini-

vening tangent is longer than 500 metres, the

mum curve length in metres should be three

appellation "broken-back" is no longer appropri-

times the design speed in km/h. The increase in

ate.


length for decreasing deflection angle also applies to these roads. In the case of a circular

Reverse curves are also a combination of two

curve without transitions, the length in question

curves but in opposite directions with an inter-

is the total length of the arc and, where transi-

vening short tangent. These curves are aes-

tions are applied, the length is that of the circu-

thetically pleasant but it is important to note that

lar curve plus half the total length of the transi-

the intervening tangent must be sufficiently long

tions.

to accommodate the reversal of superelevation between the two curves.

South African practice recommends an upper limit to the length of horizontal curves. Curves

Although compound curves afford flexibility in

to the left generally restrict passing opportuni-

fitting the road to the terrain and other ground

ties and, furthermore, dependant on their radius,

controls, their use should be avoided outside

operation on long curves tends to be erratic.

intersection or interchange areas. Once they

For this reason, it is desirable to restrict the

are on a horizontal curve, drivers expect the

length of superelevated curves to a maximum of

radius to remain unaltered hence supporting a 4-2

Chapter 4: Road Design Elements

constant speed across the length of the curve.

Obviously this would be of fairly limited duration

A compound curve is thus a violation of driver

in terms of the season and the likelihood of a

expectancy and can be expected to have a cor-

prolonged gradient of this magnitude, whereas

respondingly high crash rate.

the east-west bearing would be a problem all year round and over an extended period of time

4.2.2

Tangents

during the day. The designer should be aware

Two fundamentally different approaches can be

of this problem and, if possible, avoid selecting

adopted in the process of route determination.

bearings that reduce visibility.

In a curvilinear approach, the curves are located

problem cannot be avoided, warning signage

first and thereafter connected up by tangents.

may be considered.

If the dazzle

This approach can be adopted with advantage in mountainous or rugged terrain. A typical con-

In the second instance, a bearing at right angles

sequence of this approach is that curves tend to

to the prevailing wind direction can cause prob-

be long and tangents short.

Because of the

lems for empty trucks with closed load compart-

local topography, the approach to route determi-

ments, e.g. pantechnicons, or passenger cars

nation most frequently adopted in South Africa

towing caravans. On freight routes or routes

is for the tangents to be located first and the

with a high incidence of holiday traffic, the

curves fitted to these tangents thereafter.

designer should seek to avoid this bearing. If not possible, locating the road on the lee of a hill

Bearing

that could then offer shelter from the wind may be an option to be explored in order to offer

Economic considerations dictate that, where

some relief.

other constraints on route location are absent, roads should be as directional as possible. In

Length

consequence, tangents may be located on bearings that have an adverse impact on driver comfort and safety. Two conditions require consid-

The minimum allowable length of tangent is that

eration.

which accommodates the rollover of superele-

In the first instance, a combination of gradient, direction of travel and time of day may cause the

It has been found that extremely long tangents,

driver to be dazzled by the sun. The most obvi-

e.g. lengths of twenty kilometres or more, have

ous example is the east-west bearing where the

accident rates similar to those on minimum

vehicle would be moving towards the rising or

length tangents, the lowest accident rate occur-

setting sun. No direction of travel is completely

ring in a range of eight to twelve kilometres.

exempt from this problem. For example, when

This range is recommended for consideration in

travelling at midday in mid-winter in a northerly

fixing the maximum length of tangent on any

direction up a gradient steeper than about eight

route. This maximum is based on the assump-

per cent, the sun can present a problem.

tion of a design speed of 120 km/h or more. At



vation in a reverse curve situation.

lower design speeds, it is necessary to consider

constitute poor design so that, at a greater level

maximum lengths considerably shorter than

of precision than the proposed rule of thumb,

eight to twelve kilometres as discussed below.

three possibilities arise. These are:

•

Case 1 - The length of tangent is less

As a rough rule of thumb (which is adequate for

than or equal to the distance, Tmin,

planning purposes) the maximum length of tan-

required to accelerate from the operat

gent in metres should not exceed ten to twenty

ing peed appropriate to one curve to that appropriate to the next curve;

times the design speed in kilometres/hour. If the

•

achievable maximum length of tangent across

Case 2 - The length of tangent allows acceleration to a speed higher than that

the length of the route is regularly greater than

appropriate to the next curve but not as

this guideline value, thought should be given to

high as the desired speed remote from

consideration of a higher design speed. Rules

inhibiting curves, and

•

of thumb have their limitations and, in this case,

Case 3 - The length of tangent, Tmax,

application should be limited to design speeds

allows acceleration up to desired speed.

of 100 km/h or less. At a design speed of 120

These cases are intended to be applied in areas where curves are to design

km/h or higher, a maximum tangent length of

speeds of 100 km/h or less.

1 200 to 2 400 metres would clearly be meaningless.

To apply the guideline values of allowable speed

As an example of the application of the rule of

variations and differences, it is necessary to

thumb, a design speed of 80 km/h would sug-

estimate the operating speeds on the curves

gest that the maximum length of tangent should

preceding and following a tangent.

be in the range of 800 to 1 600 metres. In this

absence of information specific to South African

range, drivers would tend to maintain a fairly

conditions, the international value of V85 given

constant speed of about 80 km/h. At greater

in Equation 4.1 will have to be employed.

lengths of tangent, drivers would accelerate to

V85 = 105,31 + 1,62 x 10-5 x B2 - 0,064 x B

some higher speed only to decelerate at the fol-

(4.1) where V85

lowing curve. This oscillation in speed is inher-

85th percentile speed

=

(km/h)

ently dangerous as discussed in more detail


In the

B

below. Consistency of design dictates that:

•

and

The difference between design speed

=

Bendiness

=

57 300/R (degrees/km)

θ

=

Deviation angle

L

=

Total length of curve (metres).

and 85th percentile speed, and

•

Variations in 85th percentile speeds between successive elements

The general form for bendiness, B, in the case

should be limited as far as possible. Research

where the circular curve is bounded by transi-

has indicated that, ideally, these differences and

tion curves is

variations should be less than 10 km/h but that an acceptable design still results if they are less

B = (Lcl1/2R + Lcr/R +Lcl2/2R) x 180/π x 1000

than 20 km/h. Differences in excess of 20 km/h

L 4-4


(4.2)

where L

=

where

LCl1 + LCr + LCl2

C

=

average passenger car speed (km/h)

with LCl1 and LCl2 being the lengths of the preceding and succeeding transition curves and

Q

=

flow (veh/h)

LCr the length of the circular curve.

G

=

gradient (per cent)

D

=

directional split as a decimal fraction

The length of tangents is to be compared with PT

the values of Tmin and Tmax that, as suggest-

=

number of trucks in the

ed, are a function of the speeds achievable on

traffic stream as a dec

the curves preceding and following these tan-

imal fraction

gents.

PS

The values are calculated using an

acceleration or deceleration rate of 0,85

m/s2

=

number of semi trailers in the traffic stream as a decimal fraction.

(determined by car-following techniques and which also corresponds to deceleration without

In the absence of significant volumes of traffic

braking) TMIN =

2

V851 - V852

2

and on a level grade, the average speed would,

(4.3)

according to this equation, be of the order of 120

22,03

km/h, suggesting that the 85th percentile speed estimated by Eq. 4.1 is very conservative.

and TMax =

2(V85Tmax)2-(V851)2-(V852)2 22,03

If the tangent length is shorter than TMin the tan-

(4.4)

where V85Tmax =

V851

V852

=

=


for the operating speeds of the two adjacent

on long tangent, i.e.

curves to be within the difference ranges

V85, (km/h)

described above to constitute good or tolerable


design. In essence, acceleration to the operat-

on preceding curve

ing speed of the following curve could take

(km/h)

place on this curve itself. Where it is necessary


to decelerate to the operating speed of the fol-

on following curve

lowing curve, it will be necessary for the driver to

(km/h)

brake in order to achieve the appropriate speed

A tangent has a bendiness of zero so that V85

at the start of the curve. It follows that, on a two-

for TMax is, according to Eq. 4.1, 105,31 km/h.

lane two-way road, tangent lengths shorter than

South African research has derived an expres-

TMin are potentially dangerous.

sion for average speed as given in Eq. 4.5. Where the tangent length is just equal to TMax C

=

143,96 - 10,39 ln Q - 0,04 (G2 -

the vehicle will be able to accelerate from the

5,20) - 18,08 D -33,89 PT -

operating speed of the preceding curve to the

54,15 PS

desired speed and then immediately decelerate

(4.5)

to the operating speed of the following curve. In 4-5 Chapter 4: Road Design Elements


gent is non-independent and it is only necessary

this case, the difference between operating

ceeding curves. These define, in effect, the rel-

speeds on each curve and the desired speed

ative design domain of horizontal curvature on

has to be within the allowable range.

any given road, i.e. the possible range of values of radius of any curve given, the radius of the preceding curve.

In the case of a tangent length falling in the range TMin < T < TMax , it will be necessary to calculate the highest operating speed that can

The safety of any curve is dictated not only by

be reached by accelerating at a rate of 0,85

the external factors described above but also by

m/s2

from the operating speed of the first curve,

factors internal to it, namely radius, supereleva-

allowing for a deceleration at the same rate to

tion, transitions and curve widening. Of these

the operating speed of the second. It is the dif-

factors, the most significant is radius as

ference between this maximum operating speed

research carried out in Washington State shows

and the speeds on the adjacent curves that is

consistently that crash frequency increases as

critical. The maximum operating speed on a

the curve radius decreases. At present, the best

tangent of this length is calculated as

model shows that A = (0,96 L + 0,0245/R - 0,012S) 0,978(3.3 x W - 30)

V85

[11,016(T - TMin) + V8512]0,5

=

for V851 > V852

(4.7)

(4.6)

where A

=

crashes/million vehicles entering from both

4.2.3

Curves

directions

Over the years, various theoreticians have pro-

L

=

curve length (km)

posed a variety of polynomials as the most

R

=

curve radius (km)

desirable forms of horizontal curvature, with

S

=

1,

desirability presumably being determined by the

curves have

aesthetics of the end resultant and usually from

been provided

a vantage point not normally available to the driver. Accident history suggests, however, that

W

drivers have enough difficulty in negotiating sim-


if transition

=

0,

otherwise

=

roadway width (lanes plus shoulders) (m).

ple circular curves that have the property of providing a constant rate of change of bearing. It is

Using this relationship, the designer would be

recommended that anything more complex than

able to estimate the merits of increasing the

circular arcs should be avoided, the most note-

radius of a curve. This would presumably be of

worthy exception being the loop ramp on inter-

great benefit in the case where an existing road

changes.

is to be upgraded or rehabilitated.

In the preceding section, relationships between

It is necessary to determine the absolute bound-

operating speed and degree of curvature were

aries of the design domain. The upper bound is

offered,

differences

obviously the tangent in the sense that it has a

between the operating speeds observed on suc-

radius of infinite length. The lower bound is the

as

were

acceptable


minimum radius for the selected design speed

The side friction factor is a function of the condi-

and this is a function of the centripetal force nec-

tion of the vehicle tyres and the road surface

essary to sustain travel along a circular path.

and varies also with speed. For the purposes of

This force is developed in part by friction

design, it is desirable to select a value lower

between the vehicle's tyres and the road surface

than the limit at which skidding is likely to occur

and in part by the superelevation provided on

and the international general practice is to

the curve. The Newtonian dynamics of the situ-

select values related to the onset of feelings of

ation is illustrated in Figure 4.1.

discomfort. Canadian practice suggests that the

Figure 4.1: Dynamics of a vehicle on a curve side friction factor be taken as

The relationship between speed, radius, lateral

f

friction and superelevation is expressed by the

=

0,21 - 0,001xV

(4.9)

=

vehicle speed (km/h).

e+f where e

f

=

V2/127 R

=

superelevation( taken

=

where V

(4.8)

as positive when the

For any given speed, it is thus only necessary to

slope is downward

select the maximum rate of superelevation,

towards the centre of

emax, in order to determine the minimum allow-

the curve)

able radius of horizontal curvature for that

lateral or side friction

speed. This selection is based on considera-

factor

tions of the design domain as discussed in the

V

=

speed of vehicle (km/h)

following section.

In practice, four values of

R

=

radius of curvature (m).

emax are used, being 4, 6, 8, and 10 per cent. The minimum radius of curvature appropriate to

This equation is used to determine the minimum

design speeds in the range of 40 km/h to 130

radius of curvature that can be traversed at any

km/h for each of these values of emax is given in

given speed.

Table 4.1.



relationship:

Guidelines are offered in the following section

as 8 per cent, provided that this value of super-

for the selection of emax

elevation is used only between intersections and that the superelevation is sufficiently remote

4.2.4

Superelevation

from the intersections for full run-off to be achieved prior to reaching the intersection area.

The selection of the appropriate value of emax is at the discretion of the designer in terms of the

In rural areas, the range of observed speeds is

design domain concept. The higher values of

relatively limited and adequate distance to allow

emax are typically applied to rural areas and the

for superelevation development and runoff is

lower values to the urban environment.

usually available. Climatic conditions may, how-


ever, impose limitations on the maximum value The spatial constraints in urban areas will very

of superelevation that can be applied. Icing of

often preclude the development of high values

the road surface is not a typical manifestation of

of superelevation. Because of congestion and

the South African climate but has been known to

the application of traffic control devices, the

occur in various high-lying parts of the country.

speeds achieved at any point along the road

Heavy rainfall reduces the available side friction

can fluctuate between zero and the posted

and relatively light rain after a long dry spell also

speed - or even higher depending on the local

reduces side friction.

level of law enforcement. Negotiating a curve

to areas where the road surface is polluted by

with a superelevation of 10 per cent at a crawl

rubber and oil spills, as is the case in urban

speed can present a major problem to the driv-

areas and the immediately surrounding rural

er. As a general rule, urban superelevations

areas. Where any of these circumstances are

should not exceed 6 per cent although, in the

likely to occur, a lower value of e

case of an arterial, this could be taken as high

mended. 4-8


This applies particularly

max

is recom-

A lower value of emax should also be considered

•

in a road where steep gradients occur with any

applied to sustain lateral acceleration down to

frequency.

A superelevation of 10 per cent

radii requiring fmax followed by increasing e with

would present trucks with some difficulties when

reducing radius until e reaches emax. In short,

they are climbing a steep grade at low speeds.

first f and then e are increased in inverse pro-

As shown in Table 7.3, the combination of a

portion to the radius of curvature;

superelevation of 10 per cent and a gradient of

•

8 per cent has a resultant of 12,8 per cent at

of Method 2 with first e and then f increased in

approximately 45O to the centreline.

inverse proportion to the radius of curvature;

•

Method 2:

Method 3:

Method 4:

Side friction is first

Effectively the reverse

As

for

Method

3,

except that design speed is replaced by aver-

Whatever the value selected for e max, this value

age running speed, and

should be consistently applied on a regional

•

Method 5:

Superelevation

and

basis. Its selection governs the rate of superel-

side friction are in curvilinear relations with the

evation applied to all radii above the minimum.

inverse of the radius of curvature, with values

Variations in emax result in curves of equal

between those of Methods 1 and 3.

radius having different rates of superelevation. Drivers select their approach speeds to curves

These methods of distribution are illustrated in

on the basis of the radius that they see and not

Figure 4.2.

lack of consistency with regard to supereleva-

In terms of the design domain concept, Method

tion would almost certainly lead to differences in

2 has merit in the urban environment. As point-

side friction demand with possibly critical conse-

ed out earlier, provision of adequate superele-

Recommended rates of emax are

vation in an environment abounding in con-

quences.

offered in Table 4.2.

straints such as closely spaced intersections

Distribution of e and f

and driveways is problematic. It is thus sensible

There are a number of methods of distributing e

to make as much use as possible of side friction

and f over a range of curves flatter than the min-

before having to resort to the application of

imum for a given design speed. Five methods

superelevation.

are well documented by AASHTO. These are:

drivers operating at relatively low speeds in an

•

superelevation

urban environment are prepared to accept high-

and side friction are directly proportional to the

er values of side friction than they would at high

inverse of the radius;

speeds on a rural road.

Method 1:

Both


It also should be noted that


on the degree of superelevation provided. A

Method 5 is recommended for adoption in the

bution for superelevation over the range of cur-

case of rural and high-speed urban roads. In

vature.

practice it represents a compromise between Methods 1 and 4. The tendency for flat to inter-

Tables 4.3, 4.4, 4,5 and 4,6 provide values of e

mediate curves to be overdriven is accommo-

for a range of horizontal radii and values of emax

dated by the provision of some superelevation.

of 4, 6, 8, and 10 per cent respectively.

The superelevation provided sustains nearly all

Superelevation runoff

lateral acceleration at running speeds (assumed


to be about 80 per cent of design speed) with considerable side friction available for greater

In the case of a two-lane road, superelevation

speeds. On the other hand, Method 1, which

runoff (or runout) refers to the process of rotat-

Figure 4.2: Methods of distributing of e and f avoids the use of maximum superelevation for a

ing the outside lane from zero crossfall to

substantial part of the range of curve radii, is

reverse camber (RC) thereafter rotating both

also desirable. Method 5 has an unsymmetrical

lanes to full superelevation. Tangent (or crown)

parabolic form and represents a practical distri-

runoff refers to rotation of the outside lane from 4-10


zero crossfall to normal camber (NC). Rotation

drainage resulting in the possibility of storm

is typically around the centreline of the road

water ponding on the road surface. A further

although

driveway

cause of ponding could be where the centreline

entrances or drainage in the urban environment,

gradient is positive and equal to the relative gra-

may require rotation to be around the inside or

dient. In this case, the inner edge of the road

outside edge. These latter alternatives result in

would have zero gradient over the entire length

the distortion of the vertical alignment of the

of the superelevation runoff.

constraints,

such

as

road centreline and a severe slope on the road edge being rotated, with a potentially unaesthet-

Ponding is extremely dangerous for two rea-

ic end result. In the case of dual carriageway

sons. The more obvious danger is that it can

cross-sections, rotation is typically around the

cause a vehicle to hydroplane, causing a total

outer edges of the median island.

loss of traction and steering ability. If the front

The designer should be sensitive to the fact that

straight ahead when the vehicle moves out of

zero crossfall implies a lack of transverse

the ponded water, the sudden availability of fric-

Note:

NC denotes Normal Camber, i.e. 2 per cent fall from the centreline to either edge of the trav-

elled way RC denotes Reverse Camber, i.e. 2 per cent crossfall from the outer edge of the travelled way to the inner edge 4-11 Chapter 4: Road Design Elements


wheels are pointing in any direction other than

tion can lead to a sharp swerve and subsequent

which suggests that there is a maximum accept-

loss of control. The other possibility is that of

able difference between the gradients of the

one front wheel striking the water before the

axis of rotation and the pavement edge.

other, in which case the unbalanced drag could

Experience indicates that relative gradients of

also lead to the vehicle swerving out of control.

0,8 and 0,35 per cent provide acceptable runoff

The designer should therefore endeavour to

lengths for design speeds of 20 km/h and 130

avoid the combination of zero longitudinal gradi-

km/h respectively. Interpolation between these

ent and zero crossfall. A pond depth of 15 mm

values provides the relative gradients shown in

is sufficient to cause hydroplaning. In the case

Table 4.7.

of worn tyres, a lesser depth will suffice.


Many States of the United States have opted for The length of the superelevation runoff section

a standard relative gradient of 1:200, whereas

is selected purely on the basis of appearance,

Canada has elected to use a relative gradient of

Note:

NC denotes Normal Camber, i.e. 2 per cent fall from the centreline to either edge of the travelled

way RC denotes Reverse Camber, i.e. 2 per cent crossfall from the outer edge of the travelled way to the inner edge 4-12 Chapter 4: Road Design Elements

NC denotes Normal Camber, i.e. 2 per cent fall from the centreline to either edge of the trav-

elled way RC denotes Reverse Camber, i.e. 2 per cent crossfall from the outer edge of the travelled way to the inner edge 4-13 Chapter 4: Road Design Elements


Note:

1:400 in the calculation of the length of the

If the relative gradient approach to determina-

superelevation runoff.

tion of runoff length is adopted, this length is

Other widely used

options include adopting the distance travelled

calculated as

in four seconds and previous editions of the

4.10

AASHTO policy suggested the distance travelled in 2 seconds. As can be seen, there is a

where L

=

large degree of arbitrariness attaching to deter-

tion runoff (m)

mination of the length of superelevation runoff.

w

=

The designer can thus vary the relative gradient


length of supereleva width of one traffic lane, (m)

to accommodate other elements of the design,

n

=

number of lanes rotated

such as the distance between successive

ed

=

superelevation rate (per cent)

curves or the distance to the following intersec∆

tion. It is, however, suggested that the relative

=

relative gradient, (per cent)

gradients offered in Table 4.7 should provide a b

pleasing appearance and the designer should at

=

adjustment factor for number of lanes rotated

least attempt to achieve relative gradients of a similar magnitude.

Adjustment factor for number of lanes The gradient of the tangent runout is simply a continuation of whatever relative gradient was

If the above relationship is applied to cross-sec-

adopted for the superelevation runoff.

tions wider than two lanes, the length of the


superelevation runoff could double or treble and

portion of the runoff located on the tangent and

there may simply not be enough space to allow

the balance on the curve.

for these lengths. On a purely empirical basis,

shown that having about 2/3 of the runoff on the

it is recommended that the calculated lengths

tangent produces the best result in terms of lim-

be adjusted downwards by the lane adjustment

iting lateral acceleration.

factors offered in Table 4.8.

demand, deviation by about 10 per cent from

Experience has

If circumstances

this ratio is tolerable.

Location of superelevation runoff The superelevation runoff and tangent runout

The two extremes of runoff location are:

are illustrated in Figure 4.3. Figures 4.4 and 4.5

Full superelevation attained at the

show possible treatments for superelevation

beginning of the curve (BC), and

•

runoff on reverse and broken back curves. In

Only tangent runout attained at the BC.

these cases, the superelevation runoff termi-

Both alternatives result in high values of lateral

nates at a crossfall of two per cent rather than

acceleration and are thus considered undesir-

the more customary zero camber on the out-

able. The preferred option would be to have a

side lane.

Figure 4.3: Attainment of superelevation 4-15 Chapter 4: Road Design Elements


•


Figure 4.4: Superelevation runoff on reverse curves

Figure 4.5: Superelevation runoff on broken-back curves 4-16 Chapter 4: Road Design Elements

If the circular curve is preceded by a transition

cubic parabola achieves a maximum value and

curve, all of the superelevation runoff should be

then flattens out again and is thus not a true spi-

located on the transition curve.

ral and the lemniscate requires an unacceptable length of arc to achieve the desired radius. The

4.2.5

Transition curves

clothoid, which has the relationship whereby the radius, R, at any point on the spiral varies with

Any vehicle entering a circular curve does so by

the reciprocal of the distance, L, from the start of

following a spiral path. For most curves, this

the spiral, is thus the preferred option.

transition can be accommodated within the lim-

Expressed mathematically, this relationship is R

its of normal lane width. At minimum radii for the

=

A2/L

(4.11)

design speed, longer transition paths are fol-

where A is a constant called the spiral parame-

lowed and, if these occur on narrow lanes, the

ter and has units of length.

shift in lateral position may even lead to encroachment on adjacent lanes. Under these

The length of a transition curve may be based

circumstances, it may be convenient to shape

on one of three criteria. These are:

the horizontal alignment such that it more accu-

•

Rate of change of centripetal accelera-

rately reflects the path actually followed by a

tion, essentially a comfort factor, varying

vehicle entering the circular curve.

between 0,4 m/s3 and 1,3 m/s3;

• •

Various curves can be used to provide a transi-

Relative slope as proposed in Table 4.3; or Aesthetics.

tion from the tangent to the circular curve. Whatever form is used, it should satisfy the con-

Since relative slope is applied to curves where

ditions that:

transitions are not provided, its use also for tran-

• •

It is tangential to the straight;

sitioned curves would be a sensible point of

Its curvature should be zero (i.e. infinite

departure. For the various design speeds, a

radius) on the straight;

radius corresponding to a specified centripetal

The curvature should increase (i.e.

acceleration can be calculated. These radii are

radius decrease) along the transition;

• •

listed in Table 4.9 for an acceleration of 1,3

Its length should be such that, at its junction with the circular curve, the full

m/s2. There is little point in applying transition

super elevation has been attained;

curves to larger radii where the centripetal

It should join the circular arc tangential-

acceleration would be lower.

ly, and

•

The radius at the end of the transition

Setting out of transitions

should be the same as that of the circular curve.

Application of the spiral has the effect that the Candidate curves are the lemniscate, the cubic

circular curve has to be offset towards its centre.

parabola (also known as Froude's spiral), and

It is thus located between new tangents that are

the clothoid (also known as Euler's spiral). The

parallel to the original tangents but shifted from



•

4.2.6

them by an amount, s, known as the shift. The

Lane widening

value of the shift is given by When vehicles negotiate a horizontal curve, the s

=

L2/2R

rear wheels track inside the front wheels. In the

(4.12)

case of semi trailers with multiple axles and where L R

= =

selected length of tran

pivot points, this off-tracking is particularly

sition curve (m)

marked. The track width of a turning vehicle,

radius of circular curve (m)

also known as the swept path width, is the sum of the track width on tangent and the extent of

The starting point of the spiral is located at a dis-

off-tracking, with the off-tracking being a func-

tance, T, from the Point of Intersection (PI) of the

tion of the radius of the turn, the number and

original tangents with

location of pivot points and the length of the wheelbase between axles. The track width is


T

=

(R+s) tan θ/2 + L/2

calculated as

(4.13) where θ

=

U

=

u + R - (R2 - ΣLi2)0,5 (4.15)

deviation angle of the circular curve

where U

=

track width on curve (m)

u

=

track width on tangent (m)

The most convenient way to set out the spiral is

R

=

radius of turn (m)

by means of deflection angles and chords and

Li

=

wheel base of design

the deflection angle for any chord length, l, is

vehicle between suc-

given as

cessive axles and pivot

a

=

l2/6RL

points (m).

x 57,246 (4.14) 4-18 Chapter 4: Road Design Elements

Strictly speaking, the radius, R, should be the

FA

=

[R2 + A(2L + A)]0,5 - R

radius of the path of the midpoint of the front axle.

(4.16)

For ease of calculation, however, the

radius assumed is that of the road centreline.

where FA

=

width of front overhang (m)

The front overhang is the distance from the front

R

=

radius of curve (m)

axle of the vehicle to the furthest projection of

A

=

front overhang (m)

the vehicle body in front of the front axle. In the

L

=

wheel base of single

case of the turning vehicle, the width of the front

unit or tractor (m)

between the path followed by the outer front

The width of the rear overhang is the radial dis-

edge of the vehicle and the tyre path of the outer

tance between the outside edge of the inner

front wheel. The width of the front overhang is

rearmost tyre and the inside edge of the vehicle

calculated as

body. In the case of a passenger car this dis-

Figure 4.6: Typical turning path 4-19 Chapter 4: Road Design Elements


overhang is defined as the radial distance

WC

tance is typically less than 0,15 m. The width of

=

N (U + C) + FA(N - 1) + Z

truck bodies is usually the same as the wheel-

(4.18)

base width so that the width of the rear overhang is zero.

where N

=

number of lanes

and the other variables are as previously A typical turning path is illustrated in Figure 4.6.

defined.

Turning paths for numerous vehicles are provided in the 2000 edition of the AASHTO Policy on

As a general rule, values of curve widening,

geometric design of highways and streets and

being (WC - W) where W is the width of the trav-

the designer is directed towards Exhibits 2-3 to

elled way on tangent sections, that are less than

2-23 of that document.

0,6 m are disregarded. Lane widening is thus generally not applied to curves with a radius

It is necessary to provide an allowance, C, for

greater than 300 metres, regardless of the

lateral clearance between the edge of the road-

design speed or the lane width.

way and the nearest wheel path, and for the body clearance between passing vehicles.

Widening should transition gradually on the

Typical values of C are:

approaches to the curve so that the full addi-

•

0,60 m for a travelled way width of 6,0

tional width is available at the start of the curve.

m;

Although a long transition is desirable to ensure

0,75m for a travelled way width of 6,6

that the whole of the travelled way is fully

m, and

usable, this results in narrow pavement slivers

0,90 m for a travelled way width of 7,4 m.

that are difficult, and correspondingly expen-

• •


sive, to construct. In practice, curve widening is A further allowance, Z, is provided to accommo-

thus applied over no more than the length of the

date the difficulty of manoeuvring on a curve

super elevation runoff preceding the curve. For

and the variation in driver operation. This addi-

ease of construction, the widening is normally

tional width is an empirical value that varies with

applied only on one side of the road. This is

the speed of traffic and the radius of the curve.

usually on the inside of the curve to match the

It is expressed as

tendency for drivers to cut the inside edge of the travelled way.

Z

=

0,1(V/R0,5)

where V

=

design speed of the

(4.17)

In terms of usefulness and aesthetics, a tangent transition edge should be avoided. A smooth

road (km/h)

graceful curve is the preferred option and can be adequately achieved by staking a curved

By combining Eqs 4.12, 4.13 and 4.14 with the

transition by eye. Whichever approach is used,

clearance allowances, C and Z, the width of the

the transition ends should avoid an angular

travelled way can be calculated as

break at the pavement edge.


Widening is provided to make driving on a curve

constant rate of change of bearing. It thus has

comparable with that on a tangent. On older

a certain academic appeal.

roads with narrow cross-sections and low design speeds and hence sharp curves, there

The general equation of the parabola is

was a considerable need for widening on

y = ax2 + bx +c,

curves. Because of the inconvenience attached

from which it follows that the gradient, dy/dx, at

to widening the surfacing of a lane, it follows that

any point along the curve is expressed as 2ax +

the required widening may not always have

b and the rate of change of gradient, d2y/dx2, is

been provided. Where a road has to be rehabil-

2a. This has the meaning of extent of change

itated and it is not possible to increase the

over a unit distance.

radius of curvature, the designer should consid-

express the rate of change in terms of the dis-

er the need for curve widening.

tance required to effect a unit change of gradi-

Normal usage is to

ent. This expression is referred to as the KIn the case of an alignment where curves in

value of the curve and is equal to 1/2a. It fol-

need of widening of the travelled way follow

lows that the length of a vertical curve can be

each other in quick succession, the inconven-

conveniently expressed as being

ience associated with the application of curve L

widening can be avoided by constructing the

=

AxK

(4.19)

entire section of road, including the intervening tangents, to the additional width.

4.3

where L

=

curve length (m)

A

=

algebraic difference

VERTICAL ALIGNMENT

between the gradients on either side of the

Vertical alignment comprises grades (often

curve

referred to as tangents) and vertical curves.

K

4.3.1

Grades have the properties of length and gradient,

=

rate of change

General controls for vertical

senting the height in metres gained or lost over a horizontal distance of 100 metres.

On rural and high-speed urban roads, a smooth grade line with gradual changes, which are con-

Curves may be either circular or parabolic, with

sistent with the class of the road and the char-

South African practice favouring the latter. The

acter of the terrain, is preferable to an alignment

practical difference between the two forms is

with numerous breaks and short lengths of

insignificant in terms of actual roadway levels

grades and curvature. A series of successive,

along the centreline. The parabola has the

relatively sharp crest and sag curves creates a

property of providing a constant rate of change

roller coaster or hidden dip profile which is aes-

of gradient with distance, which is analogous to

thetically unpleasant.

the horizontal circular curve, which provides a

safety concern, although, at night, the loom of 4-21


Hidden dips can be a


alignment

invariably expressed as a percentage, repre-

approaching headlights may provide a visual

Where the total change of gradient across a ver-

clue about oncoming vehicles.

Such profiles

tical curve is very small, e.g. less than 0,5 per

occur on relatively straight horizontal alignments

cent, the K-value necessary to achieve the min-

where the road profile closely follows a rolling

imum length of curve would be high.

natural ground line.

these circumstances, the vertical curve could be omitted altogether without there being an

A broken-back grade line, which consists of two

adverse visual impact.

vertical curves in the same direction with a short length of intervening tangent, is aesthetically

The vertical alignment design should not be car-

unacceptable, particularly in sags where a full

ried out in isolation but should be properly coor-

view of the profile is possible. A broken plank

dinated with the horizontal alignment as dis-

grade line, where two long grades are connect-

cussed later. In addition to the controls imposed

ed by a short sag curve, is equally unaccept-

on the grade line by the horizontal alignment,

able. As a general rule, the length of a curve (in

the drainage of the road may also have a major

metres) should not be shorter than the design

impact on the vertical alignment. The top of a

speed in km/h. In the case of freeways, the min-

crest curve and the bottom of a sag imply a zero

imum length should not be less than twice the

gradient and the possibility of ponding on the

design speed in km/h and, for preference,

road surface. Where water flow off the road sur-

should be 400 metres or longer to be in scale

face is constrained by kerbs, the gradient

with the horizontal curvature. The broken-back

should be such that longitudinal flow towards

and broken-plank curves are the vertical coun-

drop inlets or breaks in the kerb line is support-

terparts of the horizontal broken-back curve and

ed.

the long tangent/small radius curve discussed

On lower-speed urban roads, drainage

design may often control the grade design.

earlier. The only difference between them is that these forms of vertical alignment are, at

There are, to date, no specific guidelines on

least, not dangerous.


Under

consistency of vertical alignment in terms of the

In theory, vertical curves in opposite directions

relative lengths of grades and values of vertical

do not require grades between them. In prac-

curvature, as is the case in horizontal alignment.

tice, however, the outcome is visually not suc-

However, where grades and curves are of

cessful. The junction between the two curves

approximately equal lengths, the general effect

creates the impression of a sharp step in the

of the grade line tends to be pleasing.

alignment, downwards where a sag curve fol-

4.3.2

lows a crest and upwards where the crest curve

Grades

follows the sag. A short length of grade between the two curves will create the impression of con-

The convention adopted universally is that a

tinuous, smoothly flowing vertical curvature.

gradient that is rising in the direction of increas-

The length of the intervening grade in metres

ing stake value is positive and a descending

need not be more than the design speed in km/h

gradient negative.

to achieve this effect.

interlinked in that steep gradients have an 4-22 Chapter 4: Road Design Elements

Gradient and length are

adverse effect on truck speeds and hence on

length of grade" typically taken as being the dis-

the operating characteristics of the entire traffic

tance over which a speed reduction of 15 km/h

stream. This effect is not limited to upgrades

occurs. For a given gradient, lengths less than

because truck operators frequently adopt the

the critical length result in acceptable operation

rule that speeds on downgrades should not

in the desired range of speeds.

exceed those attainable in the reverse direction.

desired freedom of operation is to be maintained

Where the

on grades longer than the critical length, it will It is desirable that truck speeds should not

be necessary to consider alleviating measures

decrease too markedly. Apart from the opera-

such as local reductions of gradient or the pro-

tional impact of low truck speeds, it has also

vision of extra lanes.

tion between crash rates and the speed differ-

Local research indicates that the 85th percentile

ential between trucks and passenger cars.

mass/power ratio is of the order of 185 kg/kW.

American research indicates that crash rates for

The performance of the 85th percentile truck is

speed reductions of less than 15 km/h fluctuate

illustrated in Figure 4.7. The critical lengths of

between 1 and 5 crashes per million kilometres

grade for a speed reduction of 15 km/h are

of travel increasing rapidly to of the order of 21

derived from these performance curves and are

Figure 4.7: Truck speeds on grades shown in Table 4.10.

crashes per million kilometres of travel for a speed reduction of 30 km/h. In the absence of South African research, it is presumed that a

As suggested earlier, grades longer than those

similar trend would manifest itself locally,

given in Table 4.10 may require some form of

although probably at higher crash rates. For

alleviating treatment. One such treatment is the

these reasons, reference is made to the "critical

provision of climbing lanes. This is discussed in 4-23



been established that there is a strong correla-

Section 4.4.2. Stepping the grade line, i.e. by

readily be achieved under the three sets of cir-

inserting short sections of flatter gradient, as an

cumstances. Other factors that should be borne

alleviating treatment is sometimes offered as

in mind in selecting a maximum gradient

relief to heavy trucks at crawl speeds on steep

include:

gradients. In practice, this has proved to be

•

would suggest a reduction in maximum

ineffective because drivers of heavy trucks sim-

gradient in order to maintain an accept

ply maintain the crawl speed dictated by the

able Level of Service;

steeper gradient in preference to going through

•

the process of working their way up and down

costs, being the whole-life cost of the road and not merely its initial construc-

through the gears.

tion cost;

•

Maximum acceptable gradients shown in Table

property, where relatively flat gradients in a rugged environment may result in

4.11 are dictated primarily by the topography

high fills or deep cuts necessitating the

and the classification of the road. Topography is

acquisition of land additional to the nor-

described as being flat, rolling or mountainous

mal road reserve width;

• •

which is somewhat of a circular definition in the


traffic operations, where high volumes

sense that what is really being described is not

environmental considerations; and adjacent land use in heavily developed or urban areas

the topography itself but the gradients that can


It is the designer's responsibility to select a max-

should be considered is of the order of 0,5 per

imum gradient appropriate to the project being

cent.

designed. The values offered in Table 4.11 are

kerbing, gradients should not be less than 0,5

thus only intended to provide an indication of

per cent. If the grade is longer than 500 metres,

gradients appropriate to the various circum-

increasing the camber to 2,5 or 3,0 per cent

stances.

should be considered. The latter value of cam-

It is recommended that, even without

ber should only be considered in areas subject Maximum gradients on freeways should be in

to heavy rainfall because it may give rise to

the range of three to four per cent regardless of

problems related to steering and maintaining the

the topography being traversed. Lower order

vehicle's position within its lane.

roads have been constructed to gradients as steep as twenty per cent but it is pointed out that

Crest curves are often in cut and, for the mini-

compaction with a normal 12/14 tonne roller is

mum value of K for a design speed of 120 km/h,

virtually impossible on a gradient steeper than

the gradient would be at a value of less than 0,5

about twelve per cent. It is recommended that

per cent for a distance of 55 metres on either

this be considered the absolute maximum gradi-

side of the crest. Over this distance, channel

ent that can be applied to any road.

grading should be applied to the side drains. On sag curves, the distance over which the longitu-

The minimum gradient can, in theory, be level,

dinal gradient is less than 0,5 per cent is 26

i.e. zero per cent. This could only be applied to

metres on either side of the lowest point at the

rural roads where storm water would be

minimum K-value.

and allowed to spill over the edge of the shoul-

Varying the camber between 2 per cent and 3

der. If used on a road that is kerbed, channel

per cent over a distance of about 80 metres will

grading would have to be employed. Given the

provide an edge grading at 0,5 per cent in the

limits of accuracy to which kerbs and channels

case where the centreline gradient is flat. As an

can be constructed, the flattest gradient that

alternative means of achieving adequate

Figure 4.8: Sight distance on crest curves 4-25 Chapter 4: Road Design Elements


removed from the road surface by the camber

drainage, this is more useful in theory than in

by the required sight distance is contained with-

practice because shaping and compacting the

in the length of the vertical curve.

road surface to have this wind in it is extremely

If the curve length is shorter than the required

difficult. This method is not unknown but is not

sight distance, lesser values of K can be

recommended because of the construction

employed as indicated by Equation 4.21.

problem. (4.21)

4.3.3

Curves where K

=

Distance required for a 1 % change of gradient (m)

As the parameter, K, has been described as the S

determinant of the shape of the parabolic curve,

=


it follows that some or other value of K can be

for selected design

determined such that it provides adequate sight

speed (m)

distance across the length of the curve. Sight

h1

=

Driver eye height (m)

distance is measured from the driver eye height,

h2

=

Object height (m)

h1, to a specified object height, h2. In the case

A

=

Algebraic difference in

of a crest curve, the line of sight is taken as

gradient between the

being a grazing ray to the road surface, as illus-

approaching and depart-

trated in Figure 4.7.

ing grades (%) The values of K offered in Table 4.12 are based

Vertical curvature for stopping sight distance

on a curve length longer than the required stop-

The required value of K is derived from the

ping distance with a driver eye height of 1,05

equation of the parabola as indicated in

metres and various heights of object as dis-

Equation 4.20.

cussed in Chapter 3. K=

S2 200(h10,5 + h20,5)2


where K

S

=

=

(4.20)

Vertical curvature for passing sight distance

Distance required for a

Similar calculations can be carried out based on

1 % change of gradient

passing sight distance. High values of K result

(m)

so that, in the situation where the crest of the


curve is in cut, the increase in volumes of exca-

for selected design

vation will be significant. Although the designer

speed (m)

should seek to provide as much passing sight

h1

=


distance as possible along the length of the

h2 A

= =

Object height (m) Algebraic difference in gradient between the approaching and depart ing grades (%).

road, it may be useful to shorten the crest curve in order to increase the lengths of the grades on either side rather than to attempt achieving passing sight distance over the crest curve itself.

This relationship applies to the condition where4-26


Vertical curvature for barrier sight dis-

specifically in cases where the actual speed dif-

tance

ferentials between the overtaking and the overtaken vehicles are greater than those applied in

On undivided roads, barrier sight distance (also

the derivation of passing sight distances. K val-

referred to as non-striping sight distance) indi-

ues of crest curvature corresponding to barrier

cates whether no-passing pavement markings

sight distance are offered in Table 4.13.

are required. Barrier sight distance is shorter than passing sight distance.

Sag curves

The designer

or more wherever possible because passing

During the hours of daylight or on well-lit streets

manoeuvres can often be completed in less

at night, sag curves do not present any prob-

than the calculated passing sight distance

lems with regard to sight distance. Under these



should attempt to provide barrier sight distance

circumstances, the value of K is determined by

existing pedestrian bridges, a clearance of 5,6

considerations of comfort, specifically the

metres may be accepted. This is suggested

degree of vertical acceleration involved in the

because pedestrian overpasses are relatively

change in gradient. The maximum comfortable

light structures that are unable to absorb severe

vertical acceleration is often taken as 0,3

m/s2.

impact and are more likely to collapse in such an event.

The increased vertical clearance

Where the only source of illumination is the

reduces the probability of damage to the struc-

vehicle's headlights, the line of sight is replaced

ture and improves the level of safety for pedes-

by a line commencing at headlight height, taken

trians using it.

as being 0,6 metres, and with a divergence angle of 1O relative to the grade line at the posi-

The criteria of comfort, sight distance and verti-

tion of the vehicle on the curve. This situation is

cal clearance lead to a minimum desirable


length of sag curve. A further criterion is that of drainage, whereby a minimum distance with a

Although not a frequent occurrence, sight dis-

gradient of less than 0,5 per cent is desired.

tance on a sag curve may be impaired by a structure passing over the road. Checking the

Where the stopping sight distance is less than

available sight distance at an undercrossing is best made graphically on the profile.

the length of the curve, the value of K is given

In this

by Equation 4.22.

case, the line of sight is a grazing ray to the soffit of the structure. The selected clearance is

K

thus of interest. Clearances are typically taken

=

____ S2___

(4.22)

120 + 3,5S

as 5,2 metres measured at the lowest point on the soffit. In the case of pedestrian bridges, the where S


clearance is 5,9 metres while, in the case of

=

Figure 4.9: Sight distance on a sag curve 4-28 Chapter 4: Road Design Elements

stopping distance (m)

4.4

The criterion of comfort, as expressed in

CROSS-SECTIONS

Equation 4.23, provides K-values roughly half of those dictated by considerations of stopping

The prime determinants of cross-section design

sight distance.

are:

• K

=

V2

where V

=

design speed (km/h)

/ 395

The function that the road is intended to serve;

(4.23)

•

The nature and volume of traffic to be accommodated; and

•

K-values for sag curves, as determined by

The speed of the traffic.

headlight distance and comfort, for the case where the sight distance is less than the length

Road function refers to a spectrum of needs

of the curve, are given in Table 4.14

ranging

from

accessibility

to

mobility.

Furthermore, a road can be classified on a variIn both crest and sag curves, K-values dictated

ety of different bases. A common feature of

by sight distance, where the curve length is

these considerations is that they largely concern

greater than the sight distance, can be used

addressing the needs of the occupants of a

throughout.

The alternative case, where the

moving vehicle. These needs may find expres-

sight distance is longer than the curve length,

sion in a desire for ready access to or from a

generally offers a lower K-value.

property adjacent to the road, freedom to

K-values appropriate to headlight distance

for high-speed long distance travel. High-speed

should be used in rural areas and also where

traffic requires more space than relatively slow-

street lighting is not provided.

Where street

moving traffic. Space takes the form of wider

lighting is provided, the lower K-values associ-

lanes, wider shoulders and (possibly) the inclu-

ated with the comfort criterion may be adopted.

sion of a median in the cross-section.



manoeuvre in a terminal or intersection area or

All these needs have to be met in terms of over-

these lanes or, as a further development, to pro-

all objectives of safety, economy, convenience

vide cycle paths adjacent to or, for preference,

and minimum side effects.

removed from the travelled lanes.

In urban areas, road functions also have to

Although the horizontal and vertical alignments

include considerations of living space. People

are disaggregated in the sense that they are a

enjoy casual encounters, meeting people on

combination of tangents and curves, the cross-

neutral territory, as it were, without the obligation

section is heavily disaggregated, comprising a

of having to act as host or hostess in the home.

multitude of individual elements.

The sidewalk café, the flea market and window-

ments are illustrated in Figure 4.10. Design is

shopping all have to be accommodated within

thus concerned primarily with the selection of

the road reserve. All of these activities impact

elements that have to be incorporated within the

on the cross-section, which has to be designed

cross-section, followed by sizing of these indi-

accordingly.

vidual elements.

Traffic does not exclusively comprise motorised

In spite of this disaggregated approach to

vehicles. In developing areas, it may be neces-

design, there are numerous combinations of

sary to make provision for animal-drawn vehi-

elements that occur frequently. Cases in point

cles and, in this context, developing areas are

are:

not necessarily exclusively rural. The volume of

• • •

motorised vehicles will have an impact on the design of the cross-section with regard to the

Two-lane two-way roads; Two-plus-one roads; Four-lane undivided and divided roads, and

number of lanes that have to be provided. High

•

volumes of moving vehicles will generate a need for special lanes such as for turning, pass-

Four, six (or more) lane freeways.

Each of these composite cross-sections leads to

ing, climbing or parking.


These ele-

the development of a standard road reserve

In urban areas, the presence of large numbers

width that often is enshrined in legislation. In

of pedestrians will require adequate provision to

this section, the individual elements rather than

be

the composite cross-section will be discussed.

made

in

terms

of

sidewalk

widths.

Pedestrians are also to be found on rural roads.

4.4.1

On rural roads, speeds are high so that crashes

General controls for cross-sections

involving pedestrians are inevitably fatal. It is thus sensible to make at least modest provision

Safety is a primary consideration in the design

for pedestrians on rural roads, even though their

of the cross-section. The safety of the road user

numbers may be low.

refers to all those within the road reserve, whether in vehicles or not.

Cyclists can often be accommodated on the normal travelled lanes but, when the number of

Wide lanes supposedly promote the safety of

cyclists increases, it may be necessary to widen

the occupants of vehicles although current evi4-30


dence suggests that there is an upper limit

other protected land use. During the planning

beyond which safety is reduced by further

process, it is prudent to attempt to acquire addi-

increases in lane width. The reverse side of the

tional road reserve width to allow for improve-

coin is that wide lanes have a negative impact

ments to traffic operations, auxiliary lanes, wider

on the safety of pedestrians attempting to cross

pedestrian areas, cycle paths as well as for pro-

the road or street. South Africa has a particu-

vision of utilities, streetscaping and mainte-

larly bad record in terms of pedestrian fatalities,

nance considerations.

which account for approximately half of the total

not be taken to the extreme, where large tracts

number of fatalities. In devising safe cross-sec-

of land are unnecessarily sterilised in anticipa-

tions, it is therefore necessary to consider the

tion of some or other future eventuality.

This should, however,

needs of the entire population of road users and not just those in vehicles.

The location of existing major utilities, which may be either above or below ground, and diffi-

In urban areas, it is necessary to make provision

cult or costly to relocate is a fairly common

for boarding and alighting public transport pas-

design control in urban areas. In particular the

sengers, disabled persons and other non-vehic-

location of aboveground utilities in relation to

ular users of the facility in addition to accommo-

clear zone requirements should be carefully

dating pedestrians and cyclists. In these areas,

considered.

design speed usually plays a lesser role in the design of the cross-section.

In the discussion that follows, dimensions are

Land availability is of particular importance in

field" designs. Very often, however, the design-

urban areas. Land for the road reserve may be

er does not have this level of freedom of choice.

restricted because of the existence of major

For example, rehabilitation projects often

buildings or high cost of acquisition or some or

require the creation of additional lanes without

Figure 4.10: Cross-section elements 4-31 Chapter 4: Road Design Elements


essentially discussed in terms of new or "green

the funding or space available for additional

Similarly, a fourth lane on a 13,4 metre wide

construction. The choice then is one of either

cross-section would imply lane widths of 3,1

accepting a lane or shoulder width that is nar-

metres and a zero width of shoulder if the shoul-

rower than would be desired or foregoing the

der rounding of 0,5 metres is to be maintained.

additional lanes.

The traffic volumes necessitating consideration of a four-lane cross-section would render such a

As discussed below, the preferred cross-section

configuration of lanes and shoulders highly

for a two-lane two-way road has a total width of

undesirable.

13,4 metres. This comprises lane widths of 3,7

4.4.2

metres and usable shoulder widths of 2,5 metres plus a 0,5 metre allowance for shoulder

Basic Lanes

Basic lanes are those that are continuous from

rounding. If a cross-section of this width already

one end of the road to the other. The number of

exists and it is deemed necessary to incorporate

lanes to be provided is largely determined by

a climbing lane without incurring extra construc-

traffic flow and the desired Level of Service that

tion costs, this can be achieved by accepting a

the road is to provide. Reference is thus to the

climbing lane width of 3,1 metres with the adja-

Highway Capacity Manual.

cent shoulder having a width of 1,0 metres. The through lanes have to be reduced to a width of

The anticipated traffic speed offers an indication

3,4 metres each and the opposite shoulder to a

of the required width of lane. Lane widths typi-

width of 1,5 metres thus allowing for the shoul-

cally used are 3,1 metres, 3,4 metres and 3,7

der rounding of 0,5 metres on both shoulders.

metres. Research has indicated that the crash rate starts to show a marginal increase above a

Many of the older cross-sections have a total

lane width of 3,6 metres. It is accordingly rec-

paved width between shoulder breakpoints of

ommended that widths significantly greater than

12,4 metres, comprising lanes with a width of

3,7 m should not be employed.

authorities use lane widths of 5,4 metres on

and 0,5 metres of shoulder rounding on both

major routes.

sides. Three lanes with a width of 3,1 metres


Some local

3,4 metres, shoulders with a width of 2,3 metres

The intention is apparently to

make provision for parking or cycle traffic with-

each would allow for shoulders that are approx-

out demarcating the lanes as such. In practice,

imately 1 metre wide and 1 metre of shoulder

passenger cars are very inclined to use these

rounding. It is suggested that the dimensions

lanes as though they were two unmarked lanes

offered for the three-lane cross-section consti-

2,7 metres in width. Although this is a very eco-

tute an irreducible minimum and can be consid-

nomical method for providing adequate capaci-

ered as a palliative measure only over a rela-

ty, the safety record of such cross-sections may

tively short distance, e.g. to accommodate a

be suspect and their use should be discour-

climbing lane. Accommodating a third lane on

aged.

cross-sections narrower than 12,4 metres between shoulder breakpoints should not be

The narrowest width recommended for consid-

essayed.

eration (3,1 metres) allows for a clear space of 4-32 Chapter 4: Road Design Elements

300 mm on either side of a vehicle 2,5 metres

shoulder, or a cross fall, being a slope from one

wide. This width would only be applied to roads

edge of the travelled way to the other edge.

where traffic volumes and/or speeds are expect-

These slopes are provided to ensure drainage

ed to be low.

of the road surface and are typically at two per cent although, in areas where high rainfall inten-

Where traffic volumes are such that a multi-lane

sities are likely to occur, the slope could be

or divided cross-section is required, 3,7 metres

increased to as much as three per cent.

would be a logical width to adopt. High speeds

4.4.3

would also warrant this width because, on a nar-

Auxiliary lanes

rower lane, momentary inattention by a driver could easily cause a vehicle to veer into the path

Auxiliary lanes are located immediately adjacent

of another. Intermediate volumes and speeds

to the basic lanes. They are generally short and

are adequately accommodated by a lane width

are provided only to accommodate some or

of 3,4 metres.

other special circumstance.

In the case of a rehabilitation or reconstruction

Auxiliary lanes are often used at intersections.

project, it may be necessary to add lanes to the

They can be turning lanes, either to the left or to

cross-section. The cost of the additional earth-

the right, or through lanes. The turning lanes

works may however be so prohibitive that the

are principally intended to remove slower vehi-

funds required to upgrade the road to full 3,7

cles, or stopped vehicles waiting for a gap in

metre lanes and 3,0 metre shoulders may sim-

opposing traffic, from the through traffic stream

ply not be available. The options available to

hence increasing the capacity of the through

the road authority are then either not to upgrade

lanes.

the road at all or to accept some lesser widths of

employed at signalised intersections to match

lanes and/or shoulders. It is suggested that a

the interrupted flow through the intersection

stepwise approach to the problem, first reducing

area with the continuous flow on the approach

the available width of shoulder and thereafter

lanes.

considering reductions in lane width, could be

lanes are discussed in depth in Chapter 6.

adopted by.

Auxiliary lanes are also employed at inter-

The application and design of these

safe, even at a speed of 120 km/h, but is not a

changes.

These are through lanes and are

comfortable solution when applied over an

intended to achieve lane balance where turning

extended distance. A reduction in the posted

volumes are sufficient to warrant multi-lane on-

speed limit may be desirable if the lesser lane

and off-ramps. They may also be used between

width is applied over several kilometres. At still

closely spaced interchanges to support weav-

lower lane widths, it may be advisable to provide

ing, principally replacing a merge-diverge with a

a posted speed limit of 100 km/h even for rela-

Type A weave. Auxiliary lanes at interchanges

tively short sections of road.

are discussed in Chapter 7.

Travelled lanes have either a camber, being a

Climbing lanes and passing lanes are auxiliary

slope from the centreline towards the outside

lanes employed on network links, i.e. other than 4-33



A 3,4 metre lane is reasonably

Through auxiliary lanes are often

Climbing

last-second manoeuvres occur. In the case of

lanes are often referred to as truck lanes,

merging, such manoeuvres can be extremely

crawler lanes, overtaking lanes or passing

hazardous. Reference should be made to the

lanes. The function of climbing lanes is, howev-

SADC Road and Traffic Signs Manual for the

er, very different from that of passing lanes.

recommended signage and road markings.

at intersections and interchanges.

Climbing lanes remove slower vehicles from the The lane drop should not be located so far from

traffic stream and have the effect of reducing the

the end of the need for the auxiliary lane that

number of passenger car equivalents (PCE's) in the stream.

drivers accept the increase in number of lanes

If the reduction is sufficient, the

and do not expect the reduction. Furthermore,

Level of Service (LOS) on the grade will match

the location should not be such that the lane

that on the preceding and succeeding grades.

drop is effectively concealed from the driver.

Passing lanes also remove slower traffic from

Concealment can arise from location immedi-

the stream but, in this case, the objective is pla-

ately beyond a crest curve. An "architectural"

toon dispersal, thus supporting an increase in

approach which locates the lane drop on a hor-

the capacity of the road. In short, the climbing

izontal curve and selects curve radii that result

lane seeks to match the LOS on a steep grade

in a smooth transition from two lanes to one

to that on the preceding and following flatter

across the length of the curve is a particularly

grades, whereas the passing lane improves the

subtle form of concealment that should be

capacity of the road as a whole. The ultimate

avoided at all costs. To simplify the driving task,

demonstration of the latter circumstance is the

the lane drop should be highly visible to

four-lane road, which could be described as a

approaching traffic and drivers should not be

two-lane road with continuous passing lanes in

subjected to successive decision points too

both directions.

quickly implying that lane drops should be removed from other decision points.

From a safety point of view, it is important that drivers are made aware of the start and, more

It is possible that the shoulder adjacent to an

particularly, the end of an auxiliary lane. The

auxiliary lane could be very narrow. However,


basic driver information requirements in the latter

for emergency use it is recommended that a

case are:

•

three metre wide surfaced shoulder be provided

Indication of the presence of a lane

along and extending downstream of the taper

drop;

•

for a distance equal to the stopping sight dis-

Indication of the location of the lane

tance for the design speed of the road.

drop; and

•

Indication of the appropriate action to

Climbing lanes

be undertaken. Four types of warrants for climbing lanes are in It has been observed that, without adequate

use. These are:

signposting and road markings indicating the

•

presence of a lane drop to drivers, erratic and

Reduction of truck speed through a given amount or to a specified speed;


• • •

Reduction in truck speed in association

before the climbing lane is warranted.

The

with a specified volume of traffic;

speed reduction applied is 20 km/h from an ini-

Reduction in LOS through one or more

tial 80 km/h. The volume warrant is given in

levels, and

Table 4.15 below.

Economic analysis. Truck speed reductions without reference to

The first three focus on the performance of

traffic volumes have merits in terms of safety.

trucks and infer some or other impact on pas-

Their principal benefit lies in reduction in the

senger vehicles. The fourth directly quantifies

speed differential in the through lane, thereby

the effect of slow moving vehicles (which need

reducing the probability of the occurrence of a

not necessarily be exclusively truck traffic) on

crash. It is, however, theoretically possible that

the traffic stream in terms of delay over the

a climbing lane would be considered warranted

design life of the climbing lane and compares

merely because it would lead to the required

the benefit of the removal of delay with the cost

truck speed reduction even if total traffic vol-

of providing and maintaining the climbing lane.

umes were very low with virtually no trucks in

Speed reductions adopted internationally vary in

the traffic stream. The addition of a volume war-

the range of 15 km/h to 25 km/h and are usual-

rant increases the likelihood of a reasonable

ly intended to be applied to a single grade. The

economic return on the provision of a climbing

most widely occurring value of speed reduction

lane.

is 15 km/h, based on considerations of safety.

warrants such as those described above are not

The Australian approach bases the need for

intended to be - or are ever likely to be - fully

climbing lanes on examination of a considerable

economic.

It is conceded that performance-based

lanes is based on traffic volume, percentage of

Level of Service is a descriptor of operational

trucks and the availability of passing opportuni-

characteristics in a traffic stream. An important

ties along the road. Speed reduction is to 40

feature is that it is purely a representation of the

km/h and not by 40 km/h.

driver's perception of the traffic environment and

South African practice, as described in TRH17,

is not concerned with the cost of modifying that

uses a combination of the speed and traffic vol-

environment. The warrants suggested by the

ume as a warrant, requiring both to be met

Highway Capacity Manual are: 4-35



length of road. The justification for climbing

• •

A reduction of two or more Levels of

climbing lanes are variously referred to as pass-

Service in moving from the approach

ing bays, turnouts or partial climbing lanes and

segment to the grade, or

are typically 100 to 200 metres long. Because

Level of Service E existing on the grade.

vehicles entering the turnout do so at crawl speeds, the tapers can be very short, e.g. twen-

The Highway Capacity Manual warrant is

ty to thirty metres long, corresponding to taper

severe and does not, in any event, match the

rates of 1:6 to 1:10 in the case of 3,1 metre wide

speed reduction warrant. The provision of a

lanes.

climbing lane at a specific site is thus dependent on the type of warrant selected. It is according-

Climbing lanes usually have the same width as

ly suggested that, if a designer wishes to apply

the adjacent basic lane. In very broken terrain,

this warrant, the reduction considered should be

a reduction in width to as little as 3,1 metres can

of one or more Levels of Service, and not two or

be considered because of the low speeds of

more.

vehicles in the climbing lane.

On the same

grounds, the shoulder width may also be In view of the economic restraints on new con-

reduced but to not less than 1,5 metres. If the

struction, a compromise between convenience

shoulders elsewhere on the road are three

and cost effectiveness is required. The com-

metres wide, the additional construction width

promise proposed is that, while delay - seen as

required to accommodate the climbing lane and

a major criterion of Level of Service - is

reduced shoulder is thus only 1,6 metres.


employed in determination of the need for provision of a climbing lane, the delay considered

While the decision whether or not to provide the

would not be that suffered by the individual vehi-

climbing lane could be based on the economics

cle but rather by the entire traffic stream.

of the matter, the location of its terminals is

Commercially available software calculates the

dependent purely on safety based on the oper-

value of the time saved during the design life of

ational characteristics of truck traffic. The war-

the climbing lane and relates this to input from

rant for the provision of climbing lanes in terms

the designer in respect of construction and

of truck speed reduction is set at 20 km/h as

maintenance costs of the lane.

described previously. It is suggested, as a safety measure to allow for variation in the hill climb-

In mountainous terrain, where trucks are

ing capabilities of individual trucks, that, having

reduced to crawl speeds over extended dis-

established that a climbing lane is warranted, a

tances and relatively few opportunities for over-

speed reduction of 15 km/h be used to deter-

taking exist, the cost of construction of climbing

mine the location of the climbing lane terminals.

lanes may be prohibitive. Under these circum-

Table 4.10 shows critical lengths of grade in

stances, an alternative solution to the opera-

terms of a truck speed reduction of 15 km/h for

tional problem may be to construct short lengths

various gradients. It is recommended that the

of climbing lane as opposed to a continuous

full width of the climbing lane be provided at or

lane over the length of the grade. These short

before the end of the critical length, with this 4-36


length being measured on the preceding sag

The terminal may take one of two possible

curve from a point halfway between the Vertical

forms. One option is to provide a right-to-left

Point of Intersection (VPI) and the end of the

taper merging the basic lane with the climbing

vertical curve (EVC). The full width of the climb-

lane, followed by a left-to-right taper back to the

ing lane should be maintained until the point is

two-lane cross-section. The motivation for this

reached where truck speed has once again

layout is that faster-moving vehicles find it easy

increased to be 15 km/h less than the normal

to merge with slower-moving traffic. This is its

speed on a level grade.

only advantage. Should a vehicle not manage to complete the merge before the end of the

The length of the entrance taper should be such

right-to-left taper, its only refuge is the painted

that a vehicle can negotiate the reverse curve

island, thereafter being confronted by an oppos-

path with the benefit of a 2 per cent crossfall on

ing lane situation. Furthermore, in this layout,

the first curve followed by a negative superele-

the basic lane terminates at the end of the

vation of 2 per cent on the second curve with a

climbing lane with the climbing lane thereafter

short intervening tangent to allow for the rever-

becoming the basic lane. This is a contradiction

sal of curvature. Ideally, it should not be neces-

of the fundamental definitions of the basic and

sary for the vehicle path to encroach on the

auxiliary lanes. This option is not recommend-

shoulder. These conditions can be met by a

ed for two-lane roads, although it could possibly

taper which is about 100 metres long, corre-

be used on multilane or divided cross-sections.

sponding to a taper rate of 1 : 27.

This is

approximately half the taper rate applied to

The alternative is to provide a simple taper,

interchange off-ramps, which is appropriate as,

dropping the climbing lane after it has served its

in this case, the taper addresses a reverse path

purpose. A vehicle that cannot complete the

and not a single change of direction.

merging manoeuvre at the end of the climbing lane has the shoulder as an escape route. This is a safer option than that described above.

As stated above, the exit terminal of the climbing lane should be the point at which trucks

As described with regard to the entrance taper,

have accelerated back to a speed that is, at

negotiate a taper that is 100 metres in length.

speeds on the basic lanes. If there is a barrier

However, a flatter taper would allow time to find

line at this point, the lane should be extended to

a gap in the opposing traffic. A taper rate of 1:50

the point at which the barrier line ends. The rea-

is suggested for on-ramps at interchanges

son for this is that a vehicle entering the basic

where vehicles are required to negotiate only a

lane may inadvertently force an overtaking vehi-

single change of direction. The reverse curve

cle into the opposing lane. This is a potentially

path followed in exiting from an auxiliary lane

dangerous situation and the designer must

may require a still flatter taper rate for which rea-

ensure that there is sufficient sight distance to

son a rate of the order of 1:70 is suggested,

support appropriate decisions by the drivers

leading to a taper that is approximately 200

involved.

metres in length. 4-37 Chapter 4: Road Design Elements


a vehicle exiting from the climbing lane could

most, 15 km/h slower than the truck operating

Passing lanes

there is an absence of passing opportunities. They are aimed at platoon dispersion and local

The procedure followed in the design of the

research has demonstrated that a passing lane

alignment of a road should seek, in the first

length of about one kilometre is adequate for

instance, to provide the maximum possible

this purpose.

passing opportunity. Thereafter, the need for

Numerous short passing lanes

are preferable to few long passing lanes and it is

climbing lanes should be evaluated and deci-

recommended that they be located at two, four

sions taken on where climbing lanes are to be

and eight kilometre spacings. Where traffic vol-

provided and what the lengths of these climbing

umes are low, the longest spacing can be used

lanes should be. At this stage, it is possible to

and, as traffic volumes increase, the intervening

relate the total passing opportunity to the overall

lanes can be added in a logical manner.

length of the road. With one-kilometre long passing lanes provided The analysis of two-lane roads as described in

at two-kilometre intervals, the next level of

the Highway Capacity Manual is based on two

upgrading would be a Two + One cross-section.

criteria, being percentage time spent following

In this case, the road is effectively provided with

and average travel speed. Both of these are

a three-lane cross-section from end to end with

adversely influenced by inadequate passing

the centre lane being alternately allocated to

opportunities. For example, a road with sixty

each of the opposing directions of flow. In keep-

per cent passing opportunity demonstrates a 19

ing with the spacings discussed above, the

per cent increase in time spent following by

switch in the direction of flow in the centre lane

comparison with a road with similar traffic vol-

should be at about two-kilometre intervals.

umes and unlimited passing opportunities. This assumes a 50/50 directional split. The increase

Unlike climbing lanes, passing lanes tend to

in time spent following is greater with unbal-

operate at the speeds prevailing on the rest of

anced flows. If the flow is as low as 800 vehi-

the road. Reductions in lane width are thus not

cles per hour, passing opportunities limited to 60

recommended and passing lanes should have

per cent result in a reduction of the order of

the same width as the basic lanes.


three per cent in average speeds by comparison with the speeds on roads with unlimited passing

As recommended for climbing lanes, the

opportunity. It is accordingly necessary for the

entrance taper to a passing lane could be 100

designer to carry out an analysis based on the

metres in length and the length of the exit taper

Highway Capacity Manual to establish whether

double this to allow adequate time for merging

or not additional passing opportunities should

vehicles to find a gap in the through flow.

be provided in order to maintain the desired

Seeing that both the entrance and the exit

Level of Service.

tapers signal a change in operating conditions on the road, it is recommended that decision

Passing lanes are normally provided in areas

sight distance should be available at these

where construction costs are low and where

points. 4-38


High occupancy vehicle (HOV) lanes

in the cross-section. The essential point of difference lies in the fact that the passenger car is

As described above, auxiliary lanes are short

usually taken as the design vehicle for basic

and are intended only to deal with a specific cir-

lanes whereas the HOV lanes are designed to

cumstance.

As soon as this circumstance

accommodate buses. Kerb radii at intersections

changes, the auxiliary lane is dropped. HOV

and the width of the turning lanes on bus routes

lanes, on the other hand, form part of the rapid

should be such that buses can negotiate these

transit system of a city and can thus be provid-

curves without encroaching on the adjacent

ed over a substantial distance. Whether they

lanes or, more importantly, on the sidewalks.

should be considered as auxiliary lanes could thus be debated.

Bus routes typically converge on the CBD. The HOV lanes, which, as part of the normal cross-

HOV lanes are typically applied on commuter

section, may have served well in the outlying or

routes with a view to encouraging the use of

suburban areas, could prove inadequate to

public transport or lift clubs hence reducing con-

accommodate the increased volume of bus traf-

gestion. The average occupancy of passenger

fic in the CBD. It may then be necessary to des-

cars is of the order of 1,5 persons per vehicle,

ignate various of the streets in the CBD as

whereas a municipal bus can convey 80 pas-

exclusive bus roads or, alternatively, to consider

sengers effectively replacing 50 or more pas-

a system similar to the O-Bahn routes.

senger cars in the traffic stream. In view of the fact that buses can be 2,6 metres wide, narrow

4.4.4

Kerbing

lane widths are inappropriate to HOV lanes that, ideally, should not be narrower than 3,6 metres,

Kerbs are raised or near-vertical elements that

i.e. allowing a clear space of 0,5 metres

are located adjacent to the travelled way and

between the sides of the vehicle and the lane

are usually used for:

• • •

HOV lanes have to be policed to ensure that only vehicles qualifying for the privilege use

Drainage control; Delineation of the pavement edge; and Reduction in maintenance operations by providing protection for the edge of

them. Signalisation can be employed to give

surfacing.

vehicles in HOV lanes priority over other road

Kerbing is normally only applied in urban areas

users. This is described in the South African

where vehicle speeds are relatively low.

Road Traffic Signs Manual in some detail. The combination of policing and priority usage

In rural areas, the drainage function is normally

underpins the effectiveness of HOV lanes but

accommodated by channels or open drains of

these operational issues are normally outside

various forms. Delineation is usually by means

the terms of reference of geometric design.

of an edge line or a contrasting colour on the

It is important that the designer draws a distinc-

shoulder. Protection of the edge of surfacing

tion between the basic lanes and the HOV lanes

can be by means of buried edge blocks or, more 4-39



markings.

typically, by means of a thickened edge. A thick-

drainage. Perhaps their widest application is to

ened edge is simply a bitumen-filled V-shaped

be found in residential areas, where vehicles

groove cut into the base course.

can drive off the travelled way to park on the verge.

Kerbs may be barrier or semi-mountable or mountable. Barrier and semi mountable kerbs

Channels are usually about 300 mm wide, thus

normally are accompanied by a channel (or gut-

automatically providing an offset between the

ter) whereas the mountable kerb is, in effect, a

kerb and the edge of the travelled way. Where

channel itself.

channels are not provided, the offset should still be maintained for reasons of safety.

Barrier kerbs are intended primarily to control

4.4.5

drainage as well as access and can inhibit slow-

Shoulders

moving vehicles from leaving the roadway. When struck at high speeds, barrier kerbs can

Shoulders are the usable areas immediately

result in loss of control and damage to the vehi-

adjacent to the travelled way and are a critical

cle. In spite of the name, barrier kerbs are inad-

element of the roadway cross-section.

equate to prevent a vehicle from leaving the

provide:

road after a high-speed impact. In addition, a

• •

barrier kerb can lead to a high-speed errant

A recovery area for errant vehicles; A refuge for stopped or disabled vehicles;

vehicle vaulting over a barrier or guardrail. For

•

this reason, barrier kerbing is not generally used

An area out of the travel lanes for emergency and maintenance vehicles; and

on urban freeways and is considered undesir-


They

able on expressways and arterials with design

•

speeds higher than 70 km/h. Barrier kerbs are

In addition, shoulders support use of the road by

never used in conjunction with rigid concrete

other modes of transport, for example cyclists

barrier systems.

and pedestrians.

Semi-mountable kerbs have a face slope of 25

Regulation 298 of the regulations promulgated

mm/m to 62,5 mm/m and are considered mount-

in terms of National Road Traffic Act (Act

able under emergency conditions. They are typ-

93/1996) prohibits driving on the shoulder

ically used on urban freeways and arterials and

except that this is permitted:

also in intersections areas as a demarcation of

• • •

raised islands.

Lateral support of the roadway structure.

On a two-lane road; Between the hours of sunrise and sunset, While being overtaken by another vehicle.

Mountable kerbs have a relatively flat sloping

provided this can be done without endangering

face of 10 mm/m to 25 mm/m and can be

the vehicle, other vehicles, pedestrians or prop-

crossed easily by vehicles. They are particular-

erty and if persons and vehicles on the road are

ly useful as a form of lane demarcation on high-

clearly discernable at a distance of at least 150

speed roads but are not effective as a form of

metres. 4-40


Considering the above applications of the shoul-

Between the two extremes of 3,0 metres and

der, a stopped vehicle can be accommodated

1,0 metres, shoulder widths of 1,5 or 2,5 metres

on a shoulder that is three metres wide. There

could be used in the case of intermediate traffic

is no merit in adopting a shoulder width greater

volumes and speeds. These alternative shoul-

than this. The shoulder should, however, not be

der widths would not normally be used for the

so narrow that a stopped vehicle could cause

inner shoulders of a dual carriageway road.

congestion by forcing vehicles travelling in both

Table 4.16 illustrates the application of the vari-

directions into a single lane. A partly blocked

ous shoulder widths on undivided rural roads.

lane is acceptable under conditions of low speed and low traffic volume. Assuming the

Paved widths of between 1,5 and 2,5 metres

narrowest width of lane, i.e. 3,1 metres, it would

should be avoided. The presence of the paving

be possible for two vehicles to pass each other

may tempt a driver to move onto the shoulder to

next to a stopped car if the shoulder were not

allow another vehicle to overtake, but these

less than 1,0 metres wide. Hazards, including

widths cannot accommodate a moving vehicle

the edges of high fills, cause a lateral shift of

with any safety.

vehicles if closer to the lane edge than 1,5 metres. Allowing for shoulder rounding of 0,5

These shoulder widths are recommended for

metres, the usable shoulder is thus 1,0 metres

adoption for new construction. In the case of

wide and this should be considered the irre-

rehabilitation or reconstruction projects, there

ducible minimum width of shoulder.

may not be sufficient width of cross-section to accommodate the desirable widths and some

Where the traffic situation demands a dual-car-

lesser width will have to be considered.

riageway cross-section, the greatest width of

pointed out in Section 4.4.2, it may be advisable

shoulder, i.e. three metres, is called for. This

to first reduce the shoulder width before consid-

width would apply to the outer shoulder. The

ering reductions of lane width.

As

inner shoulder need only be one metre wide: to protect the integrity of the pavement-

The shoulder breakpoint is usually about 500

layers;

• •

mm beyond the edge of the usable shoulder to

to avoid drop-offs at the lane edge; and

allow for shoulder rounding.

provide space for roadmarkings

provided the median island is not kerbed, thus

Where guardrails or other roadside appurte-

allowing a disabled vehicle to be moved clear of

nances have to be provided, these are located

the adjacent lane. If a barrier, such as kerbing

300 to 500 mm beyond the usable shoulder.

or a guardrail, makes the median island inac-

The shoulder breakpoint should be a further 500

cessible, the full shoulder width should be pro-

mm beyond these appurtenances, as a lesser

vided in the case of a six-lane cross section

distance will not provide the support needed by

because negotiating two lanes to reach the

a guardrail when hit by an errant vehicle.

safety of the outside shoulder (with a disabled vehicle) could be difficult. 4-41



•

The surfacing of shoulders is recommended:

should constitute adequate grounds for full sur-

• • •

facing of the shoulders.

For freeways; In front of guardrails; Where the total gradient, being the

Full surfacing implies continuous surfacing

resultant of the longitudinal gradient and

along the length of the road and not necessarily

the camber or superelevation, exceeds

across the full width of the shoulder, although

six per cent;

•

this is the desirable option.

Where the materials with which the

above that the minimum recommended width of

shoulders are constructed are readily

• • •

It is suggested

erodible, or where the availability of

shoulder is 1,0 metres. If it were considered

material for maintenance of the shoul-

necessary to surface the shoulder at all, there

ders is limited;

would be little or no operational advantage in

Where heavy vehicles would tend to use

surfacing a lesser width than this. In the case of

the shoulder as an auxiliary lane;

new construction, the designer has the option of

In mist belts; or

considering the economic merits of a relatively

Where significant usage by pedestrians

narrow surfaced shoulder vis-à-vis a wide

occurs.

unsurfaced shoulder. In the case of rehabilita-


tion projects, it may be decided to retain the full A patchwork of surfaced shoulders would be

3,0 metre shoulder but, as a cost-saving meas-

both unsightly and unsafe. Where the interven-

ure, to surface only half of the total width.

ing lengths of unsurfaced shoulders are short, it is suggested that they also be surfaced. As a

The cross fall on surfaced shoulders is normally

guideline, it is proposed that if surfacing sixty or

an extension of that on the travelled lanes.

more per cent of the shoulder is warranted, this

Where shoulders are not surfaced, the cross fall 4-42


is normally one per cent steeper than that on the

maximum stem thickness of 175 to 200 mm,

lane to allow for the rougher surface and the

corresponding to the diameter of a guardrail

consequently slower rate of flow of storm water

post, is recommended.

off the road surface. A further application of medians refers to access

At night or during inclement weather it is impor-

management, where right turns into or out of

tant that the driver should be able to distinguish

local land uses are often discouraged on high-

clearly between the shoulder and the lane. This

speed roads.

can be accomplished by the use of a shoulder material of a contrasting colour or texture. Edge

From the above it can be inferred that medians

marking is a convenient way of indicating the

are typically applied in the case of high speed or

boundary between the lane and the shoulder. Rumble strips can also be used and have been

high volume roads with a basic function of

shown to reduce the rate of run-off-road inci-

mobility.

dents by twenty per cent or more. Rumble strips can be raised or grooved. Being intended to

Median islands can be as narrow as one metre,

provide the driver with an audible warning, the

which is sufficient to contain a median barrier

noise level they generate is unacceptable in

comprising back-to-back guardrails. This sug-

urban areas and should therefore only be used

gests that, including minimum width median

in rural areas.

shoulders, the minimum width of the median should be not less than three metres.

4.4.6

Inner

shoulders are often not provided in the urban

Medians

cross-section but kerbing would require an offThe median is the total width between the inner

set of about 300 mm. The minimum width of an

edges of the inside traffic lanes and includes the

urban median should thus be 1,6 metres.

long ago as the early 1930s it was proposed to

Research has found that few out-of-control vehi-

"separate the up-traffic from the down-traffic", a

cles travel further than nine metres from the

function which the median fulfils to this day.

edge of lane, so that this width of median would

This separation is intended to reduce the proba-

be sufficient to avoid most head-on crashes.

bility of head-on crashes and also to reduce the nuisance of headlight glare (usually by the

In urban areas, medians often contain right-turn-

planting of shrubs on the central island). The

ing lanes.

reduction in head-on crashes is achieved

shoulder is invariably replaced by kerbing so

through selection of a suitable width of median

that the median would be the sum of the lane

or the use of median barriers. Shrubs can also

width plus the width required to provide a pedes-

serve as a barrier to prevent cross-median acci-

trian refuge. Pedestrians do not feel safe on

dents but the stems of the shrubs should not

median islands narrower than about two metres,

grow so thick as to become a further hazard. A

suggesting that the median should have a width 4-43


In intersection designs, the inner


central island and the median shoulders. As

of the order of 5,5 to 6,0 metres. This width is

piped drainage system is generally available.

adequate to accommodate pedestrians as well

The depressed median allows the roadbed to

as the right-turning lane.

If intersections are

drain into the median, specifically on curves

closely spaced, it may be necessary to apply

where water from the outer carriageway is pre-

this width to the full length of the median, where-

vented from draining across the inner carriage-

as with widely spaced intersections, e.g. 500

way. In the urban context, kerbing includes drop

metres or more between intersections, a lesser

inlets directing storm water into the underground

width can be applied between the intersections

system.

with the median being flared out by means of Urban median islands are usually narrower than

active tapers at the intersections.

their rural counterparts and do not normally Medians with a width of nine metres or more

have barriers. The barriers have to be terminat-

allow for individual grading of the two carriage-

ed at every intersection and at some entrances

ways, which can be useful in rolling terrain. In

so that the safety offered by the barrier is more

addition, these medians lend themselves to

than offset by the hazard of the barrier ends.

landscaping and to the creation of a park like

Kerbing offers a modest degree of protection to

environment. Unfortunately, they create prob-

pedestrians who may be on the median while

lems at intersections by virtue of the long travel

crossing the road. In addition, kerbing can, to a

distances that they impose on turning vehicles.

limited extent, redirect errant vehicles back into

The incidence of crashes at intersections

their own lanes.


increases with increasing width of median and, at widths of 20 metres, the intersection should

The speeds on rural roads make kerbing inap-

be designed as two intersections back-to-back,

propriate in this environment, as the driver of a

with traffic control on the roadway crossing the

vehicle striking a kerb at high speed would

median. The wide rural median does not trans-

almost certainly lose control of the vehicle, with

late well to the urban environment so that roads

this problem being compounded by the

on the outskirts of urban areas should be

inevitable damage to the front wheels of the

designed with medians appropriate to a future

vehicle.

urban characteristic. Depressed medians generally have flat slopes Medians may be either depressed or raised.

and a gently rounded bottom so that the driver

Depressed medians are normally used in rural

of a vehicle leaving the road has an opportunity

areas and raised medians in urban areas. This

to regain control, minimising occupant injury and

differentiation between rural and urban areas

vehicle damage. Overturning crashes are more

arises for two reasons: drainage and safety.

frequent on slopes steeper than 1 : 4 for median widths of six to twelve metres.

This should,

Storm water drainage in rural areas is generally

therefore, be considered the steepest allowable

above the surface for ease of maintenance

slope, with slopes of 1 : 6 or flatter being pre-

whereas, in urban areas, a well-developed

ferred. 4-44


4.4.7

Outer separators

At an intersection, the frontage road should either be terminated or moved a substantial dis-

The outer separator is the area between the

tance away from the through lanes.

This is

edges of the travelled way of the major road and

intended to safeguard the operation of the inter-

the adjacent parallel road or street. It compris-

section because vehicles attempting to turn

es the left shoulder of the major road, an island

from the through road to a frontage road could

and the right shoulder of the adjacent road or

very easily generate a queue that backs up onto

street. The outer separator serves as a buffer

the through lanes. Not only is this operationally

between through traffic and local traffic on a

undesirable but it could also be unsafe.

frontage or service road. It is typically applied where the corridor has to serve the two func-

Where it is anticipated that a road will have to be

tions of long distance travel and local accessi-

widened at some time in the future, the width of

bility. An arterial passing through a local shop-

the outer separator should be such that it can

ping area is an example of this application.

accommodate the additional lane, hence minimising the extent of damage to the rest of the road cross-section.

If travel on the frontage road is one way and in the same direction as that on the adjacent

4.4.8

through lane, the outer separator can be as nar-

Boulevards

row as three metres or, if barriers are provided, Boulevards are only used in urban areas and

two metres.

are similar to outer separators with regard to At night, drivers on the through lane would find

their function and location. The principal differ-

an opposing direction of flow on the frontage

ence is that they separate a sidewalk and not a

road very confusing, being confronted by head-

frontage

lights both to the right and to the left. Under

Boulevards are a desirable feature because:

these circumstances, the width of the outer sep-

•

road

from

the

through

lanes.

The separation between the sidewalk and the vehicular traffic provides

arator should be substantially increased, prefer-

increased safety for pedestrians and

ably doubled, to minimise the effect of the

•

on non-illuminated sections of the road.

The probability of a pedestrian/vehicle collision is reduced as the sidewalk is

Plantings or dazzle screens on the outer sepa-

placed some distance from the kerb;

•

rator are recommended for the same reason.

Pedestrians are less likely to be splashed by passing vehicles in wet weather;

On rural freeways, the outer separator should

•

be at least nine metres wide, based on the dis-

Space is provided for street furniture and

tance that an out-of-control vehicle is able to

streetscaping as well as for surface and

move away from the edge of the through lane.

underground utilities, and

•

Reference in the literature is to outer separator

Changes to the cross-slope of the side-

widths of twenty to thirty-five metres in rural

walk to provide for appropriate driveway

areas.

gradients are minimised using the 4-45 Chapter 4: Road Design Elements


children at play;

approaching traffic, particularly headlight glare

boulevard area to effect the gradient

have to be provided across boulevards, the vari-

change.

ation in slope should not be so drastic that vehicles cannot traverse the area without scraping

The verge, showing the location and dimensions

their undersides on the ridge between the

of the boulevard, is illustrated in Figure 4.11.

boulevard and the sidewalk.

Aesthetic considerations in the urban environ-

Boulevards are as wide as the road reserve

ment are important, particularly when major

allows.

streets pass through or are adjacent to parkland and residential areas.

than two to three metres.

Desirably, the positive

aesthetic qualities of the adjacent land use are

4.4.9

carried over into the verge and boulevard areas


Ideally, they should not be narrower

Bus stops and taxi lay-byes

of the street cross-section. As a feature of the

Bus stops and lay-byes can be located within

urban landscape, boulevards are usually

the width of the boulevard. In this case, grass-

grassed or landscaped. If the boulevards are

ing of the boulevard is discontinued and the

narrower than 1,5 metres, they are surfaced

area surrounding the bus stop is paved as an

rather than grassed because of the mainte-

extension of the sidewalk to provide users of

nance difficulties associated with narrow strips.

public transit with all-weather access to buses.

The entire area from the reserve boundary to

Pedestrian accidents often occur at bus stops.

the road edge is normally sloped towards the

This can be attributed to the fact that buses fre-

road to assist drainage, not only of this area but

quently stop too close to the road edge, thus

of the adjacent development as well. Because

obstructing oncoming drivers' view of pedestri-

Figure 4.11: Verge area indicating location of boulevard of the impedance offered by grass to overland

ans crossing the road.

flow, the slope of the boulevard should be at

stances, a pedestrian stepping out from behind

least four per cent. Local circumstances may

the bus would be moving directly into the path of

require steeper slopes but, where driveways

an oncoming vehicle. 4-46


Under these circum-

Two approaches can be adopted to minimise

The location of bus stops can have an adverse

this problem. If the bus stop is provided with

impact on safety. A bus at a stop located imme-

adequate entrance and exit tapers, it is easy for

diately in advance of intersections would force

buses to move well clear of the travelled way. If

left-turning vehicles into a situation of heavily

space permits, a painted island can be provided

reduced sight distance.

between the bus stop and the travelled way so

pulling out of the stop, the bus could seriously

that the stop is, in effect, a short length of auxil-

influence the operation of the intersection as a

iary lane. In addition to an approach aimed at

whole.

Furthermore, while

the physical dimensions of the bus stop, a further safety measure could be the provision of

Best practice suggests that bus stops should be

barriers preventing bus passengers from cross-

located beyond intersections.

ing the road until they have moved clear of the

should not be located more than about fifty

bus stop itself.

metres

the

nearest

intersection.

Figure 4.12: Typical layout of a bus stop A proposed typical layout of a bus stop is illus-

Destinations for bus passengers may be on the

trated in Figure 4.12. If the frequency of service

bus route itself but are more likely to be to one

on a particular road is high, e.g. where two or

side or the other of the route. The close prox-

more bus routes have converged upstream of

imity of the bus stop to an intersection offers

the bus stop, the length of the bus stop should

passengers a convenient route to their final des-

be increased to 25 metres to accommodate two

tination. However, it is suggested that a bus

buses. If necessary, the tapers can be reduced

stop should not be located closer than about 15

to not less than 1 : 3 for design speeds of 70

metres from the kerb line of the intersecting

km/h or less.

road or street. A lesser spacing would make it



from

However, they

difficult for a left-turning bus to enter the bus

driveway entrances may have to have a steeper

stop and, furthermore, may result in encroach-

cross-slope than this to match the gradient of

ment on the sight triangle required by a driver on

the driveways but should not exceed a cross-

the intersecting road or street.

slope of five per cent.

4.4.10 Sidewalks

Kerbs, raised medians and channelising islands can be major obstructions to the elderly and

Pedestrian traffic is not encouraged in the road

people with disabilities, particularly those in

reserves of freeways, expressways or other

wheelchairs.

high-speed arterials and accommodation of

minimising the impact of these obstacles is to

pedestrian traffic is usually handled elsewhere.

provide ramps, also referred to as kerb cuts,

On all other urban streets, pedestrian traffic can

dropped kerbs or pram dips.

be expected and it is necessary to provide side-

have a slope of not more than about six per

walks.

cent. A kerb height of 150 millimetres would

The most common method for

Ramps should

thus require a ramp length of 2,5 metres. There should be a clear sidewalk width of 1,5 metres

In commercial areas or areas where the road

beyond the top end of the ramp so that, where a

reserve width is restricted, sidewalks may

ramp is provided, the overall sidewalk width

extend from the kerb to the road reserve bound-

should be not less than four metres.

ary. As discussed above, there is distinct merit

chairs may be 0,75 metres wide so that two

in placing a boulevard between the sidewalk

wheelchairs passing each other on the ramp

and the travelled way, but pedestrian volumes

would require a ramp width of the order of 2 to

may be so high that the entire available width

2,5 metres. If it is not possible to provide this

would have to be utilised for the sidewalk. The

width, a width of not less than 1,5 metres should

width of a sidewalk should not be less than 1,5

be considered.

metres and a minimum width of two metres should be provided near hospitals and old-age

The designer should be aware that, in accom-

homes where wheelchair traffic could be expect-

modating one group of pedestrians with disabil-

ed. If the sidewalk is immediately adjacent to


Wheel

ities, a different group might be disadvantaged

the kerbing, the minimum width should be

in the process. Visually impaired pedestrians

increased by about 0,6 to 1 metre. This is to

would have trouble in locating the kerb face in

make provision for fire hydrants, street lighting

the presence of a ramp.

and other road furniture. It also allows for the

As a vertical face

across the sidewalk would be unexpected, even

proximity of moving vehicles and the opening of

by sighted pedestrians, the sides of the ramp

car doors.

should also be sloped.

The normal cross-slope on a sidewalk is 2 per

Sidewalks are not normally provided in rural

cent. Cross-slopes steeper than this present a

areas. It should, however, be noted that approx-

problem to people with walking impairments or

imately half the fatalities on the South African

who are in wheel chairs. Sidewalks crossing

road network are pedestrians, with many of 4-48


these fatalities occurring in rural areas.

pacted regularly to provide pedestrians with a

Provision should therefore be made for pedes-

hard surface to walk on. In high rainfall areas, a

trian safety outside urban areas. Paved foot-

portion at least of the shoulder should be paved,

ways could be considered under the warranting

with this paved area being at least 1,5 metres

conditions listed in Table 4.17.

wide. Furthermore, the road shoulder should be well drained to prevent the accumulation of

Footways can be as little as one metre wide, but

water, which would force pedestrians to walk on

a width of 1,8 metres would allow two people to

the carriageway.

walk side by side.

4.4.11 Cycle paths The safest location for footways is at the edge of the road reserve. This location is not popular

Changes from single to multiple land usage will

with pedestrians because the footway then fol-

result in shorter trip lengths, making the bicycle

lows all the variations in the natural ground

a more popular form of transport. In addition,

level. In rolling or mountainous terrain through

people are becoming conscious of the need for

cuts and fills, such a footway would not make for

exercise and of the bicycle as means of exer-

comfortable walking. Even if this footway were

cise. Finally, unlike the motor vehicle, bicycles

provided, pedestrians would almost certainly

are environmentally friendly.

prefer to walk on the more level surface of the

If adequately

possible, be situated at least three metres away

can play an important role in the transportation

from the travelled way. This corresponds to a

system. It is important to realise that cyclists

location immediately outside the edge of the

need sufficient space to operate with safety and

usable shoulder in the case of a high volume

convenience rather than simply being assigned

high-speed road.

whatever space is left over after the needs of

In cases where footways are not warranted but

vehicular traffic has been accommodated.

where a large number of pedestrians walk alongside the road, the road shoulder should be

The basic requirements of cyclists are:

upgraded to cater for them. The minimum width

• • •

of these shoulders should be three metres. If not surfaced, they should be bladed and com-

Space to ride; A smooth surface; Speed maintenance, and



planned, designed and maintained, cycle paths

shoulder. In level terrain, the footway should, if

•

safely. The surface of a cycle path should not

Connectivity.

deviate from a three-metre straightedge by The bicycle design envelope and clearances are

more than 5 mm and should also be shaped to

illustrated in Figure 4.13. The one metre enve-

existing features to within 5 mm.

lope allows for the width of the bicycle as well as Adequate clearances to

For bicycles to be effective as a means of trans-

fixed objects and passing vehicles should be

portation, cyclists must be able easily to main-

provided, in addition to the one metre envelope.

tain a steady speed with ease. Cyclists typical-

for erratic tracking.


ly travel at speeds of twenty to thirty km/h someBicycles have narrow tyres inflated to high pres-

times reaching 50 km/h on downgrades. Once

sures and have no suspension system to speak

slowed or stopped it takes considerable time

of. A smooth surface is therefore desirable for

and effort to regain the desired operating speed.

bicycles to be used effectively, comfortably and

Bicycle routes should thus be designed for con-

Figure 4.13: Bicycle envelope and clearances 4-50 Chapter 4: Road Design Elements

tinuous movement, avoiding steep gradients,

from the roadway and from which all

rough surfaces, sharp corners, intersections or

motor traffic, with the exception of main-

the need to give way to other road users.

tenance vehicles, is excluded. Cycle paths may be located within the road reserve or in an independent reserve.

It should be possible to undertake and complete journeys by bicycle.

Bicycle routes on road-

Cycle lanes should have the widths indicated in

ways or separate paths should form a connect-

Table 4.18.

ed network on which bicycle trips can be made effectively and conveniently. Connectivity is an

The geometry dictated by motor vehicles is gen-

important aspect of effective bicycle routes and

erally adequate for bicycles except that bicycles

should be given careful consideration during the

have a longer stopping sight distance.

planning process. Facilities for bicycles can take the form of a:

becomes the design vehicle, with dimensions,

•

Shared roadway/cycle lane where motor

performance, stopping sight distance, minimum

vehicles and bicycles travel in a com-

horizontal and vertical curvature, and clear-

mon lane;

•

ances.

Cycle lane, which is part of the travelled way but demarcated as a separate lane;

•

4.4.12 Slopes

Shoulder lane, which is a smooth, paved portion of the shoulder, properly demar-

The slopes of the sides of the road prism are,

cated by pavement markings or traffic

like those of medians, dictated by two different

signs. As the shoulder provides a useful

conditions.

area for cycling with few conflicts with

safety and a slope of 1:4 is the steepest accept-

fast-moving motor vehicles, this facility

•

Shallow slopes are required for

is very useful in rural areas.

able slope for this purpose. The alternative is to

Cycle path, which is physically removed

accept a steeper slope and provide for safety by 4-51



In the case of separate cycle paths, the bicycle

some other means, such as barriers. In this

to provide the road and its appurtenant works.

case the steepest slope that can be used is dic-

Utilities not directly connected with the road,

tated by the natural angle of repose and erodi-

e.g. telephone or power lines are normally locat-

bility of the construction material.

ed in the verge.

Non-cohesive materials require a slope of 1:2,

In urban areas, the area between the edge of

whereas cohesive soft materials can maintain a

the travelled way and the road reserve bound-

slope of 1:1,5. Cuts in firm cohesive materials

ary provides space for a variety of elements

such as stiffer clays can be built to a slope of

that, for convenience, are summarised in Table

1:1. Rock cuts can be constructed to a slope of

4.19.

1:0,25 (4:1) provided that the material is reaSome of these elements have been discussed

sonably unfissured and stable.

previously.

The intention is to provide the

It is stressed that the slopes suggested are only

designer with a checklist of elements that should

an indication of normally used values.

The

be accommodated.

Some elements will be

detailed design of a project should therefore

mutually exclusive.

For example, the use of

include geotechnical analysis, which will indi-

barrier kerbs indicates that mountable kerbs

cate the steepest slopes appropriate to the con-

cannot be present.

struction or in-situ material. Economic analysis

temporary change in cross-section, for example

will indicate the height of fill above which a slope

the boulevard being replaced by a bus embay-

of 1:4 should be replaced by a steeper slope

ment. Yet others may overlap, for example the

and alternative provision made for safety. As a

driveway approach that crosses a sidewalk.

Others may represent a

rule of thumb, the transition from the flat slopes to slopes dictated by the materials typically

Elements that are most likely to be accommo-

occurs at a fill height of about 3 m.

dated in the verge are;

•

Berms intended to function as barriers protecting the surrounding development

4.4.13 Verges


from visual intrusion or noise;

• • •

The verge is defined as the area between the longitudinal works and the road reserve boundary. The limit of the longitudinal works in the

Cut and fill slopes; Driveway approaches, and Underground services.

case of the rural cross-section is often at the top

Even in the unlikely event that none of these

of cut or the toe of fill. Where drainage works,

elements have to be provided, there has to be a

such as side drains or catch water drains, are

clear space between the edge of the travelled

required, these form part of the longitudinal

way and the road reserve boundary. This space

works, which may thus be wider than the actual

would provide sight distance in the case of hor-

road prism. In rural areas the verge simply rep-

izontal curvature and also allow for emergency

resents the difference in width between a statu-

stopping. Furthermore, there should be some

tory road reserve and the width actually needed

flexibility to accommodate future unknowns. It 4-52


is suggested that this clear space should ideally

road itself. However, to the driver the presence

be a minimum of five metres wide with an

of a pole is a hazard to be avoided and whether

absolute minimum width of three metres.

the pole is carrying a power line or a streetlight is a matter of indifference. Given this broader

4.4.14 Clearance profiles

approach to utilities, surface utilities typically located in the reserve include:

• • • • •

exclusively reserved for provision of the road. It defines the lowest permissible height of the soffit of any structure passing over the road and also the closest approach of any lateral obstacle

Electrical transmission lines; Telephone lines; Street lighting; Traffic signal poles, and Fire hydrants.

to the road cross-section. Clearance profiles Underground utilities include:

are described in detail in Chapter 10.

• • • • •

4.4.15 Provision for utilities

Both surface and underground utilities are often

Storm and foul water sewers; Water reticulation; Buried telephone lines; Gas pipelines, and Power transmission cables.

located within the road reserve. Utilities convey the sense of services not directly related to the

Most urban authorities have guidelines for the



The clearance profile describes the space that is


Figure 4.14: Collision rate

Figure 4.15: Prediction of utility pole crashes placement of utilities. The use of an integrated

utilities is by manholes and open manholes are

process in the planning and location of road-

an unnecessary hazard to pedestrians. In older

ways and utilities is encouraged in order to

municipal areas, services were sometimes

avoid or at least to minimise conflicts.

located under the roadway itself. This practice should be actively discouraged as it places both

As a rule, underground utilities should be locat-

maintenance workers and passing vehicles at

ed in the verges or boulevards. Access to these

risk. Every time the road is resurfaced, it is nec4-54


4.4.16 Drainage elements

essary to remove the manholes and replace them at the new level. This operation carries an element of risk but, if not carried out, the man-

The process of design of storm water drainage

hole is at a lower level than the road surface and

systems is exhaustively discussed in the South

the drop could be sufficient for a driver to lose

African Roads Agency Drainage Manual, 1986.

control of the vehicle. The lower level of the manhole would, during rainy weather, lead to

In this section, discussion is limited to the ele-

the creation of a pond of water that could slow-

ments that the designer should incorporate in

ly drain into the conduits of the buried utility,

the cross-section to ensure adequate drainage

possibly leading to disruption of the service

of the road reserve and the adjacent land.

being provided. A distinction is drawn between rural and urban drainage. Rural drainage focuses largely on the

The problem with surface utilities is that they are

swift removal of storm water from the travelled

carried on poles that can be hit by errant vehi-

way onto the verge and on its movement to a

cles. Research has indicated that the frequen-

point where it can be taken from the upstream to

cy of crashes is a function of the pole density in

the downstream side of the road. In an urban

poles per kilometre and the average pole offset

environment, the road reserve serves as the

from the travelled way. The crash frequency is

principal conduit of storm water from surround-

typically of the order of 0,1 crashes per kilome-

ing properties and its conveyance to a point

tre per year with a pole spacing of less than 20

where it can be discharged into natural water-

poles per kilometre and an offset of eight

courses.

metres. When the pole density is higher than 30

Rural drainage is, in short, the

removal of water from the road reserve whereas

poles per kilometre and the offset less than one

urban drainage attracts water to the reserve.

metre, the collision rate climbs to a high of 1,5 crashes per kilometre per year. This is illustrat-

In both rural and urban environments, storm

ed in Figure 4.14.

water drainage is aimed both at the safety of the road user and the integrity of the design layers

A nomograph predicting utility pole crashes is

of the road.

given in Figure 4.15.

exclusively directed towards safeguarding the design layers. It was previously common prac-

The example shows that a road with an ADT of

tice to recommend a minimum depth of drain.

11 000 vehicles and a pole density of 40 poles

The safety of the road user dictates rather that a

per kilometre will experience 0,75 crashes per

maximum depth of drain be specified. The rec-

kilometre per year if the pole offset is 1,5

ommended maximum depth is 500 mm.

metres. If the designer were to increase the

volume of water to be conducted by a drain thus

pole offset to 3 metres, the crash rate would

indicates the required width of the drain rather

reduce to 0,5 crashes per kilometre per year, an

than its depth, since the need to keep the design

improvement of 33 per cent.

layers unsaturated has not changed.


The


Previously, rural drainage was

Rural drainage

scour is likely to occur are given in Table 4.20. Conventional open-channel hydraulics will, in

Rural drainage is normally by means of unpaved

conjunction with Table 4.20, indicate when

open drains, which may either silt or scour,

either silting or scouring is likely, and hence

depending on the flow speeds in them. Both

whether it is necessary to pave a drain or not.

silting and scouring of a drain increase the hazard to the road user. Scour would lead to the

As a rough guide to longitudinal slopes, it is sug-

creation of a deep channel that would be impos-

gested that unpaved drains should not be steeper

sible to traverse with any degree of safety. It

than 2 per cent, or flatter than 0,5 per cent.

may also cause erosion of the shoulder and ulti-

Paved drains should not be flatter than 0,3 per

mately threaten the integrity of the travelled way

cent. Practical experience indicates that it is dif-

itself. Silting may block the drain, so that water

ficult to construct a paved drain accurately to the

that should have been removed would be dis-

tolerances demanded by a slope flatter than 0,3

charged onto the road surface.

per cent, so that local imperfections may cause silting of an otherwise adequate drain.

The effectiveness of the drain depends on water speed, which is a function of longitudinal slope,

Where the longitudinal slope is so flat that self-

as well as of other variables. There is a range

cleansing water speeds are not achieved, even

of slopes over which the flow velocity of water

with paving, it will be necessary to consider a

on in-situ materials will be so low that silting

piped drainage system.

occurs, and another range where the flow velocOn

As an alternative to lining a material subject to

slopes between these two ranges neither silting

scour, it is possible to reduce flow velocity by

nor scouring will occur and unpaved drains will

constructing weirs across an unpaved drain.

be effective.

The drain will then in effect become a series of

ity will be high enough to cause scour.


stilling basins at consecutively lower levels. Paving solves some of the problems caused by

While this could be an economical solution in

both silting and scouring. Paving generally has

terms of construction cost, it has the disadvan-

a lower coefficient of roughness than in-situ

tage that an area of deep localised erosion,

materials, so that water speeds are higher in a

immediately followed by a stone-pitched or con-

paved drain than in an unpaved drain with the

crete wall, would confront an errant vehicle. If

same slope. Furthermore, it is possible to force

this alternative is to be considered at all, it

higher speeds in the paved drain by selection of

should be restricted to roads with very low traf-

the channel cross- section. The problem of silt-

fic volumes and the weirs should be spaced as

ing can be resolved, at least partially, by paving

far apart as possible.

the drain. Drains constructed through in-situ materials The flow velocity below which silting is likely to

generally have flat inverts so that, for a given

occur is 0,6 m/s. Flow velocities above which

flow, the flow velocity will be reduced. The flat 4-56


inverts reduce the possibility of scour and are

reduce the likelihood of a vehicle digging its

easy to clear if silting occurs. Paved drains, not

front bumper into the far side of the drain and

being susceptible to scour, have a V-profile.

somersaulting.

Self-cleansing velocities are thus achieved at relatively small flows and the need for mainte-

Typical drain profiles are illustrated in Figure 4.16.

nance is reduced.

The sides of the drain should not be so steep as

(a)

to be dangerous to the road user; a maximum

Side drains are located beyond the shoulder

Ideally, both

breakpoint and parallel to the centre line of the

sides of the drain should be designed to this

road. While usually employed in cuts, they may

slope or flatter. Where space for the provision of

also be used to run water along the toe of a fill

the drain is restricted, the slope closest to the

to a point where the water can conveniently be

road should remain at 1:4 and the outer slope

diverted, either away from the road prism or

steepened. This has the effect of positioning the

through it, by means of a culvert.

drain as far as possible from the path of vehi-

in conjunction with fills, side drains should be

cles. One example of this is a side drain in a

located as close to the edge of the reserve

cut, where the outer slope of the side drain forms an extension of the cut face.

When used

boundary as is practicable to ensure that ero-

These

sion of the toe of the fill does not occur. Side

slopes, in combination with the flat invert, give

drains are intended as collectors of water and

the trapezoidal profile of an unpaved drain.

the area that they drain usually includes a cut face and the road surface.

It is recommended that the bottom of a lined Vprofile and the junctions between the sides and

(b)

Edge drains

bottom of an unlined trapezoidal profile be slightly rounded.

The rounding will ease the

Edge drains are intended to divert water from fill

path of an errant vehicle across the drain, and

slopes that may otherwise erode either because 4-57



slope of 1:4 is recommended.

Side drains


Figure 4.16: Typical drain profiles of the erodibility of the material or because they

located almost under a guardrail would heighten

are subjected to concentrations of water and

the possibility that a vehicle wheel might snag

high flow velocities.

under the guardrail.

Guardrail posts tend to

serve as points of concentration of water, so that, as a general rule, edge drains are warrant-

Edge drains are constructed of either concrete

ed when the fill material is erodible or when

or premixed asphalt. Premix berms normally

guardrails are to be installed.

have a height of 75 to 80 mm, and are trapezoidal in profile with a base width of 250 mm and

Edge drains should preferably be raised rather

a top width of 100 mm. Concrete edge drains

than depressed in profile. A depressed drain

are normal barrier kerbs and channels. These 4-58


require a properly compacted backing for stabil-

used, a transverse slope flatter than 1:10 may

ity and are, therefore, not as easy to construct

make it difficult to protect the design layers of

as premix berms.

the road.

Unlike side drains, median drains,

whether lined or not, are generally constructed

(c)

Catch water drains

with a shallow V-profile with the bottom gently rounded.

The catch water drain, a berm located at the top of a cut, is to the cut face what the edge drain is

(e)

Chutes

to a fill. It is intended to deflect overland flow from the area outside the road reserve away

Chutes are intended to convey a concentration

from the cut face. Even if the cut is through

of water down a slope that, without such protec-

material which is not likely to scour, the catch

tion, would be subject to scour. They may vary

water drain serve to reduce the volume of water

in size from large structures to half-round pre-

that would otherwise have to be removed by the

cast concrete products, but they are all open

side drain located at the bottom of the cut face.

channels. Flow velocities are high, so that stilling basins are required if down-stream erosion

Catch water drains are seldom, if ever, lined.

is to be avoided. An example of the application

They are constructed with the undisturbed top-

of chutes is the discharge of water down a fill

soil of the area as their inverts and can readily

slope from an edge drain. The entrances to

be grassed as a protection against scour.

chutes require attention to ensure that water is

Transverse weirs can also be constructed to

deflected from the edge drain into the chute,

reduce flow velocities, since the restrictions pre-

particularly where the road is on a steep grade.

viously mentioned in relation to weirs do not

It is important that chutes be adequately spaced

apply to catch water drains. The cut face and

to remove excess water from the shoulders of

the profile of the drain reduce the probability of

the road. Furthermore, the dimensions of the

a vehicle entering the drain but, should this hap-

chutes and stilling basins should be such that

pen, the speed of the vehicle will probably be

these drainage elements do not represent an

low.

excessive risk to errant vehicles.

they should be as shallow as is compatible with

(d)

their function and depths in excess of 150 mm

Median drains

should be viewed with caution. Median drains do not only drain the median but also, in the case of a horizontal curve, prevent

Because of the suggested shallow depth of

water from the higher carriageway flowing in a

chutes, particular attention should be paid to

sheet across the lower carriageway. The space

their design and construction to ensure that the

available for the provision of median drains

highly energised stream is not deflected out of

makes it possible to recommend that the trans-

the chute. This is a serious erosion hazard that

verse slopes should be in the range of 1:4 to

can be obviated by replacing the chute with a

1:10. If the narrowest median recommended is

pipe. 4-59



Generally,

(f)

Mitre banks

requirements for these forms of protection are usually conflicting. For major storms, the runoff

As their name implies, these banks are con-

should be retarded to reduce flood peaks and,

structed at an angle to the centre line of the

for minor storms, the runoff is best handled by

road. They are intended to remove water from

rapid removal. The solution is to provide two

a drain next to the toe of a fill and to discharge

separate but allied drainage systems.

it beyond the road reserve boundary. Several

ajor storms involve considerations such as

mitre banks can be constructed along the length

attempting to achieve rates of runoff that do not

of a drain, as the concentration of water in the

exceed pre-development levels.

drain should ideally be dispersed and its speed

achieved in part through the application of

correspondingly reduced before discharge.

detention and retention ponds.

Speed can be reduced not only by reducing the

the layout of road patterns can be coordinated

volume, and hence the depth, of flow but also by

with the run-off requirements of the system to

positioning the mitre bank so that its toe is virtu-

increase the time of concentration, hence reduc-

ally parallel to the natural contours.

ing the risk of flood hazards

The

This is

Furthermore,

upstream face of a mitre bank is usually protected by stone pitching, since the volume and

Accommodation of minor storms is achieved

speed of flow of water that it deflects may cause

with kerbs and channels, drop inlets and under-

scour and ultimately lead to breaching of the

ground reticulation. The runoff is initially col-

mitre bank.

lected in channels until the flooded width of the road reaches a specified limit and then dis-

(g) Rural underground systems

charged into the underground system, which is


connected to an outfall point - typically a natural The geometric designer is not directly con-

watercourse. In the case of high-speed routes,

cerned with the underground system, except for

such as freeways, expressways and major arte-

its inlets. These should be hydraulically efficient

rials, no encroachment of storm water onto the

and correctly positioned to ensure that water

travelled lanes can be considered. Minor arteri-

does not back up onto the road surface or satu-

als and collectors should have one clear lane of

rate the design layers. To restrict the hazard to

3,0 metres minimum width in each direction and

the road user, inlets that are flush with the sur-

local streets need only have one clear lane of

face drain invert are preferable to raised struc-

3,0 metres minimum width. In all cases, the

tures.

100-year storm should not cause a barrier kerb to be overtopped. The designer should ensure,

Underground reticulation is costly both to pro-

therefore, that the resultant of gradients and

vide and to maintain.

The designer should

crossfall is sufficiently steep and the spacing of

therefore, without violating the principles dis-

drop inlets sufficiently short to ensure that the

cussed above, attempt to reduce the use of

recommended lateral spreads of water are not

underground drainage as far as possible by the

exceeded.

discerning use of surface drainage.

Urban drainage Urban drainage entails the provision of protection from major and minor storms and the basic 4-60 Chapter 4: Road Design Elements

TABLE OF CONTENTS 5. 5.1 5.2 5.3 5.4

5.5

ALIGNMENT DESIGN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 VISION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 DESIGN APPROACH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3 FITTING THE ROAD TO THE LANDSCAPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5 5.4.1 . . . Freeway design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5 5.4.2 . . . Single carriageway roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9 5.4.3 . . . Perspectives in road design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9 INTEGRATION WITH THE ENVIRONMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-20

LIST OF FIGURES Figure 5-1: Radii for acceptable curve lengths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6 Figure 5-2: 1/R diagram for short curve/long tangent alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7 Figure 5-3: 1/R diagram for long curve/short tangent alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7 Figure 5.4:. 1/R diagram for curvilinear alignment with transitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-8 Figure 5.5: 1/R diagrams for alternative alignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-8 Figure 5-5: Short sag curve on long tangent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10 Figure 5-6: Short humps on long horizontal curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11 Figure 5-7: Short vertical curves preceding a long horizontal curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12 Figure 5-8: Distorted alignment at bridge crossing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13 Figure 5-9: Broken-back curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-14 Figure 5-10: Out-of-phase vertical and horizontal alignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15 Figure 5-11: Minor rolling on long horizontal curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-16 Figure 5-12: Break in horizontal alignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-17 Figure 5-13: Well-coordinated crest and horizontal curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-18 Figure 5-14: Well coordinated sag and horizontal curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-18 Figure 5-15: N2 North: Curvilinear alignment in a valley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-19 Figure 5-16: N2 North - Median widening to accommodate stream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-19 Figure 5-17: N2 - Combining horizontal and vertical alignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-20 Figure 5-18: Blending of fills into landscape. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-21 Figure 5.19: Contour plans of cut slopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-22 Figure 5.20: Cuts with constant or varying cut batters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-23

Chapter 5 ALIGNMENT DESIGN 5.1

INTRODUCTION

5.2

VISION

Most of the chapters in this guide assist the

The eye is a truly remarkable instrument sensi-

designer by describing the principles on which

tive enough for the dark-adapted eye to detect a

design parameters are based and by offering

single photon. It is also backed up by enormous

ranges and guidance in the selection of suitable

computing power and a memory bank of visual

standards. In this section an attempt is made to

images. Although it is capable of processing

stand away from the detail and discuss highway

fine detail in real time, it remains subject to the

design holistically.

laws of physics.

South Africans are blessed with a beautiful

Despite the difference in scale, the stars over-

country. There are thus boundless opportunities

head can be brighter than the spark from a

to develop highway location and design as an

campfire. This demonstrates better than words

art form in this country's varied landscape from

that seeing depends on the energy of the light

mountains and plains to deserts and seas.

received by the eye. Also there has to be a rea-

When properly applied, there can be benefits

sonable contrast between the source and the

both to the user and to the landscape.

background.

Stars are not visible during the

Creating a harmonious alignment is architec-

per cent can be perceived. At twilight, much

ture. It requires the ability to visualize the final

greater differences are required. The cones of

form both from the perspective of the driver and

the eye's retina which perceive colour also

the outside observer as well as a grounding in

require significant light energy and do not oper-

engineering principles The ability to capture the

ate at low levels at night. The yellow and green

final form in the mind's eye and translate the

colours at the centre of the spectrum appear

design into two-dimensional representations

significantly brighter than the reds and blues

defining the layout represents creativity at a high

which have longer and shorter wavelengths

level. Most texts on the subject of the aesthet-

respectively.

ics of alignment design stress the role of experience. Undoubtedly, we all learn as we grow in

Although very quick, our visual response is not

the profession. However, in our society at pres-

immediate. Approximately 0,2 of a second is

ent we do not have the luxury of widely avail-

required to fix on an object.

able, experienced engineers. Designers often

changing environment we can at best process

have to tackle difficult problems for the first time

twelve images per second.

and at short notice. In what follows we have

from the dashboard clock to a road sign takes

therefore concentrated on describing what it is

about one second. From there, reaction time

important to know and on giving some guidance

requires 1,3 seconds thus leading to a final safe

as to what is, and what is not, good practice.

reaction/response time of 2,5 seconds. A com5-1

Chapter 5: Alignment Design

However, in a Changing focus


day. In daylight, a contrast in brightness of five

designers and are further summarized below:

plex environment, where the desired response is not immediately obvious, may require a significantly longer reaction time as discussed in

1.

objects and incidences that must be

Chapter 3.

reacted to increases proportionately.

The driver sees the road and surrounds as if he

Concentration focuses on the approach-

were stationary in a 3D movie. Nearby objects

ing road and traffic in the immediate

speed past in a blur. Objects in the foreground

vicinity.

can only be seen briefly. Only objects at the

tion becomes more and more dangerous. It follows that the driver will only be

Under conditions of good lighting, people with

able to see interesting objects in the

good eyesight can observe an object that sub-

centre of his visual field and therefore

tends an angle of one arc minute or 29 millime-

the road should aim the eye towards

Lines can be perceived

objects of interest and create variety

more acutely. However, they should subtend an

through curvature.Planes that stand per-

angle of at least 4 arc seconds to be visible.

pendicular to the road are prominent

People with good eyesight are not the norm and

while parallel ones are not.

allowance has to be made for lower acuity when

2.

designing objects and messages that are impor-

As speed increases the eyes seem to focus further and further ahead. Drivers

tant to the driver in a high-speed environment

anticipate the distance ahead that they

and a constantly changing visual field.


Observing irrelevant objects

outside of the necessary area of atten-

infinity focus can be scanned at leisure.

tres at 100 metres.

As speed increases, the number of

will require to respond to emergen-

Because of the ability of both the head and eyes

cies.At 70 km/h the focal point of the eye

to swivel, it is difficult to place boundaries on the

is approximately 400 metres ahead

driver's field of view. However, the sensitive part

while at 120 km/h the focal point can be

of the retina at the centre of the visual field has

up to 1 000 metres ahead. From the

a relative arc of 2,5 degrees. The peripheral

principle of visual acuity, it follows that

vision lies outside this cone where little detail

anything that has to be brought to the

can be perceived. The visual field also dimin-

driver's attention must lie close to the

ishes the more finely we focus.

axis of vision and also be large enough to be recognized at a long distance. 3.

From these general principles of vision, J R

As the level of concentration increases,

Hamilton and L L Thurston (in a paper entitled

the total visual field decreases with the

"Human Limitations in Automobile Driving" pub-

result that the peripheral vision diminishes as

lished in 1937) enunciated five propositions that

speed increases.

are applicable to the highway environment.

referred to as tunnel vision and, unless

These were summarized in "Man Made

the point of concentration is made to

America" by Tunnard and Pushkarev (1963),

move through an arc by means of a

which should be required reading for all highway

curving roadway, driving along a straight 5-2


This is sometimes

4.

and uneventful highway can become-

networks, drivers have to concentrate in order to

hypnotic.

survive. They must focus as far ahead as pos-

As speed increases, foreground fea-

sible to anticipate the approaching vehicles and

tures begin to fade because the driver is

changes in alignment.

not able to see clearly except at a dis-

sudden events in the foreground and both their

tant focus. Foreground detail is greatly

peripheral vision and space perception are

diminished at 80 km/h, and beyond 100

impaired.

They cannot cope with

km/h reception of the foreground is negligible.

Thus, only at a distance of

With modern virtual reality techniques it is also

between 50 metres and 100 metres

possible to show that, at 120 km/h, the paved

does vision become adequate at 100

area and median across a 30 metre wide free-

km/h. It follows that emphasizing elabo-

way takes up thirty per cent of the visual field,

rate detail is meaningless for the driver.

the roadside about fifteen per cent, and the sky

Only large simple shapes are usually

dominates at fifty five per cent. When the road-

perceived and particularly the geometry

way is only 15 metres wide at somewhat lower

of the paved road at the centre of the dri-

speeds the roadway takes up about fifteen per

ver's vision. Outside the road, only the

cent, the sky approximately twenty five per cent,

general outline and form of the land

and the roadside the remaining sixty per cent.

together with objects on the horizon are distinguishable. This leads us to the conclusion that our

Space perception also becomes impaired as

approach to aligning a divided freeway should

speed increases. This is a complex sub-

be very different from that for a two lane main

ject and is related to the fact that we can-

road. For freeways, the pavement and its medi-

not perceive small relative changes in objects at long distances.

an dominate and the designer should focus his

A person

attention on the architecture of the alignment as

requires clues from the surrounding land-

it is moulded into the land form. On the other

scape to perceive motion. The movements

hand, for the narrow pavements typical of much

of objects travelling parallel and closest

of our network, the opportunity to use the road

to the axis of vision cannot be perceived

as a platform for viewing the environment can

beyond 250 metres on either side. As speed

be used to dramatic effect in road architecture.

increases the time interval between first discerning movement and passing the object reduces.

5.3

It follows therefore that the

DESIGN APPROACH

highway should offer as many clues as possible to allow the driver to judge his

In engineering terms, the geometric elements of

speed and remain in tune with reality as

the highway are simple. We use tangents, hor-

the space around him changes continuously.

izontal curves, grades and vertical curves.

When taken together, these principles confirm

However, the use of these elements in combi-

that, at the high speeds on main and freeway

nation can be nearly infinite: The result can be 5-3



5.

pleasing or discordant; economical or expen-

11.

sive; safe or dangerous.

The history, competency and funding of road maintenance in the region.

In

The two main factors that dominate alignment

context-sensitive

information

is

required which should be collated and summa-

design are the purpose of the road and the site.

rized on plans and in tables to build up an

The standards for a particular facility are usual-

understanding of the site and used to underpin

ly determined at the early planning stage and

the design trade-offs that will invariably follow.

are based on the highway’s position in the road hierarchy and on the budget available.

short,

Decisions such as which side of a ridge to fol-

This

low, where and at what angle to cross a river,

leads to a definition of standards in terms of

where material can be borrowed to support a

width, design speed, road surface, maximum

grade separation structure, where intersections

gradient, accessibility and drainage standards

or interchanges can be safely located, are all

as input to the initial design.

commonplace when a road alignment is designed. As they are also usually inter-related,

With these standards in mind, it is then the task

comprehensive information is essential to effec-

of the designer to understand the site as holisti-

tive and efficient design.

cally and in as much detail as possible. We need to know about:

With the purpose of the road established and

1.

The landform and how it was sculptured

the opportunities and constraints understood,

by nature;

the next step is to identify broad corridors or

2.

The underlying geomorphology;

avenues for further investigation both from the

3.

The engineering properties of the sur-

information gathered and from on-site recon-

face materials including erodibility;

naissance. Within each corridor, sketch-plan-

The cadastral layout and ownership pat-

ning techniques can be employed, ideally at a

terns;

scale of 1:5 000 to enable the alternatives to

The use of the land and the natural veg-

more closely examined. The ranking of the pre-

etation;

ferred options that flow from this examination

The normal and extreme weather cond -

should be rigorous and scientific. It should deal

ions;

with all issues, including aesthetics, engineer-

The drainage patterns, streams and

ing, socio-political issues and the life cycle cost.

peak discharges of the important catch-

This process, with its plethora of feedback loops

ments including their silt load;

and consultation, invariably leads to the selec-

Sensitive environmental areas that

tion of one preferred corridor, or at most two, in

require protection or preservation;

which an alignment can be designed at a scale

9.

The location of road building material;

suitable for final construction drawings.

10.

The need for access to and through the

process should be followed, even if there is only

road including an understanding of local

one obvious and preferred corridor, as it is part

circulation; and

of the process of understanding the site.

4.

5.


6. 7.

8.

5-4 Chapter 5: Alignment Design

This

5.4

FITTING THE ROAD TO THE

fall relative to the centreline. The notion is thus

LANDSCAPE

that of an abstract ribbon in space, which the designer should be able to visualise from study

The appearance of the road to the traveller and

of the survey plan, the longitudinal section and

its appearance in the landscape depend on how

the cross-section. As a rule, the most pleasing

we string the tangents, grades and curves

appearance results when the horizontal and ver-

together.

tical curves are of approximately equal length

These elements form an inclined

and in phase with one another.

plane, a simple geometric form common to all engineering design. However, nature did not create the surface of the earth using pencil and

External harmony describes the match or mis-

paper. The inclined plane is not nature's way of

match between the road and its environment.

folding the landscape, forming escarpments and

The achievement of external harmony results in

incising the rivers. A road with its rigid geomet-

the road being an enhancement of the land-

ric form made up of vectors and circular arcs is

scape rather than a scar across it. An example

thus a discordance in nature. It is the designer's

of the latter is a long tangent at right angles to

task to minimise that discord. This can be done

the natural contours and with a succession of

by the judicious use of curvature.

short crest and sag curves closely following the ground line.

This results in a roller coaster

Returning to the earlier discussion on what the

appearance totally at odds with the form of the

eye sees at speed and how vehicles behave, we

landscape.`

concluded that there is a vast scale difference

5.4.1

between a divided freeway and a two-lane road.

Freeway design

while the other is typically 13,7 metres wide or at

In locating a freeway or divided roadway, the

most 15 if there are sidewalks or surfaced

appearance of the road as a ribbon in the land-

drains. On a freeway, the paved ribbon domi-

scape from the viewpoint of the driver is crucial.

nates the driver's view, whereas, on a two-lane

To create a continuous and homogenous

road, the roadside takes up most of the visual

appearance, sudden breaks, kinks or abrupt

field.

It follows, therefore, that the design

changes in the alignment must be avoided. This

approaches for these two cases need to be very

invariably requires the use of above minimum

different indeed.

standards for horizontal and vertical curves and the use of the clothoid spiral to form transitions

This difference in approach is captured in the

between tangents and curves. To create inter-

twin concepts of internal and external harmony

est in a changing landscape the designer should

of the alignment.

strive for the curvilinear or 'splined' alignment. Figure 5-1 illustrates a range of acceptable

Internal harmony describes the drivers' view of

curve lengths for varying deflections and radii

the road itself as the centreline rises and falls or

that are visually desirable. Curve radii of up to

changes direction and the road edges rise and

18 000 metres with lengths in excess of 4 000 5-5



The former can easily be 35 to 40 metres wide

metres have been used successfully in South

the designer must be aware of the optical illu-

Africa.

sion that results from this combination. A crest curve causes a horizontal curve to appear to

Visualising the roadway from the three dimen-

have a larger radius than it has in reality and a

sional viewpoint of the driver is important. The

sag curve causes the horizontal curve to appear

Figure 5-1: Radii for acceptable curve lengths road that flows through the landscape, avoids

sharper than it really is. The lower the K-value

major obstacles and is in scale with the terrain

of the vertical curvature, the more pronounced

should be the designer's objective.

the effect is. If the horizontal curve has a mini-


mum radius, the crest curve could tempt a drivThe co-ordination of the vertical and horizontal

er to maintain a speed higher than is safe

alignment ensures that the scale of the plan and

whereas the sag curve could lead to unneces-

profile view are in harmony. However, it is not

sary and sharp braking which would not neces-

always possible to ensure that horizontal and

sarily be anticipated by following drivers.

vertical curves coincide. When these elements

Correct phasing of the vertical and horizontal

are out of phase or out of scale the designer

alignment should thus be accompanied by the

should take particular care to avoid unpleasant

use of horizontal radii that are well above the

effects.

minimum for the design speed of the road.

Several examples of good and bad

practice are illustrated and discussed in Section A useful tool for analysing the curvilinear nature

5.4.3.

of an alignment is the 1/R diagram in which the Although the aesthetics of the 3-D alignment of

inverse of the radius is plotted against the cen-

the road are enhanced by having the vertical

treline distance. In the diagrams shown in the

curves contained within the horizontal curves,

following figures tangents have a zero value and 5-6


the curves a constant value that is inversely pro-

Figure 5-5 illustrates alternative 1/R diagrams

portional to the radius. Transitions appear as

for a group of successive tangents employing:

sloping lines.

1.

Minimum radius curves;

2.

Curves approximately equal in length to

The examples in Figures 5-2 to 5-4 illustrate

the adjacent tangents; and

how 1/R diagrams are used to visualise the

3.

Long curves with the intervening tan-

curvilinear nature of an alignment. Not only is

gents only long enough to accommo-

the disjointed nature of the alignment shown in

date superelevation development

Figure 5-2 immediately apparent but the total area enclosed by the 1/R line and the horizontal

A fourth 1/R diagram illustrates the effect of

Figure 5-3: 1/R diagram for long curve/short tangent alignment axis in Figure 5-4 is significantly less than the

removing the broken-back curve between SV 1

same areas in Figures 5-2 and 5-3.

800 and SV 6 000. Depending on topographic 5-7 Chapter 5: Alignment Design


Figure 5-2: 1/R diagram for short curve/long tangent alignment

Figure 5-4: 1/R diagram for curvilinear alignment with transitions restraints, this alignment would be the preferred

ficient for aesthetic design. It is essential that

option. It illustrates two sets of true S-curves,

the road flow with the landscape.

being reverse curves with equal radii. The first

approaching an escarpment from rolling terrain

S-curve has radii of approximately 3 000 metres

the curve radii and curve length should be grad-

and the other radii of approximately 1 100 metres.

ually decreased in the transition zone until the

When


road enters the pass. This continuity of alignRelating the areas to the deflection angles and

ment enhances both the aesthetic and operating

the remaining tangents by simply making the

characteristics by reducing the element of sur-

curves as long as possible is not, of course, suf-

prise. It is under these conditions that co-ordi-

Figure 5.5: 1/R diagrams for alternative alignments 5-8 Chapter 5: Alignment Design

nating the horizontal and vertical alignment is

As the roadside takes up more than half the dri-

particularly important, as the gradients will also

ver's visual field when driving on a single car-

change in keeping with the horizontal alignment.

riageway road, there is significant benefit to be gained by maintaining the grade line above

Treatment of the median can play an important

ground level for as much of the alignment as

role in the design of a divided highway. Medians

possible. Roadways elevated in this way create

that are less than 10 metres wide should never

greater interest and improve overall visibility in

be narrowed and should usually be treated as

addition to the obvious benefit of having suffi-

unifying the two carriageways. When median

cient height to accommodate drainage struc-

widths of 15 metres or more are used, the car-

tures.

riageways can be treated independently and

5.4.3

some variation of median width or even separation of the carriageways can assist in fitting the

To illustrate the advantages of visualising the

highway to the landscape.

5.4.2

Perspectives in road design

alignment in three dimensions and to guide the design towards good practice, a number of

Single carriageway roads

alignment combinations are shown in Figures 55 to 5-17.

The advantages of curvilinear, co-ordinated alignments apply equally well to single carriageway roads. However, because the carriageway is not as wide as that of a dual carriageway, geometric standards that are well above the minimum are not demanded by the scale of the road but are certainly not precluded.

In designing the road using curves to fit the landscape, the chosen radii should allow, whermay require the day lighting of shallow cuts. With sufficiently long radii, proper sight distance can be achieved.

The alternative method of

achieving an adequate percentage of passing sight distance is by reducing the radius and hence the length of the horizontal curve enabling provision of passing sight distance on the adjacent tangents. The result may provide adequate sight distance but possibly at the cost of aesthetics.



ever possible, for passing sight distance. This

Figure 5-5 shows the advantage of maintaining

result in a disjointed alignment will be there for

a constant, uniform grade for as long as possi-

the life of the road.

ble.

Local dips to minimise earthworks that


A: Local dip on long grade

B: Local dip eliminated on long grade

Figure 5-5: Short sag curve on long tangent 5-10 Chapter 5: Alignment Design

Short crests and sags should also be avoided

Maintaining a constant grade is the preferred

on horizontal curves, as shown in Figure 5-6.

option.

B: Removal of humps on horizontal curve

Figure 5-6: Short humps on long horizontal curve 5-11 Chapter 5: Alignment Design


A: Short humps on long horizontal curve

A short discontinuity or dip in the alignment pre-

advance and following the sag curve improves

ceding a horizontal curve creates a particularly

the appearance, as shown in Figure 5-7.

discordant view. Eliminating the crest curves in


A: Short hump and dip preceding horizontal curve

B: Long sag curve linking into horizontal curve

Figure 5-7: Short vertical curves preceding a long horizontal curve 5-12 Chapter 5: Alignment Design

A common fault in road alignment is illustrated in

The advantages in the alignment aesthetics of a

Figure 5-8. The roadway is often unnaturally

skew crossing often far outweigh the savings

curved to cross a small stream or grade separa-

deriving from a square crossing

tion at right angles.

A: Distorted alignment to create square river crossing

Figure 5-8: Distorted alignment at bridge crossing 5-13 Chapter 5: Alignment Design


B: Skew crossing improves horizontal alignment

Figure 5-9A illustrates the broken-back horizon-

back effect. The advantages of using a single

tal curve, or two curves in the same direction

radius curve throughout are illustrated in Figure

separated by a short tangent. The sag curve on

5-9.B.

the separating tangent intensifies the broken-


A: Broken-back curve

B: Replacement of broken-back curve by single radius long curve

Figure 5-9: Broken-back curve 5-14 Chapter 5: Alignment Design

A sag curve at the start of a horizontal curve has

the horizontal curve would cause it to start earlier.

the effect of enhancing the sharp angle appear-

Applying both remedial measures should result

ance as shown in Figure 5-10, and should be

in a better phasing of the horizontal and vertical

avoided. Raising the preceding grade will move

alignments.

the sag curve downstream. A longer radius on



Figure 5-10: Out-of-phase vertical and horizontal alignments

Minor changes in grade or rolling of the vertical

avoided on long horizontal curves.

alignment as shown in Figure 5-11 should be

A: Rolling gradeline

B: Elimination of rolling gradeline


Figure 5-11 B illustrates the advantages of co-ordinating the horizontal and vertical alignments.

Figure 5-11: Minor rolling on long horizontal curve 5-16 Chapter 5: Alignment Design

crest and the continuation of the curve is visible

horizontal curve is hidden by an intervening

in the distance. The road appears disjointed.

Figure 5-12: Break in horizontal alignment



Figure 5-12 shows the effect when the start of a


Figure 5-13: Well-coordinated crest and horizontal curves

Figure 5-14: Well coordinated sag and horizontal curves Figure 5-13 and Figure 5-14 illustrate the

vertical alignment.

advantages of co-ordinating the horizontal and

curve is contained within the horizontal curve. 5-18


In each case the vertical

Figure 5-15: N2 North: Curvilinear alignment in a valley in practice.

Figure 5-16: N2 North - Median widening to accommodate stream 5-19 Chapter 5: Alignment Design


Figures 5-15 to 5-17 illustrate examples where excellent aesthetic designs have been achieved

Figure 5-17: N2 - Combining horizontal and vertical alignment

5.5

INTEGRATION WITH THE ENVI-

In locating a road, it is necessary to synthesise

RONMENT

land use planning and transportation planning.


The traffic engineer sees the road as a conduit A road is never the exclusive preserve of the

for goods and people. Estimates are made from

user as it affects everybody in its surroundings.

the factors governing trip generation and trip ori-

The impacts on the non-user are usually a com-

gins and destinations. These ultimately result in

bination of social, economic and environmental

expected and future traffic volumes. The land

factors that act together in complex ways. They

use planner on the other hand is concerned with

can be negative as in the scarring of a hillside,

the organisation of the areas and their relation-

the diversion of a stream or the closing of an

ship to one another. Although making a neces-

access. They can also be positive:

sary contribution to communication, roads, par-

•

When the space used by the road is

ticularly those of an arterial or freeway nature,

orderly;

are viewed as a barrier. They should never cut

The alignment imaginative;

across homogenous areas of whatever type.

• • •

It appears to belong where it is; and It serves the transportation needs of the

The challenge of reconciling the conflicts

community.

between the disciplines can best be tackled by a 5-20 Chapter 5: Alignment Design

team approach where all participants strive to

guardrails.

understand the viewpoints and constraints of

more readily into the natural contour of the land.

the various disciplines.

Integration with the The general rule is that the lower the cut or fill

social environment should be the primary objective.

Flat, rounded slopes also blend

the flatter the slope should be. On fills with a

Of the social values, the aesthetic and

height of 8 to 10 metres, slopes of 1 : 1,1/2 to1

visual impacts are often the most important. It is

: 2 appear acceptable. For heights of 4 to 5

the detail of embankments, road signs, bridges,

metres slopes of 1 : 4 should be the goal.

the planting of the median and a host of other

Minor cuts and fills should be blended into the

design features that attract the eye. All these

landscape so that they are hardly noticeable.

should be integrated with the immediate envi-

This concept is shown in Figure 5-18. In the first

ronment of the road.

sketch, the continuity of the space is preserved in the cross-section whereas, in the second, the

Of all these, the form of the cross section is the

space is chopped up into discontinuous ele-

main element under the control of designer that

ments.

can be manipulated to soften the impact of the road. Flattening the slopes of cut and fill and rounding

The use of contour plans to grade cut and fill

the changes of slopes to create smooth con-

slopes are particularly useful when designing

tours has many benefits. It reduces soil erosion,

interchanges to ensure a pleasing layout of

minimises the risk of injury when a vehicle

what are often large ground areas.

leaves the road and reduces the warrants for 5-21



Figure 5-18: Blending of fills into landscape


Figure 5.19: Contour plans of cut slopes

Figures 5-19 and 5-20 illustrate the advantages

through a natural feature whereas the second

of transitioning the slopes into a deep cut. The

illustrates what, at best, would be a scar on the

approach shown in the first figure could lead to

landscape.

the appearance of the road being located


B: Cut with varying cut batters

Figure 5.20: Cuts with constant or varying cut batters



A: Cut with constant cut batters

TABLE OF CONTENTS 6

INTERSECTION DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1 6.1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

6.2

DESIGN PRINCIPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 6.2.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 6.2.2 Elements affecting design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 6.2.3 Traffic manoeuvres and conflicts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3 6.2.4 Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4 6.2.5 Intersection types and selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5 6.2.6 Location of intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7 6.2.7 Spacing of intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8 6.2.8 Channelisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11

6.3

GEOMETRIC CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12 6.3.1 Angle of intersection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12 6.3.2 Horizontal and vertical alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12 6.3.3 Lane widths and shoulders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13

6.4

SIGHT DISTANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13 6.4.1 General Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13 6.4.2 Sight Triangles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14 6.4.3 Intersection control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-15

6.5

CHANNELISATION ELEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-26 6.5.1 Channelising islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-26 6.5.2 Turning roadway widths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-31 6.5.3 Tapers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-32

6.6

ROUNDABOUTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-34 6.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-34 6.6.2 Operation of roundabouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-34 6.6.3 Design speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-36 6.6.4 Sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-37 6.6.5 Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-37

LIST OF TABLES Table 6.1: Values of human factors appropriate to intersection design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 Table 6.2 : Vehicle characteristics applicable to design of channelised intersections . . . . . . . . . . . . . . . . . . 6-3 Table 6.3: Features contributing to accidents at intersections and remedial measures . . . . . . . . . . . . . . . . . 6-4 Table 6.4: Typical maximum traffic volumes for priority intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6 Table 6.5: Minimum radii for location of intersections on curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7 Table 6.5: Recommended minimum access separations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10 Table 6.5: Recommended sight distances for intersections with no traffic control (Case A) . . . . . . . . . . . . . 6-17 Table 6.6: Adjustment factors for approach sight distances based on approach gradient . . . . . . . . . . . . . . 6-17 Table 6.7: Travel Times Used to Determine the Leg of the Departure Sight Triangle along the Major Road for Right and Left Turns from Stop-Controlled Approaches (Cases B1 and B2). . . . . . . . . 6-19 Table 6.8: Travel times used to determine the leg of the departure sight triangle along the major road . to accommodate crossing manoeuvres at stop-controlled intersections (Case B3). . . . . . . . . . . 6-21 Table 6.9: Leg of approach sight triangle along the minor road to accommodate crossing manoeuvres . from yield-controlled approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-23 Table 6.10:Travel times used to determine the sight distance along the major road to accommodate right turns from the major road (Class F). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-24 Table 6.11:Turning roadway widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-32 Table 6.12:Taper rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-33

LIST OF FIGURES Figure 6.1: Typical intersection types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1 Figure 6-2: Classes of intersections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10 Figure 6.3: Desirable signal spacings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11 Figure 6.4: Fitting minor road profiles to major road cross-sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14 Figure 6.5: Sight triangles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16 Figure 6.6: Effect of skew on sight distance at intersections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-25 Figure 6.7: General types and shapes of islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-27 Figure 6.8: Typical directional island . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-28 Figure 6.9: Typical divisional (splitter) island . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-29 Figure 6.10:Typical median end treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-31 Figure 6.11: Elements of roundabouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-35 Figure 6.12: Intersection conflict points. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-36

Chapter 6 INTERSECTION DESIGN 6.1

INTRODUCTION

The design of an at-grade intersection requires

physical layout of an intersection is defined by

understanding of the principles of both traffic

its horizontal and vertical alignment, roadway

and highway engineering. The operation of an

cross-sections, surface texture and drainage.

lengths and delays, accident potential, vehicle

The successful integration of all these factors is

operating characteristics and traffic control. The

required for good design, which must overcome

Figure 6.1: Typical intersection types 6-1 Chapter 6: Intersection Design


intersection is influenced by its capacity, queue

the potential safety and operation conflicts that

of 2,5 seconds is normally used as an input to

are inherent when traffic streams interact at

models determining intersection sight distance.

intersections.

However, because of heightened awareness at controlled intersections, circumstances may

Although some guidance on capacity and traffic

indicate a lower acceptable value of 2,0 sec-

control is offered, the focus of this chapter is on

onds at busy urban intersections.

application of the geometric principles that govern the physical layout and location of an inter-

The concept of driver expectancy is crucial in

section.

the evaluation of drivers' response and tasks within intersections.

6.2

DESIGN PRINCIPLES Vehicle Characteristics

6.2.1

General


The size and manoeuvrability of vehicles is a The unique characteristic of intersections is that

governing factor in intersection design, particu-

vehicles, pedestrians and bicycles travelling in

larly when channelisation features are being

many directions, must share a common area,

selected. Because of the importance of vehicle

often at the same time. The mitigation of the

characteristics in the operation of an intersec-

resulting conflicts is a major objective of inter-

tion, the selection of an appropriate vehicle, as

section design.

This conflict resolution is, in

described in sub-section 3.4.4, will influence the

turn, influenced by construction and mainte-

elements in the above table. In selecting an

nance costs, environmental factors and the

appropriate vehicle, the designer should careful-

ease of implementation.

ly evaluate the expected traffic mix in context.

6.2.2

Various vehicle characteristics and their influ-

Elements affecting design

ence on the design of channelised intersections Human factors

are described in Table 6.2.

The driver's response, discussed in sub-section

Environmental Influences

3.2.1, is a major factor in intersection design. The type of highway and area, surrounding land

The recommended perception and reaction time 6-2

Chapter 6: Intersection Design

use and the prevailing climate all have an influ-

The type of area and adjacent land use governs

ence on the type of design selected. Flexibility

the selection of an appropriate intersection. In

of approach is essential and the concepts of

urban areas, pedestrian flows, on-street parking

context-sensitive design as outlined in Chapter

and bus and taxi activity are commonplace. In

2 should be applied

residential areas, bicycles and school crossings need to be considered. They are usually absent

Functional classification is a key to applying the

in rural areas, where utility and delivery vehicles

appropriate design standards. Primary arterials

are more common.

speeds and are often used by drivers unfamiliar

Local climate can influence design decisions.

with them. Large trucks and buses are common

Where the presence of mist is a frequent occur-

and there is a driver expectancy for route conti-

rence, sight distance would be reduced. Heavy

nuity and a high level of service. Intersection

rainfalls can obscure signs and road markings

design should reflect and make provision for the

and reduce pavement friction.

operating characteristics of drivers and their

6.2.3

expectations in the various classes of roads. Channelisation

should

accommodate

Traffic manoeuvres and conflicts

the

expected vehicles in a simple and direct man-

Typical manoeuvres that result in vehicle conflict

ner. Decision sight distance is an important ele-

at intersections are:

ment and traffic control devices and pavement

• •

markings should be placed with care.

Crossing; Merging;

6-3 Chapter 6: Intersection Design


carry high traffic volumes, operate at high

• •

Diverging; and

Traffic volumes are the most important determi-

Weaving.

nant of intersection accidents. This is not surprising as the accident potential caused by con-

Diverging and merging may be either to the left

flict increases as traffic on the approach legs

or right, mutual or multiple. Crossings may be

increases.

direct, if the angle of skew is between 75o and 120o, or oblique if the angle is in the range of

The type of traffic control also influences acci-

60o

Oblique skews should be avoided if

dents. More rear-end and sideswipe accidents

at all possible. If the angle of skew is less than

tend to occur at signalised intersections than at

60o,

the possibility of replacing the skew by a

other types of control. Stop and Yield controls

staggered intersection should be considered.

tend to increase the frequency of crossing acci-

to

75o.

Angles of skew greater than

120o

should be

dents. Table 6.3 lists many of the condition that

replaced by relocation of the intersection to an

can lead to accidents and also the geometric

angle of skew closer to

90o.

and control measures that are used to mitigate


poor accident experience.

Weaving is a combination of merging and

6.2.4

diverging traffic moving in the same direction. It

Capacity

may be simple or complex. To operate successfully, an intersection must be The conflict at intersections created by the vari-

able to handle peak traffic demands. The analy-

ous manoeuvres leads to a unique set of opera-

sis of capacity is based on the operational char-

tional characteristics. Understanding these is

acteristics of conflicting vehicles separated by

central to intersection designs and the most

the time constraints imposed by traffic control

important characteristics are safety and capacity.

devices. The measuring and forecasting of traf6-4


fic flows and capacity analysis is a specialised

Unsignalised intersections

subject and designers should refer to the manuals and references commonly used. The follow-

The capacity of the major road at Stop- and

ing is a brief summary of capacity as it relates to

Yield-controlled intersections is not affected by

design.

the presence of the intersection. The capacity of the minor road is dependent on the distribution of gaps in the major road traffic and the gap

Signalised intersections

acceptance of the minor road traffic. acceptance

The idealised flow rate through an intersection

and vehicle length.

green time. Initial driver reaction, vehicle accel-

described in Section 6.4

signal cycle in accordance with C

=

capacity (veh/h)

s

=

saturated flow

Factors affecting capacity include:

• • • •

rate (veh/h) g/c

=

the ratio of green time to

Operational speed of the major road; Intersection sight distance; Radii of turning roadways; Intersection layout and number of lanes;

• •

signal cycle time.

Type of area; and Proportion of heavy vehicles

The critical factors are intersection sight dis-

The important factors affecting saturation flow

tance and the number and arrangement of traf-

are:

fic lanes.

Number of lanes; Widths of lanes;

6.2.5

Proportion of heavy vehicles;

Intersection types and selection

Gradients in excess of 3%; On-street parking;

There are four basic types or classes of inter-

Pedestrian activity; and

section:

Type and phasing of signals.

three-legged T and Y intersections;

four legged intersections with a defined crossing path; multi-legged intersections; and round-

The critical factors are the total number of lanes

abouts, with the last-mentioned encompassing

and the need for exclusive turning lanes at each

all three of the previous types.

approach.

These are

shown schematically in Figure 6.1. The design-



• • • • • • •

Gap

to compute intersection sight distance as

al to the green time for that approach within the

where

It is not a function of

are usually somewhat shorter than those used

approach or leg of an intersection is proportion-

s x g/c

the

acceptance times used in determining capacity

The capacity of an

=

on

approach speed on the major road.

eration and the behaviour of following vehicles

C

dependent

reaction/response time, vehicle acceleration

is known as the saturation flow rate per hour of

all affect this flow rate.

is

Gap

er's selection of the basic intersections type is

At intersections carrying light crossing and turn-

normally predicated on the design context, as

ing volumes the capacity figures for uninterrupt-

intersections can vary greatly in scope, shape,

ed flow generally apply. Table 6.4 below is a

degree of channelisation and traffic control

guide to the maximum traffic volumes that these

measures.

intersections can accommodate. When volumes exceed the above, the capacity

Important factors to be considered in the selec-

of the intersection should be analysed in detail.

tion of an intersection type are:

• • • •

Cost of construction;

As safety at intersections is of key importance,

Type of area;

the following summary of the interrelationship

Land use and land availability;

between intersections and accidents can act to

Functional classes of the intersecting

guide the selection of an intersection type and

roads;

• • •

layout:

Approach speeds;

•

Proportion of traffic on each approach;

The U.S. National Safety Council esti-

and

mated that 56 per cent of all urban acci-

Volumes to be accommodated.

dents and 32 per cent of all rural accidents occur at intersections.

•

Careful consideration of these factors, together

to the volume and distribution of traffic

with the warrants for selecting appropriate traffic

on the major and minor roads.

control devices, will lead to an appropriate

•

choice or to a limited number of alternatives


The number of accidents is proportional

Roundabouts have considerable safety advantages over other types of at grade

from which to make the final selection. The crit-

intersections.

ical factors are cost and capacity.

•

Poor sight distance leads to significantly higher injury and total accident rates.

A worldwide review of intersection design prac-

However, on roundabout approaches,

tice reported that, "typically the cheapest inter-

accidents may actually increase with

section type providing the required level of serv-

increasing sight distance.

ice is chosen". This cost is usually the sum of

•

the design, construction and right-of-way costs.

Medians should be as wide as practical

This view is consistent with South African expe-

at rural, unsignalised intersections but

rience, where the cost to the road authority is

not wider than necessary at signalised

often the governing factor in the choice of inter-

intersections.

section type.

•

Channelisation is usually beneficial but


• 6.2.6

kerbed islands in the major road may be

the alignment of either the major or the minor

hazardous in rural areas.

road, or both, to ensure that adequate sight dis-

The hazard of an intersection increases

tance is available. If this is not possible, the

as the approach speed increases.

options available to the designer are to:

Location of intersections

• • •

relocate the intersection; provide all-way Stop control; or provide a Jug handle or Quarter link

Given the fact that intersections are the most

interchange, as described in Section

dangerous part of any road network, it follows

7.6.4.

that their location deserves serious attention by

Where heavy earthworks, possibly beyond the

the designer. It is necessary to minimise both

normal limits of the road reserve, are required to

the likelihood of crashes occurring and the con-

provide adequate sight distances, relocation

sequences of the crashes that do occur. There

may be an option.

are thus various restraints on the location of intersections that should be considered.

The location of an intersection on a curve can create problems for the drivers on both legs of

The need for drivers to discern and perform the

the minor road. Drivers on the minor road leg on

manoeuvres necessary to pass safely through

the inside of the curve will have difficulty in see-

an intersection demands that decision sight dis-

ing approaching traffic because this traffic will

tance be available on the major road approach-

be partly behind them.

es. The driver on the minor road requires ade-

effect, an artificial angle of skew. Furthermore,

quate intersection sight distance, as well as the

there is a possibility that a portion of the sight tri-

sight triangles described in Section 6.4, in order

angle may fall outside the limits of the road

either to merge with traffic on the major road or

reserve, which could hamper efforts to obtain a

to cross safely. It may be necessary to modify

clear line of sight for the driver on the minor



6-7

This constitutes, in

road. For these, reasons, intersections should

The location of an intersection may require

not be located on horizontal curves with radii

modification to improve the angle of skew

less than those indicated in Table 6.5.

between the intersecting roads, as discussed in Section 6.3.1. If the angle of skew is less than

Drivers on the outside of a curve typically have

60O, the intersection can be replaced by two rel-

little or no difficulty in seeing opposing vehicles

atively closely spaced T-intersections. A vehicle

on the major road. The opposing vehicles are

on the minor road would thus follow the route

partly in front of them and they have the addi-

comprising a right turn onto the major road, fol-

tional height advantage caused by the superel-

lowed by a left turn off it. Any delay to the minor

evation of the curve. However, they have to

road vehicle would occur clear of the high-

negotiate the turn onto the major road against

speed traffic on the major road. If the angle of

an adverse superelevation. The risks involved

skew is greater than 120O, relocation should be

in sharp braking during an emergency should

to a four-legged intersection in preference to the

also be borne in mind when an intersection is

two T-intersections, because these would result

located on a curve. In general, an intersection

in the minor road traffic following the route com-

should not be located on a curve with a superel-

prising a left turn onto the major road and a right

evation greater than 6 per cent.

turn off it with consequent delays to the major road traffic and increased risk to the vehicle

Stopping sight distances increase with steepen-

waiting to complete the right turn off the major

ing negative gradient. Stopping sight distance

route. A right-left stagger or offset is, in short, to

required on a gradient of -3 per cent is approxi-

be preferred to a left-right stagger.

mately 6 per cent longer than that on a level grade, whereas, on a -6 per cent gradient, it is

A further limitations on the location of intersec-

approximately 16 per cent longer. It is suggest-

tions - being the spacing of successive intersec-

ed that, as a safety measure, intersections

tions - is discussed in the following section.

should not be located on gradients steeper than

6.2.7

three per cent. The gradient is more critical on

Spacing of intersections


the minor road than on the major road because all vehicles on the minor road have to stop or

Designers seldom have influence on the spac-

yield.

ing of roadways in a network as it is largely predicated by the original or developed land

One of the consequences of a collision between

use. Nevertheless, the spacing of intersections

vehicles is that either or both may leave the

impacts significantly on the operation, level of

road. It is therefore advisable to avoid locating

service and capacity of a roadway. It follows

intersections on high fills if at all possible. The

that intersection spacing should, inter alia, be

obstruction of sight distance by bridge parapets

based on road function and traffic volume. The

should also be borne in mind. In the case of the crossing road ramp terminal at a narrow dia-

principles described in the National Guidelines

mond interchange, both these problems may

for Road Access Management in South Africa

arise.

should therefore play a role in the determination 6-8 Chapter 6: Intersection Design

of the location of individual intersections. This is

Along signalised arterials, intersection spacing

of particular concern when the provision of a

should be consistent with the running speed and

new intersection on an existing road is being

signal cycle lengths, which are variables in

considered.

themselves. If the spacing of the intersections is based on acceptable running speeds and

Access management is aimed at maintaining an

cycle lengths, signal progression and an effi-

effective and efficient transportation system for

cient use of the roadway can be achieved.

the movement of people and goods, simultaneously supporting the development of the adja-

In Figure 6.2, these three variables are com-

cent land use.

bined in a chart allowing the selection of a suit-

usage generally leads to demands for improved

able intersection spacing.

road infrastructure and the improved infrastruc-

transpires that the minimum spacing on arterial

ture makes access to it very attractive. Allowing

roads should be at least 400m. Where spacings

access simply on the basis of its meeting some

closer than the minimum exist, a number of

or other minimum geometric requirement results

alternative actions can be considered, for exam-

in increasing traffic conflicts and reduction in

ple two-way flows can be converted to one-way

capacity so that the benefit of the original

operation or minor connecting roads can be

improvement is lost.

closed or diverted, and channelisation can be

This then leads to

From this figure it

demands for further road improvements.

used to restrict turning movements.

This cycle can only be broken by the develop-

Where the crossing road of an interchange is an

ment of a proper Access Management Plan by

arterial, the suggested minimum distance along

the local authority concerned. This plan speci-

the arterial from the ramp terminal to the next

fies where intersections may be located.

intersection is 200m in the case of a collector

Furthermore, it defines the class of intersection

road. If the next intersection is with an arterial

that may be considered. Three classes of inter-

the spacing between the ramp and the intersec-

sections are defined in the National Guidelines.

tion should be increased to 600 metres for a

These are:

Class 3 arterial and 800 metres for a Class 2

•

arterial.

Full access, which allows for all possible movements at an intersection or access;

•

•

Partial access, which allows left-in, left-

The spacing between successive unsignalised

out and right-in movements to and from

intersections is measured by the separation

a development or access road; and

between them with separation being defined as

Marginal access, which allows only left-

the distance between their reserve boundaries.

in and/or left-out movements to and from

Recommended access separations are provid-

a development or access road.

ed in Table 6-5. The left-in left-out class of access would typical-

These are illustrated in Figure 6.2.

ly only be applied under circumstances where 6-9 Chapter 6: Intersection Design


Increasingly intensive land

speeds or traffic volumes or both are high. This

and deceleration lanes using taper rates as list-

is in order to minimise the disruption that would

ed in Table 6-12 and lengths listed in Table 7-5

be caused by right-turning vehicles. As such, it

or 7-6


would normally be provided with acceleration

Figure 6-2: Classes of intersections 6-10 Chapter 6: Intersection Design

6.2.8

•

Channelisation

Undesirable or wrong-way movements should be discouraged or prohibited;

• •

The purpose of channelisation is to achieve safe and efficient operation by managing the conflicts that are inherent to intersections.

Vehicle paths should be clearly defined; The design should encourage safe vehicle speeds;

NCHRP

Figure 6.3: Desirable signal spacings

•

Report 279 reports that the objectives of good

•

Reduction of the number of points of

right angles and merge at flat angles;

potential conflict to the minimum com-

•

patible with efficient operation;

•

•

Limitation of the frequency of actual con-

The design should be in the context of the traffic control scheme;

•

flicts; and

•

High priority flows should have the greater degree of freedom;

Reduction of the complexity of conflict areas whenever possible;

•

Traffic streams should cross at close to

Decelerating, slow-moving or stopped

Limitation of the severity of those con-

vehicles should be separated from high-

flicts that do occur.

er-speed through lanes; and

•

Refuge for pedestrians and the handi-

To achieve these objectives there are nine prin-

capped should be provided where

ciples of channelisation design:

appropriate. 6-11 Chapter 6: Intersection Design


of conflict whenever possible;

intersection design are:

•

Channelisation should separate points

The tools available to apply these principles are:

have difficulty at this angle of skew in seeing

• • • • •

Defining and arranging traffic lanes;

vehicles approaching from their left.

Traffic islands of all sizes and types;

designer should be able to specifically justify

Median islands;

using an angle of skew less than 75°. In the

Corner radii;

remodelling of existing intersections, the acci-

Horizontal and vertical approach geom-

dent rates and patterns will usually indicate

etry;

whether a problem exists and provide evidence

Pavement tapers and transitions; and

on any problems related to the angle of skews

• •

The

Traffic control devices.

6.3.2

Horizontal and vertical alignment

The first six elements are a range of physical features while traffic control devices are an inte-

The horizontal and vertical alignments through

gral part of any intersection. These six elements

and approaching an intersection are critical fea-

are discussed in Section 6.5.

tures. Simple alignment design allows for early recognition of the intersection and timely focus

6.3

on the intersecting traffic and manoeuvres that

GEOMETRIC CONTROLS

must be prepared.

6.3.1

Angle of intersection The following are specific operational requirements at intersections:

The angle of intersection of two roadways influ-

•

ences both the operation and safety of an inter-

required sight distance;

section. Large skews increase the pavement

•

area and thus the area of possible conflict.

The alignments should allow for the fre-

Operationally they are undesirable because:

quent braking and turning associated

•

with intersections; and

Crossing vehicles and pedestrians are

•

exposed for longer periods;

•


The alignments should not restrict the

•

The driver's sight angle is more con-

undue direct attention to be detracted

strained and gap perception becomes

from the intersection manoeuvres and

more difficult;

conflict avoidance.

Vehicular movements are more difficult

As a general guide, horizontal curve radii at

and large trucks require more pavement

intersections should not be less than the desir-

area; and

•

The alignments should not require

able radius for the design speed on the

Defining vehicle paths by channelisation

approach roads.

is more difficult. For high-speed roads with design speeds in

For new intersections the crossing angle should

excess of 80km/h, approach gradients should

preferably be in the range 75° to 120°. The

not be greater than - 3 per cent. For low-speed

absolute minimum angle of skew is 60° because

roads in an urban environment this can be

drivers, particularly of trucks with closed cabs,

increased to - 6 per cent. 6-12


distance at the intersection to above-minimum

Stopping sight distance should be provided con-

requirements.

tinuously on all roadways including at the

For new intersections, the gradient on the minor

approaches to intersections. However, in rural

roadway is normally adjusted to form a smooth

areas or when approach speeds are in excess

profile, as suggested in Figure 6.4.

of 80 km/h, the decision sight clearance set out in Section 3.5.8 should be provided on all approaches to intersections for safe operation,

Where major roadways intersect, the profiles of

particularly where auxiliary lanes are added to

both roads are usually adjusted in approximate-

the intersection layout to accommodate the turn-

ly equal manner. When significant channelisa-

ing movements.

tion is introduced in association with complex gradients, intersections should be designed on an elevation plan to avoid discontinuities and

In addition to these forms of sight distance, it is

ensure free drainage.

necessary

to

Distance (ISD).

6.3.3

Lane widths and shoulders

provide

Intersection

Sight

This is the sight distance

required by drivers entering the intersection to enable them to establish that it is safe to do so

Where intersecting roadways have shoulders or

and then to carry out the manoeuvres neces-

sidewalks, the main road shoulder should be

sary either to join or to cross the opposing traf-

continued through the intersection. Lane widths

fic streams.

should be 3,7 m for through lanes and 3,6

derived from elaborate models based on

metres for turning lanes. Where conditions are

assumptions of reaction times, speeds and

severely constrained, lane widths as low as 3,3

acceleration rates of turning vehicles and the

metres can be considered provided that

deceleration rates of the opposing vehicles, etc.

approach speeds are below 80 km/h. In con-

The distances offered in sub-section 6.4.3 are

stricted urban conditions on low speed-road-

derived from research into gap acceptance as

ways, lane widths of 3,0 metres should be the

reported in NCHRP Report 383 "Intersection

minimum adopted. Offsets from the edge of the

Sight Distance".

Previously, values of ISD were

1,0 metres.

6.4

SIGHT DISTANCE

6.4.1

General Considerations

The provision of adequate sight distances and appropriate traffic controls is essential for safe intersection operation.



turning roadway to kerb lines should be 0,6 to


Figure 6.4: Fitting minor road profiles to major road cross-sections

6.4.2

Sight Triangles

Two different forms of sight triangle are required.

In the first instance, reference is to

Each quadrant of an intersection should contain

approach sight triangles. The approach triangle

a clear sight triangle free of obstructions that

will have sides with sufficient lengths on both

may block a driver's view of potentially conflict-

intersecting roadways such that drivers can see

ing vehicles on the opposing approaches.

any potentially conflicting vehicle in sufficient 6-14


time to slow, or to stop if need be, before enter-

truck driver is considerably greater than that

ing the intersection. For the departure sight tri-

required by the driver of a passenger car. For

angle, the line of sight described by the

design purposes, the eye height of truck drivers

hypotenuse of the sight triangle should be such

is taken as 1,8 metres for checking the avail-

that a vehicle just coming into view on the major

ability of sight distance for trucks.

road will, at the design speed of this road, have

6.4.3

a travel time to the intersection corresponding to

Intersection control

the gap acceptable to the driver of the vehicle on the minor road. Both forms of sight triangle

The recommended dimensions of the clear sight

are required in each quadrant of the intersec-

triangles vary with the type of traffic control used

tion.

at an intersection because different types of control impose different legal constraints on

The line of sight assumes a driver eye height of

drivers resulting in different driver behaviour.

1,05 metres and an object height of 1,3 metres.

Sight distance policies for intersections with the following types of traffic control are presented

The approach and departure sight triangles are

below:

illustrated in Figure 6.5. The areas shown shaded should be kept clear of vegetation or any

• •

other obstacle to a clear line of sight. To this end, the road reserve is normally splayed to

Intersections with no control (Case A); Intersections with Stop control on the minor road (Case B);

ensure that the entire extent of the sight triangle

o

is under the control of the road authority.

Right turn from the minor road (Case B1);

Furthermore, the profiles of the intersecting

o

Left turn from the minor road (Case B2);

roads should be designed to provide the o

required sight distance. Where one or other of

Crossing manoeuvre from the minor road (Case B3);

the approaches is in cut, the affected sight tri-

•

angles may have to be "day lighted", i.e. the nat-

Intersections with Yield control on the minor road (Case C);

ural material occurring within the sight triangles

o

may have to be excavated to ensure intervisibil-

Crossing manoeuvre from the

o

Left or right turn from the minor road (Case C2);

•

Sight distance values are based on the ability of

Intersections with traffic signal control (Case D); and

the driver of a passenger car to see an

•

approaching passenger car. It is also necessary

Intersections with all-way Stop control (Case E).

to check whether the sight distance is adequate for trucks. Because their rate of acceleration is lower than that of passenger cars and as the

A sight-distance policy for stopped vehicles turn-

distance that the truck has to travel to clear the

ing right from a major road (Case F) is also pre-

intersection is longer, the gap acceptable to a

sented.



minor road (Case C1);

ity between the relevant approaches.

Figure 6.5: Sight triangles


Intersections with no control (Case A)

If sight triangles of this size cannot be provided, the lengths of the legs on each approach can be

Uncontrolled intersections are not used in con-

determined from a model that is analogous to

junction with the main road network, but are

the stopping sight distance model, with slightly

common in rural networks and access roads to

different assumptions.

rural settlements. In these cases, drivers must be able to see potentially conflicting vehicles on

Field observations indicate that vehicles

intersecting approaches in sufficient time to stop

approaching uncontrolled intersections typically

safely before reaching the intersection. Ideally,

slow down to approximately 50 per cent of their

sight triangles with legs equal to stopping sight

normal running speed. This occurs even when

distance should be provided on all the

no potentially conflicting vehicles are present,

approaches to uncontrolled intersections.

typically at deceleration rates of up to 1.5m/s. 6-16


Braking at greater deceleration rates, which can

from a speed less than the normal running

approach those assumed in the calculation of

speed.

onds after a vehicle on the intersecting

Table 6.5 shows the distance travelled by an

approach comes into view. Thus, approaching

approaching vehicle during perception, reaction

vehicles may be travelling at less than their nor-

and braking time as a function of the design

mal running speed during all or part of the per-

speed of the roadway on which the intersection

ception-reaction time and can brake to a stop

approach is located. These distances should be

Note: Based on ratio of stopping sight distance on specified approach grade to stopping sight distance on level terrain. 6-17 Chapter 6: Intersection Design


stopping sight distances, begins up to 2,5 sec-

used as the legs of the sight triangles shown in

intersections because all minor-road vehicles

Figure 6.4.

should stop before entering or crossing the major road.

Where the gradient of an intersection approach exceeds three per cent, the leg of the clear sight

Vehicles turning right from the minor road have

triangle along that approach should be adjusted

to cross the stream of traffic approaching from

by multiplying the sight distance in Table 6.5 by

the right and then merge with the stream

an adjustment factor in Table 6.6.

approaching from the left. Left-turning vehicles need only merge with the stream approaching

If these sight distances cannot be provided,

from the right.

As the merging manoeuvre

advisory speed signing to reduce speeds or

requires that turning vehicles should be able to

installing Stop signs on one or more approach-

accelerate approximately to the speed of the

es should be investigated.

stream with which they are merging, it necessitates a gap longer than that for the crossing manoeuvre.

Uncontrolled intersections do not normally require departure sight triangles because they typically have very low traffic volumes.

Right turn from the minor road (Case B1)

If a

motorist finds it necessary to stop at an uncon-

A departure sight triangle for traffic approaching

trolled intersection because of the presence of a

from the left, as shown in Figure 6.4(B), should

conflicting vehicle, it is unlikely that another

be provided for right turns from the minor road

potentially conflicting vehicle will be encoun-

onto the major road for all Stop-controlled

tered as the first vehicle departs the intersec-

approaches.

tion.

Field observations of the gaps accepted by the

Intersections with Stop control on the minor road

drivers of vehicles turning to the right onto the

(Case B)

major road have shown that the values in Table 6.7 provide sufficient time for the minor-road


Departure sight triangles for intersections with

vehicle to accelerate from a stop and merge

Stop control on the minor road should be con-

with the opposing stream without undue inter-

sidered for three situations:

•

ference. These observations also revealed that

Right turns from the minor road (Case

major-road drivers would reduce their speed to

B1);

• •

Left turns from the minor road (Case

some extent to accommodate vehicles entering

B2); and

from the minor road. Where the gap accept-

Crossing the major road from the minor-

ance values in Table 6.7 are used to determine

road approach (Case B3).

the length of the leg of the departure sight triangle along the major road, most major-road driv-

Approach sight triangles, as shown in Figure

ers need not reduce speed to less than 70 per-

6.4(A), need not be provided at Stop-controlled

cent of their initial speed. 6-18


Table 6.7 applies to passenger cars. However,

If the sight distances along the major road

for minor-road approaches from which substan-

based on Table 6.7 (including the appropriate

tial volumes of heavy vehicles enter the major

adjustments) cannot be provided, consideration

road, the values for single-unit trucks or semi-

should be given to the installation of advisory

trailers should be applied.

speed signs on the major-road approaches.

Table 6.7 includes adjustments to the accept-

Dimension "a" in Figure 6.4 (b) depends on the

able gaps for the number of lanes on the major

context within which the intersection is being

road and for the approach gradient of the minor

designed. In urban areas, drivers tend to stop

road. The adjustment for the gradient of the

their vehicles immediately behind the Stop line,

minor-road approach need be made only if the

which may be located virtually in line with the

rear wheels of the design vehicle would be on

edge of the major road. A passenger car driver

an upgrade steeper than 3 per cent when the

would, therefore, be located about 2,4 metres

vehicle is at the stop line of the minor-road

away from the Stop line. In rural areas, vehicles

approach.

usually stop at the edge of the shoulder of the major road. In the case of a three metre wide

The length of the sight triangle along the major

shoulder the driver would thus be approximate-

road (distance "b" in Figure 6.4) is the product of

ly 5,4 metres away from the edge of the trav-

the design speed of the major road in

elled way.

metres/second and the critical gap in seconds



as listed in Table 6.7.

Where the major road is a dual carriageway, two

be provided, it should be kept in mind that field

departure sight triangles have to be considered:

observations indicate that, in making left turns,

a sight triangle to the right, as for the crossing

drivers generally accept gaps that are slightly

movement (Case B3) and one using the accept-

shorter than those accepted in making right

able gap as listed in Table 6.7 for vehicles

turns.

approaching from the left.

This presupposes

decreased by 1,0 to 1,5 seconds for left turn

that the width of the median is sufficient to pro-

manoeuvres, where necessary, without undue

vide a refuge for the vehicle turning from the

interference with major-road traffic. When the

minor road. If the median width is inadequate,

recommended sight distance for a left-turn

the adjustment in Table 6.7 for multilane major

manoeuvre cannot be provided, even with a

roads should be applied with the median being

reduction of 1,0 to 1,5 seconds, consideration

counted as an additional lane.

should be given to the installation of advisory

The travel times in Table 6.7 can be

speed signs and warning devices on the majorThe departure sight triangle should be checked

road approaches.

for various possible design vehicles because the width of the median may be adequate for

Crossing manoeuvre from the minor road (Case

one vehicle type and not for another so that two

B3)

different situations have to be evaluated. In most cases it can be assumed that the deparLeft turn from the minor road (Case B2)

ture sight triangles for right and left turns onto


the major road, as described for Cases B1 and A departure sight triangle for traffic approaching

B2, will also provide more than adequate sight

from the right, as shown in Figure 6.4, should be

distance for minor-road vehicles crossing the

provided for left turns from the minor road. The

major road. However, it is advisable to check

lengths of the legs of the departure sight triangle

the availability of sight distance for crossing

for left turns should generally be the same as

manoeuvres:

those for the right turns used in Case B1.

•

Where right and/or left turns are not per

Specifically, the length of the leg of the depar-

mitted from a particular approach and

ture sight triangle (dimension "b") along the

crossing is the only legal manoeuvre;

•

major road should be based on the travel times

Where the crossing vehicle has to cross four or more lanes; or

in Table 6.7, including appropriate adjustment

•

factors.

Where substantial volumes of heavy vehicles cross the highway and where there are steep gradients on the depar-

Dimension "a" depends on the context of the

ture roadway on the far side of the inter

design and can vary from 2,4 metres to 5,4

section that might slow the vehicle while

metres.

its rear is still in the intersection.

Where sight distances along the major road

Table 6.8 presents travel times and appropriate

based on the travel times from Table 6.7 cannot

adjustment factors that can be used to deter6-20


mine the length of the leg of the sight triangle

or cross the major road without stopping. The

along the major road to accommodate crossing

sight distances needed by drivers on Yield-con-

manoeuvres.

trolled approaches exceed those for Stop-controlled approaches because of the longer travel time of the vehicle on the minor road.

At divided highway intersections, depending on the width of the median and the length of the design vehicle, sight distance may be needed

For four-legged intersections with Yield control

for crossing both roadways of the divided high-

on the minor road, two separate sets of

way or for crossing the near lanes only and

approach sight triangles as shown in Figure

stopping in the median before proceeding.

6.5(A) should be provided: one set of approach

a For minor-road approach gradients that exceed +3 per cent, multiply by the appropriate adjustment fac- t o r from Table 6.6.

Intersections with Yield control on the minor

sight triangles to accommodate right and left

road (Case C)

turns onto the major road and the other for crossing movements. Both sets of sight trian-

Vehicles entering a major road at a Yield-con-

gles should be checked for potential sight

trolled intersection may, because of the pres-

obstructions.

ence of opposing vehicles on the major road, be Crossing manoeuvres (Case C1)

required to stop. Departure sight triangles as described for Stop control must therefore be provided for the Yield condition. However, if no

The lengths of the leg of the approach sight tri-

conflicting

drivers

angle along the minor road to accommodate the

approaching Yield signs are permitted to enter

crossing manoeuvre from a Yield-controlled

vehicles

are

present,



b Travel time applies to a vehicle that slows before crossing the intersection but does not stop.

approach (distance "a" in Figure 6.5(A) are

These equations provide sufficient travel time

given in Table 6.9. The distances in Table 6.9

for the major road vehicle, during which the

are based on the same assumptions as those

minor-road vehicle can:

for Case A control except that, based on field

(1)

observations, minor-road vehicles that do not

intersection, while decelerating at the rate of

stop are assumed to decelerate to 60 per cent

1.5m/s² to 60 per cent of the minor-road design

of the minor-road design speed rather than to 50

speed; and then

per cent. The distances and times in Table 6.9

(2)

should be adjusted for the gradient of the minor

same speed.

Travel from the decision point to the

Cross and clear the intersection at the

road approach, using the factors in Table 6.6. Field observations did not provide a clear indi-

The length of the leg of the approach sight tri-

cation of the size of the gap acceptable to the

angle along the major road to accommodate the

driver of a vehicle located at the decision point

crossing manoeuvre (distance "b" in Figure

on the minor road. If the required gap is longer

6.5(A)) should be calculated using the following

than that indicated by the above equations, the

equations:

driver would, in all probability, bring the vehicle to a stop and then select a gap on the basis of

6.1

Case B. If the acceptable gap is shorter than that indicated by the above equations, the sight

6.2

distance provided would, at least, provide a margin of safety.

where: tc

b


ta

=

= =

travel time to reach and clear the

If the major road is a divided highway with a

major road in a crossing manoeu-

median wide enough to store the design vehicle

vre (sec)

for the crossing manoeuvre, then only crossing

length of leg of sight triangle along

of the near lanes need be considered and a

the major road (m)

departure sight triangle for accelerating from a

travel time to reach the major road

stopped position in the median should be pro-

from the decision point for a vehi-

vided, based on Case B1.

cle that does not stop (sec) (use Left and right-turn manoeuvres (Case C2)

appropriate value for the minorroad design speed from Table 6.9, adjusted for approach grade,

To accommodate left and right turns without

where appropriate)

stopping (distance "a" in Figure 6.5(A)), the

width of intersection to be crossed

length of the leg of the approach sight triangle

(m)

along the minor road should be 25 metres. This

length of design vehicle (m)

distance is based on the assumption that drivers

Vminor =

design speed of minor road (km/h)

making right or left turns without stopping will

Vmajor =

design speed of major road (km/h)

slow to a turning speed of 15 km/h. The length

w La

= =


of the leg of the approach sight triangle along

Since approach sight triangles for turning

the major road (distance "b" in Figure 6.5(B)) is

manoeuvres at Yield-controlled are larger than

similar to that of the major-road leg of the depar-

the departure sight triangles used at Stop-con-

ture sight triangle for Stop-controlled intersec-

trolled intersections, no specific check of depar-

tions in Cases B1 and B2.

ture sight triangles at Yield-controlled intersections should be necessary.

For a Yield-controlled intersection, the travel times in Table 6.7 should be increased by 0,5

Intersections with traffic signal control (Case D)

seconds. The minor-road vehicle requires 3,5 intersection. These 3,5 seconds represent addi-

In general, approach or departure sight triangles

tional travel time that is needed at a Yield-con-

are not needed for signalised intersections.

trolled intersection (Case C).

However, the

Indeed, signalisation may be an appropriate

acceleration time after entering the major road is

accident countermeasure for higher volume

3,0 seconds less for a Yield sign than for a Stop

intersections with restricted sight distance and a

sign because the turning vehicle accelerates

history of sight-distance related accidents.

from 15 km/h rather than from a stop. The net 0,5 seconds increase in travel time for a vehicle

However, traffic signals may fail from time to

turning from a Yield-controlled approach is the

time. Furthermore, traffic signals at an intersec-

difference between the 3,5 second increase in

tion are sometimes placed on two-way flashing

travel time on approach and the 3,0 second

operation under off-peak or night time condi-

reduction in travel time on departure explained

tions. To allow for either of these eventualities,

above.

the appropriate departure sight triangles for 6-23 Chapter 6: Intersection Design


seconds to travel from the decision point to the

Case B, both to the left and to the right, should

required by a stopped vehicle, the need for sight

be provided for the minor-road approaches.

distance design should be based on a right turn by a stopped vehicle.

Intersections with all-way Stop control (Case E) The sight distance along the major road to At intersections with all-way Stop control, the

accommodate right turns is the distance that

first stopped vehicle on each approach would be

would be traversed at the design speed of the

visible to the drivers of the first stopped vehicles

major road in the travel time for the appropriate

on each of the other approaches. It is thus not

design vehicle given in Table 6.10. This table

necessary to provide sight distance triangles at

also contains appropriate adjustment factors for

intersections with All-way Stop control. All-way

the number of major-road lanes to be crossed

Stop control may be an option to consider where

by the turning vehicle.

the sight distance for other types of control cannot be achieved. This is particularly the case if

If stopping sight distance has been provided

signals are not warranted.

continuously along the major road and, if sight distance for Case B (Stop control) or Case C (Yield control) has been provided for each

Right turns from a major road (Case F)

minor-road approach, sight distance should generally be adequate for right turns from the


Right-turning drivers need sufficient sight dis-

major road. However, at intersections or drive-

tance to enable them to decide when it is safe to

ways located on or near horizontal or vertical

turn right across the lane(s) used by opposing

curves on the major road, the availability of ade-

traffic. At all locations, where right turns across

quate sight distance for right turns from the

opposing traffic are possible, there should be

major road should be checked. In the case of

sufficient sight distance to accommodate these

dual carriageways, the presence of sight

manoeuvres. Since a vehicle that turns right

obstructions in the median should also be

without stopping needs a gap shorter than that

checked. 6-24


At four-legged intersections, opposing vehicles

area within each sight triangle should be clear of

turning right can block a driver's view of oncom-

sight obstructions, as described above.

ing traffic. If right-turn lanes are provided, offsetting them to the right, to be directly opposite

At skew intersections, the length of the travel

one other, will provide right-turning drivers with

paths

a better view of oncoming traffic.

increased. The actual path length for a crossing

for

crossing

manoeuvres

will

be

manoeuvre can be calculated by dividing the total width of the lanes (plus the median width,

Effect of skew on sight distance

where appropriate) to be crossed by the sine of the intersection angle and adding the length of

When two highways intersect at an angle out75O

120O

and where

the design vehicle. The actual path length divid-

realignment to increase the angle of intersection

ed by the lane width applied to the major road

side the range of

to

is not justified, some of the factors for determi-

cross-section gives the equivalent number of

nation of intersection sight distance will need

lanes to be crossed. This is an indication of the

adjustment.

number of additional lanes to be applied to the adjustment factor shown in Table 6.8 for Case B3.

Each of the clear sight triangles described above is applicable to oblique-angle intersections. As shown in Figure 6.6, the legs of the

The sight distances offered for Case B can,

sight triangle will lie along the intersection

regardless of the form of control, also accom-

approaches and each sight triangle will be larg-

modate turning movements from the minor road

er or smaller than the corresponding sight trian-

to the major road at skew intersections. In the

gle would be at a right-angle intersection. The

obtuse angle, drivers can easily see the full 6-25



Figure 6.6: Effect of skew on sight distance at intersections

sight triangle and, in addition, often accelerate

flicts that are inherent in any intersection. In that

from the minor road at a higher rate than when

sub-section, nine principles of channelisation

they have to negotiate a ninety-degree change

are listed.

of direction. In the acute-angle quadrant, driv-

process whereby a vehicle can be guided safe-

ers are often required to turn their heads con-

ly through the intersection area from an

siderably to see across the entire clear sight tri-

approach leg to the selected departure leg.

angle. For this reason, it is suggested that Case

Guidance is offered by lane markings that clear-

A should not be applied to oblique-angle inter-

ly define the required vehicle path and also indi-

sections.

Stop or Yield control should be

cate auxiliary lanes for turning movements. A

applied and the sight distances appropriate to

variety of symbols is also used as road mark-

either Case B or Case C provided. Even in a

ings to indicate inter alia that turns, either to the

skew intersection it is usually possible for driv-

left or to the right, from selected lanes are

ers to position their vehicles at approximately

mandatory. At intersections that are complex or

90O to the major road at the Stop line, offering

have high volumes of turning traffic, it is usually

added support for the application of Case B for

necessary to reinforce the guidance offered by

skew intersections.

road markings by the application of:

• • • • • •

When driving through a deflection angle greater than 120O, the right turn to the minor road may be undertaken at crawl speeds.

Allowance

could be made for this by adding the time,


equivalent to that required for crossing an addi-

In essence, channelisation is the

Channelising islands; Medians and median end treatments; Corner radii; Approach and departure geometry; Pavement tapers and transitions; Traffic control devices including signs and signals; and

tional lane, to the acceptable gap.

•

Arrangement and position of lanes

6.5

6.5.1

Channelising islands

CHANNELISATION ELEMENTS

At-grade intersections with large paved areas,

Islands are included in intersections for one or

such as those with large corner radii or with

more of the following purposes:

angles of skew differing greatly from 90O, permit

• • • •

unpredictable vehicle movements, require long pedestrian crossings and have unused pavement areas. Even at a simple intersection there

Control of angle of conflict; Reduction of excessive pavement areas; Regulation of traffic and indication of the proper use of the intersection;

may be large areas on which vehicles can wander from natural or expected paths.

Separation of conflicts;

•

Under

Arrangement to favour a predominant turning movement;

these circumstances it is usual to resort to chan-

• •

nelisation.

Protection of pedestrians; Protection and storage of turning vehicles; and

•

As stated in sub-section 6.2.8, the fundamental

Location of traffic control devices.

function of channelisation is to manage the con6-26 Chapter 6: Intersection Design

The three main functions of channelising islands

Islands may be kerbed, painted or simply non-

are thus:

paved. Kerbed islands provided the most posi-

•

Directional - to control and direct move-

tive traffic delineation and are normally used in

ments, usually turning;

urban areas to provide some degree of protec-

•

Division - which can be of opposing or

tion to pedestrians and traffic control devices.

same direction, usually through, move-

Painted islands are usually used in suburban

ments; and

•

areas where speeds are low, e.g. in the range of

Refuge - either of turning vehicles or of

50 km/h to 70 km/h and space limited. In rural

pedestrians.

areas, kerbs are not common and, at the Typical island shapes are illustrated in Figure

speeds prevailing in these areas, typically 120

6.7.

km/h or more, they are a potential hazard. If it

The designer should bear in mind that islands

section, the use of mountable kerbing should be

are hazards and should be less hazardous than

considered. As an additional safety measure, a

whatever they are replacing.

kerbed island could be preceded by a painted

Figure 6.7 General types and shapes of islands 6-27 Chapter 6: Intersection Design


is necessary to employ kerbing at a rural inter-

island. Non-paved islands are defined by the

metres in area to ensure that they are easily vis-

pavement edges and are usually used for large

ible to approaching drivers.

islands at rural intersections.

These islands

Directional islands are typically triangular with

may have delineators on posts and may be

their dimensions and exact shape being dictat-

landscaped.

ed by:

Islands are generally either long or triangular in

• •

shape, with the circular shape being limited to

The corner radii and associated tapers; The angle of skew of the intersection; and

•

application in roundabouts. They are situated in

The turning path of the design vehicle.

areas not intended for use in vehicle paths.


A typical triangular island is illustrated in Figure Drivers tend to find an archipelago of small

6.8. The approach ends of the island usually

islands confusing and are liable to select an

have a radius of about 0,6 metres and the offset

incorrect path through the intersection area. As

between the island and the edge of the travelled

a general design principle, a few large islands

way is typically 0,6 to 1,0 metres to allow for the

are thus to be preferred to several small islands.

effect of kerbing on the lateral placement of

Islands should not be less than about 5 square

moving vehicles.

Figure 6.8: Typical directional island 6-28 Chapter 6: Intersection Design

Where the major road has

shoulders, the nose of the island is offset about

teardrop shape. They are often employed on

one metre from the edge of the usable shoulder,

the minor legs of an intersection where these

the side adjacent to the through lane being

legs have a two-lane, two-way or four-lane undi-

tapered back to terminate at the edge of the

vided cross-section. The principle function of a

usable shoulder, thus offering some guidance

dividing island is to warn the driver of the pres-

and redirection. A kerbed cross-section on the

ence of the intersection. This can be achieved

major road suggests that the nose of the island

by the left edge of the island being, at the widest

should be offset by about 1,6 metres from the

point of the island, in line with the left edge of the

edge of the travelled way, with the side adjacent

approach leg.

to the through lane being tapered back to termi-

would thus appear as though the entire lane had

Figure 6.9: Typical divisional (splitter) island

nate 0,6 metres from the edge of the through

been blocked off by the island. If space does

lane.

not permit this width of island, a lesser blocking

Dividing, or splitter, islands usually have a

width would have to be applied but it is doubted 6-29



To the approaching driver, it

whether anything less than half of the approach

right, both from the minor road to the major road

lane width would be effective. The taper that

and from the major road to the minor.

can be employed to achieve this effect safely is discussed in Section 6.5.3. A typical dividing

Median islands are discussed in Section 4.4.6

island is illustrated in Figure 6.9.

and outer separator islands in Section 4.4.7. At intersections, the end treatment of median

The shape of the splitter island discussed in

islands is important. The width of the opening

Section 6.6.5 is derived from the need to redi-

between two median ends should match the

rect vehicles entering a roundabout through an

width of the minor road, including its shoulders,

angle of not more than 30 . Although it serves

or, where the minor road is kerbed, the opening

to create the illusion of about a half of the

should not be narrower than the surfaced width

approach lane being blocked off, its true func-

of the minor road plus an offset of 0,6 to 1,0

tion is to achieve the desired extent of deflec-

metres.

O

tion.

Furthermore, this form of splitter island

does not accommodate vehicles turning from

The median end treatment is determined by the

the left. In effect, it has a teardrop shape, albeit

width of the median. Where the median is three

distorted by its abutting a curving roadway with

metres wide or less, a simple semicircle is ade-

a relatively short radius rather than a straight

quate. For wider medians, a bullet nose end

road.

treatment is recommended. The bullet nose is formed by arcs dictated by the wheel paths of

The kerb height should ensure that the island

turning vehicles and an assumed nose radius of

would be visible within normal stopping sight

0,6 to 1,0 metres. It results in less intersection

distance. However, it may be advisable to draw

pavement area and a shorter length of opening

the driver's attention to the island by highlighting

than the semicircular end.

the kerbs with paint or reflective markings.

width of five metres, the width of the minor road

Above a median


controls the length of the opening. A flattened As in the case of the triangular island, the nose

bullet nose, using the arcs as for the conven-

of the dividing island should be offset by about

tional bullet nose but with a flat end as dictated

one metre but, in this case, to the right of the

by the width of the crossing road and parallel to

centreline of the minor road. Dividing islands

the centreline of the minor road, is recommend-

are usually kerbed to enhance their visibility and

ed.

the offset between the kerbing and the edge of

Figure 6.10.

These end treatments are illustrated in

the travelled way should thus be 0,6 metres as discussed above. For the sake of consistency,

The bullet nose and the flattened bullet nose

the radius of the nose should be of the order of

have the advantage over the semicircular end

0,6 metres.

treatment that the driver of a right turning vehicle has a better guide for the manoeuvre for

The balance of the shape of the island is defined

most of the turning path. Furthermore, these

by the turning paths of vehicles turning to the

end treatments result in an elongated median, 6-30


which is better placed to serve as a refuge for

envisaged. Reference is typically to three types

pedestrians crossing the dual carriageway road.

of operation, being:

An additional disadvantage of the use of the

Case 1

One-lane one-way travel with

semicircular end treatment for wide medians is

no provision for passing

that, whereas the bullet nose and the flattened

stopped vehicles; One-lane one-way travel with

left of the centreline of the minor road, the semi-

provision for passing a stopped

circular end treatment tends to direct the vehicle

vehicles; and Case 3

into the opposing traffic lane of the minor road.

Two-lane one-way operation

Figure 6.10: Typical median end treatments 6.5.2

Turning roadway widths

Three traffic conditions should also be considered, being: Condition A

Directional islands are bounded by the major

Insufficient SU vehicles in the

and minor roads and by a short length of one-

turning traffic stream to influ-

way, typically one-lane, turning roadway. The

ence design; Condition B

width of the turning roadway is defined by the

Sufficient SU vehicles to influence design; and

swept area of the design vehicle for the selectCondition C

ed radius of curvature and the type of operation 6-31


Sufficient

semi-trailers

to


Case 2

bullet nose both guide the vehicle towards the

way compared to the 6,4 metre lane width of the

influence design

single curve. The three-centred curve is particTurning roadways are short so that design for

ularly useful for Case C conditions because

Case 1 is usually adequate. It is reasonable to

semi-trailers require an inordinate width of turn-

assume, even in the absence of traffic data, that

ing roadway. For example, the required width of

there will be enough trucks in the traffic stream

turning roadway for Case 1, Condition C and a

to warrant the application of Condition B to the

design speed of 20 km/h is 7,9 metres whereas,

design. Turning roadway widths are listed in

under the same circumstances, passenger cars

Table 6.11

require only 4,0 metres. Drivers of passenger cars could thus quite easily perceive the turning

Three-centred curves are an effective alterna-

roadway as being intended for two-lane opera-

tive to the single radius curves listed in Table

tion.


6.11.

These curves typically have a ratio of

3:1:3 between the successive radii. However,

It is not possible to list all the possible alterna-

asymmetric combinations, e.g. 2:1:4, have also

tive three-centred curve combinations. When

proved very useful in the past. These curves

three-centred curves are considered, the

closely follow the wheel path of a vehicle nego-

designer should determine the required road-

tiating the turn thus enabling the use of narrow-

way width by the use of templates.

er lanes than with a single radius curve. In addition, three-centre curves allow the use of small-

6.5.3

Tapers

er central radii than do the equivalent single curves. Under Case C conditions, a 55:20:55

There are two types of taper, each with different

metre radius three-centred curve is the equiva-

geometric requirements. These are:

lent of a thirty metre single radius curve and per-

•

mits the use of a 6,0 metre wide turning road-

The active taper, which forces a lateral transition of traffic; and


•

The passive taper, which allows a later-

should not fall outside the range of 75O to 120O,

al transition of traffic.

it is important to note that a very short active taper may result in the creation of a local angle

The active taper is used to narrow a roadway or

of skew of less than 75O. This would make it

lane or as a lane drop, i.e. when two lanes

very difficult for the driver on the turning road-

merge into one.

The passive taper either

way to observe opposing traffic on the through

widens or adds a lane. Active tapers constitute

lane. At an angle of skew of less than 5O the

a hazard insofar as that a driver that fails to per-

driver should, using the rear view mirror, be able

ceive the change in circumstances, may either

to observe opposing traffic comfortably.

drive off the travelled way or hit the adjacent

short, tapers in the range between 1:10 and 1:

kerbing. Passive tapers, on the other hand, cre-

0,3 should be avoided (the latter corresponding

ate space on the travelled way and, thus, are

to an angle of skew of 75O).

not hazards.

In

Consequently, active tapers

should be long and passive tapers may be short.

Acceptable tapers rates are suggested in Table 6.12.

lane to allow for deceleration or followed by an

In entering a deceleration lane, a vehicle follows

auxiliary lane allowing for acceleration, these

a reverse or S-curve alignment, which is effec-

lanes will be added or dropped by means of

tively a passive taper immediately followed by

passive and active tapers respectively. In the

an active taper. Four different combinations of

absence of the auxiliary lanes, the turning road-

taper can be employed. These are:

• • • •

way can, in the case of a left turn, be created by a passive taper from the left edge of the through lane to the left edge of the turning roadway.

A straight-line taper; A partial tangent taper; A symmetrical reverse curve; or An asymmetrical reverse curve.

The turning roadway may be terminated by an

In urban areas, short straight-line tapers appear

active taper connecting its left edge to the left

to offer better targets for the approaching driver.

edge of the through lane. Apropos the sugges-

Urban intersections operate at slow speeds dur-

tion that the angle of skew of the intersection

ing peak periods and, particularly for right-turn6-33



If a turning roadway is preceded by an auxiliary

ing traffic, the need for storage may be more

vehicles already in the circle had to give way to

important than the ability to enter the decelera-

those wishing to enter it. Not surprisingly, grid-

tion lane at relatively high speeds. Tapers could

lock resulted at heavy flow rates. Ultimately,

therefore be as sharp as 1:2, which is about the

traffic circles fell into disfavour and were

limit of manoeuvrability of a passenger car at

replaced by conventional three- and four-legged

crawl speeds.

intersections.

As speeds are higher in rural than in urban

Modern roundabouts differ from traffic circles in

areas, the other forms of taper listed above may

their uniform characteristics and operation.

warrant consideration. The partial tangent taper

Internationally, roundabouts operate on the

is a straight line taper preceded by a short

"Yield on entry" rule so that, where vehicles

curved section with a radius such that the

drive on the left, vehicles yield to the right and

desired taper rate is achieved at a point about

vice versa. South Africa applies the same rule

one third of the way across the width of the aux-

except that, in the case of the mini-roundabout,

iliary lane. The symmetrical reverse curve taper

the rule is slightly modified by the use of the

has curved sections of equal radius at either

R2.2 sign which "....indicates to the driver of a

end. The guideline suggested for the partial tan-

vehicle approaching a traffic circle that he shall

gent, i.e. the curve traversing one third of the

yield right of way to any vehicle which will cross

lane width, applies to the entry and the exit

any yield line at such junction before him and

curves of the taper. The asymmetrical reverse

which, in the normal course of events, will cross

curve usually has an entry curve radius about

the path of such driver's vehicle."

twice that of the exit curve. Drivers are thus inclined to adopt the approach

6.6

that the rule of first-come-first-served applies at

ROUNDABOUTS

mini-roundabouts except that, in the case of

6.6.1

simultaneous arrivals at the mini-roundabout,

Introduction

drivers will yield to the vehicle on the right.


Traffic circles constructed in the 1930s and

A number of geometric elements are incorporat-

1940s were intended to operate in a weaving

ed in the design of roundabouts and all ele-

mode. As such, the diameters of the circles were large.

ments appear in all roundabouts. These ele-

No clear guidance was offered

ments are illustrated in Figure 6.11.

regarding priority of one vehicle over another and, in consequence, accident rates at traffic

6.6.2

circles tended to be higher than those at con-

Operation of roundabouts

ventional intersections. In an effort to rectify this

Roundabouts operate by deflecting the vehicle

situation, it was decided (in the situation of driv-

path so as to slow traffic and promote yielding.

ing on the left) that vehicles should yield to

The roadway entry is usually flared to increase

those on their left.

capacity.

In effect, this meant that


Delays at roundabouts are usually less than at

In spite of their undoubted advantages, round-

conventional intersections and, in consequence,

abouts are not appropriate to every situation.

capacity is higher. The proviso is that the com-

They may be inappropriate:

bined intersection flow should be less than 3

•

500 veh/h.

Where spatial restraints (including cost of land), unfavourable topography or

Reduced delays improve vehicle

high construction costs make it impossi-

operating costs.

ble to provide an acceptable geometric design;

Roundabouts have less potential conflict points

•

than conventional intersections. In the case of

Where traffic flows are unbalanced, with

Figure 6.11: Elements of roundabouts are replaced by 8, as illustrated in Figure 6.12.

•

In both roundabouts and conventional intersec-

At intersections of major roads with minor roads, where roundabouts would

tions, the diverge is also counted as being a

cause serious delays to traffic on the

conflict point. The safety performance of round-

major roads;

abouts is often superior to that of most conven-

•

tional intersections and the reduced number of

Where there are substantial pedestrian flows;

conflict points at roundabouts result in an

•

observable reduction in accident rates.

As an isolated intersection in a network of linked signalised intersections;



high flows on one or more approaches;

the four-legged intersection, 32 conflict points

• •

In the presence of reversible lanes;

the intersection flow is greater

Where semi-trailers and/or abnormal

than 1 500 veh/h or; o

vehicles are a significant proportion of

greater than 2 000 veh/h;

the total traffic passing through the inter-

•

section and where there is insufficient

•

on four-legged intersections, is

One major flow has a predominant

space to provide the required layout; and

through movement that is:

Where signalised traffic control down

o

Between 50 and 80 per cent of the approach volume; or

stream could cause a queue to back-up o

through the roundabout.

Between 25 and 40 per cent of the intersection volume; and

o

Roundabouts can be considered when:

•

High volumes of right-turning movements, i.e. more than 25

Intersection volumes do not exceed 3 000

Figure 6.12: Intersection conflict points veh/h at three-legged or 4 000 veh/h at

per cent of the approach vol-


four-legged intersections;

•

ume, occur and which experi-

The proportional split between the vol-

ence long delays (e.g. 15 sec-

umes on the major and minor road does

onds per vehicle) and a high

not exceed 70/30; o

incidence of right-angled acci-

where, on three-legged inter-

dents.

sections, the intersection flow is less than 1 500 veh/h or, o

6.6.3

on four-legged intersections, is less than 2 000 veh/h;

•

Design speed

The proportional split between the vol

The design speed within the roundabout should

umes on the major and minor road does

ideally

not exceed 60/40 where;

Unfortunately, this suggests a radius of between

o

60 and 80 metres hence requiring an overall

on three-legged intersections,

range


between

40

to

50

km/h.

diameter of the roundabout of the order of 150

intersection at all times. This requirement sug-

metres. Very often, the space for this size of

gests that the elaborate landscaping schemes

intersection will simply not be available and

sometimes placed on the central islands of

some lesser design speed will have to be

roundabouts are totally inappropriate to the

accepted.

intended function of the layout.

Where the design speeds on the approaches

Decision sight distance for intersections as

are high, e.g. more than 15 km/h faster than the

described in Table 3.5 should be provided on

design speed within the facility, it may be nec-

each approach to the roundabout to ensure that

essary to consider forcing a reduction in vehicle

drivers can see the nose of the splitter island. It

speed. This could be by means of horizontal

follows that roundabouts should not be located

reverse curvature. The ratio between the radii

on crest curves.

of successive curves should be of the order of 1,5:1.

6.6.5

Components

Speed humps should not be employed as

The various components of a roundabout are

speed-reducing devices on major roads or on


bus routes. Where design speeds are of the Deflection

order of 100 km/h or more, the speed hump would have to be long and the height low to ensure that the vertical acceleration caused by

A very important component is the deflection

the speed hump does not cause the driver to

forced on vehicles on the approach to the

lose control. It is suggested that, in practice, a

roundabout.

suitable profile would be difficult to construct.

speed of vehicles so that, within limits, the

Bus passengers, particularly those sitting in the

greater the deflection the better. The limit is that

rear overhang of the vehicle, would find travers-

the minimum acceptable angle of skew at an

ing a speed hump distinctly uncomfortable.

intersection is 60O, as discussed previously in

Speed humps should thus only be used at

this chapter. This corresponds to a deflection

roundabouts in residential areas or where the

on entry of 30O. The approach radius should not

intention is to apply traffic calming.

exceed 100 metres, which corresponds to the recommended design speed of 40 to 50 km/h.

6.6.4

Sight distance Entries and exits

Site visibility is important in the design of roundabouts. Specifically, approaching drivers should

The widths of single lane approaches to round-

have a clear view of the nose of the splitter or

abouts are typically of the order of 3,4 to 3,7

separator island. At the yield line and while tra-

metres.

versing the roundabout, they should have an

important factors in increasing the capacity of

uninterrupted view of the opposing legs of the

the roundabout and can be increased above the

The entry width is one of the most



The intention is to reduce the

width of the approach by flaring, i.e. by provid-

side and inside kerbs. For a vehicle with an

ing a passive taper with a taper rate of 1:12 to

overall width of 2,6 metres, the offset should

1:15. The recommended minimum width for a

thus be not less than 1,6 metres with 2,0 metres

single-lane entry is 5 metres.

being preferred. To ensure that vehicles do not travel faster than the design speed, the maxi-

If demanded by high approach volumes, the

mum radius on the vehicle path should be kept

flaring could add a full lane to the entry to

to 100 metres or less.

increase capacity. The width of a two-lane entry should be of the order of 8 metres. A variation

As a general guideline, the circulatory roadway

on the two-lane entry is to have, in effect, a sin-

should be sufficiently wide to allow a stalled

gle-lane circulatory roadway with an auxiliary

vehicle to be passed but without sufficient trucks

lane provided for the benefit of vehicles turning left at the roundabout.

in the traffic stream to influence design (normal-

The auxiliary lane is

ly described as Case 2, Condition A in the case

shielded from traffic approaching from the right

of turning roadways). The minimum roadway

by moving the end of the splitter island forward

width for single-lane operation under these cir-

to provide a circulatory road width adequate

cumstances would be of the order of 6,5 metres

only for single-lane travel. This approach could

between kerbs.

be adopted with advantage when left-turning

Two-lane operation would

require a roadway width of about 8 metres. If

traffic represents 50 per cent or more of the

trucks are present in the traffic stream in suffi-

entry flow or more than 300 veh/h during peak

cient numbers to influence design, the circulato-

hours.

ry road width should be increased by 3 metres both in the single-lane and in the two-lane situ-

Circulatory roadway

ation. A significant proportion of semi-trailers would require the width of the circulatory road

The circulatory roadway width is a function of

width to be increased to 13 metres and 16

the swept path of the design vehicle and of the

metres in the single-lane and the two-lane situ-

layout of the exits and entries and generally

ation respectively.

should be either equal to or 1,2 times the width


of the entries. The width should be constant A circulatory road width of 13 metres would

throughout the circle.

make it possible for passenger cars to traverse In the construction of the swept path of the

the roundabout on relatively large radius curves

design vehicle, it should be noted that drivers

and at correspondingly high speeds. To avoid

tend to position their vehicles close to the out-

this possibility, the central island should be mod-

side kerbs on entry to and exit from the round-

ified as discussed below.

about and close to the central island between these two points. The vehicle path, being the

The cross-slope on the roadway should be

path of a point at the centre of the vehicle,

away from the central island and equal to the

should thus have an adequate offset to the out-

camber on the approaches to the intersection.


cyclists and a place to mount traffic

Central island

signs. The central island consists of a raised non-traversable area, except in the case of mini-round-

The sizes of splitter islands are dictated by the

abouts where the central island may simply be a

dimensions of the central island and inscribed

painted dot. The island is often landscaped but

circle. As a general guideline, they should have

it should be ensured that the landscaping does

an area of at least 10 square metres so as to

not obscure the sight lines across the island.

ensure their visibility to the oncoming driver.

Historically, central islands were often square or,

The length of splitter islands should be equal to

if they had more than four entries, polygonal.

the comfortable deceleration distance from the

Negotiating the right angle bends was only pos-

design speed of the approach to that of the

sible at crawl speeds and this led to substantial

roundabout.

delays and congestion. It is now customary to Ideally, the nose of the splitter island should be

provide circular islands.

offset to the right of the approach road centreWhile, for semi-trailers, the width of the circula-

line by about 0,6 to 1 metre.

tory road between kerbs would have to be 13 metres in a single-lane configuration, all other vehicles could be served by a road width of 9,5 metres. A mountable area or apron could thus be added to the central island to accommodate this difference. The apron should have crossfall steeper than that of the circulatory road, principally to discourage passenger vehicles from driving on it and a crossfall of 4 to 5 per cent is recommended.

Splitter islands should be provided on the approaches to roundabouts to:

•

Allow drivers to perceive the upcoming roundabout and to reduce entry speed;

•

Provide space for a comfortable deceleration distance;

•

Physically separate entering and exiting traffic;

•

Prevent deliberate and highly dangerous wrong-way driving;

• •

Control entry and exit deflections; and Provide a refuge for pedestrians and 6-39 Chapter 6: Intersection Design


Splitter islands


INTERCHANGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 7.1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 7.1.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 7.1.2 Design principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

7.2

INTERCHANGE WARRANTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3 7.2.1 Traffic volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3 7.2.2 Freeways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4 7.2.3 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4 7.2.4 Topography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4

7.3

WEAVING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4

7.4

LOCATION AND SPACING OF INTERCHANGES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6

7.5

BASIC LANES AND LANE BALANCE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-9

7.6

AUXILIARY LANES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10 7.6.1 The need for an auxiliary lane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10 7.6.2 Auxiliary lane terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13 7.6.3 Driver information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13

7.7

INTERCHANGE TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13 7.7.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13 7.7.2 Systems interchanges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-15 7.7.3 Access and service interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-17 7.7.4 Interchanges on non-freeway roads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-23

7.8

RAMP DESIGN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-23 7.8.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-23 7.8.2 Design speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-24 7.8.3 Sight distance on ramps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-25 7.8.4 Horizontal alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-26 7.8.5 Vertical alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-27 7.8.6 Cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-28 7.8.7 Terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-29

7.9

COLLECTOR - DISTRIBUTOR ROADS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-35

7.10

OTHER INTERCHANGE DESIGN FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-35 7.10.1Ramp metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-35 7.10.2Express-collector systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-36

LIST OF TABLES Table 7.1: Interchange spacing in terms of signage requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-7 Table 7.2: Ramp design speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-24 Table 7.3: Maximum resultant gradients. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-27 Table 7.4: K-Values of crest curvature for decision sight distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-28 Table 7.5: Length of deceleration lanes (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-31 Table 7.6: Length of acceleration lanes (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-31 Table 7.7: Taper rates for exit ramps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-32

LIST OF FIGURES Figure 7.1: Type A weaves: (a) ramp-weave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5 . . (b) major weave with crown line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5 Figure 7.2: Type B weaves: (a) major weave with lane balance at exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5 . . (b) major weave with merging at entrance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5 . . (c) major weave with merging at entrance and lane balance at exit . . . . . . . . . . . . . . . . . . . . . . 7-5 Figure 7.3: Type C weaves: (a) major weave without lane balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6 . . (b) two-sided weave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6 Figure 7.4: Weaving distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-7 Figure 7.5: Relationship between interchange spacing and accident rate. . . . . . . . . . . . . . . . . . . . . . . . . . . 7-9 Figure 7.6: Coordination of lane balance with basic number of lanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-11 Figure 7.7: Four-legged systems interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-16 Figure 7.8: Three-legged systems interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-17 Figure 7.9: Diamond interchanges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-20 Figure 7.10: Par-Clo A interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-21 Figure 7.11: Par-Clo B interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-22 Figure 7.12: Par-Clo AB interchanges and rotaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-22 Figure 7.13: Jug Handle interchange. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-23 Figure 7.14: Single lane exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-32 Figure 7.15: Two-lane exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-32 Figure 7.16: One lane entrance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-33 Figure 7:17: Two-lane entrance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-33 Figure 7.18: Major fork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-34

Chapter 7 INTERCHANGES 7.1

INTRODUCTION

7.1.1

General

The various types of interchange configuration are illustrated in Section 7.6. Each basic form can be divided into sub-types. For example, the Diamond Interchange is represented by the nar-

The principal difference between interchanges

row diamond, the wide diamond and the split

and other forms of intersection is that, in inter-

diamond. The most recent development in the

changes, crossing movements are separated in

Diamond interchange form is the Single Point

space whereas, in the latter case,they are sep-

Diamond Interchange. This form is also referred

arated in time. At-grade intersections accom-

to as the Urban Interchange.

modate turning movements either within the limitations of the crossing roadway widths or through the application of turning roadways

Historically, the type of interchange to be

whereas the turning movements at interchanges

applied at a particular site would be selected as

are accommodated on ramps.

The ramps

an input into the design process. In fact, like the

replace the slow turn through an angle of skew

cross-section, the interchange is the aggrega-

that is approximately equal to 90 by high-speed

tion of various elements.

merging and diverging manoeuvres at relatively

approach is thus to select the elements appro-

flat angles.

priate to a particular site in terms of the topog-

O

A more sensible

raphy, local land usage and traffic moveGrade separations, discussed further in Chapter

ments and then to aggregate them into some or

10, provide spatial separation between the

other type of interchange.

crossing movements but do not make provision

7.1.2

for turning movements. They, therefore, do not

Design principles

qualify for consideration as a form of intersection.

high speeds close to the freeway and over relaThe first interchange ever built was at

tively short distances. It is therefore important

Woodbridge, New Jersey, and provided (in the

that drivers should experience no difficulty in

context of driving on the right) loops for all left

recognising their route through the interchange

turns and outer connectors for all right turns

irrespective of whether that route traverses the

thus creating the Cloverleaf Interchange. Since

interchange on the freeway or diverts to depart

then, a variety of interchange forms has been

from the freeway to a destination that may be to

developed. These include the:

the left or the right of the freeway. In following

• • •

Diamond;

their selected route, drivers should be disturbed

Par-Clo (from Partial Cloverleaf); and the

as little as possible by other traffic.

Directional

requirements can be met through the applica7-1 Chapter 7: Interchanges

These


Manoeuvres in an interchange area occur at

tion of the basic principles of interchange

Route continuity substantially simplifies the nav-

design.

igational aspects of the driving task. For example, if a driver simply wishes to travel on a free-

The driver has a number of tasks to execute

way network through a city from one end to the

successfully to avoid being a hazard to other

other it should not be necessary to deviate from

traffic. It is necessary to:

one route to another.

• •

select a suitable speed and accelerate or decelerate to the selected speed

Uniformity of signing practice is an important

within the available distance;

aspect of consistent design and reference

select the appropriate lane and carry out

should be made to the SADC Road Traffic Signs

the necessary weaving manoeuvres to

Manual.

effect lane changes if necessary; and

•

diverge towards an off-ramp or merge

Ideally, an interchange should have only a sin-

from an on-ramp with the through traffic.

gle exit for each direction of flow with this being located in advance of the interchange structure.

To maintain safety in carrying out these tasks,

The directing of traffic to alternative destinations

the driver must be able to understand the oper-

on either side of the freeway should take place

ation of the interchange and should not be sur-

clear of the freeway itself. In this manner, driv-

prised or misled by an unusual design charac-

ers will be required to take two binary decisions,

teristic. Understanding is best promoted by con-

(Yes/No) followed by (Left/Right), as opposed to

sistency and uniformity in the selection of types

a single compound decision. This spreads the

and in the design of particular features of the

workload and simplifies the decision process,

interchange.

hence improving the operational efficiency of the entire facility. Closely spaced successive

Interchange exits and entrances should always

off-ramps could be a source of confusion to the

be located on the left.

driver leading to erratic responses and manoeu-

Right-hand side

entrances and exits are counter to driver

vres.


expectancy and also have the effect of mixing high-speed through traffic with lower-speed

Single entrances are to be preferred, also in

turning vehicles. The problem of extracting turn-

support of operational efficiency of the inter-

ing vehicles from the median island and provid-

change. Merging manoeuvres by entering vehi-

ing sufficient vertical clearance either over or

cles are an interruption of the free flow of traffic

under the opposing freeway through lanes is not

in the left lane of the freeway. Closely spaced

trivial. The application of right-hand entrances

entrances exacerbate the problem and the

and exits should only be considered under

resulting turbulence could influence the adja-

extremely limiting circumstances. Even in the

cent lanes as well.

case of a major fork where two freeways are diverging, the lesser movement should, for pref-

From the standpoint of convenience and safety,

erence, be on the left.

in particular prevention of wrong-way move-

7-2 Chapter 7: Interchanges

•

ments, interchanges should provide ramps to

The crossing road ramp terminals may

serve all turning movements. If, for any reason,

include right and left turn lanes, traffic

this is not possible or desirable, it is neverthe-

signals and other traffic control devices.

less to be preferred that, for any travel move-

Not being obstructed by bridge piers

ment from one road to another within an inter-

and the like, these would be rendered

change, the return movement should also be

more visible by taking the crossing road

provided.

over the freeway. The other design principles, being continuity of

Provision of a spatial separation between two

basic lanes, lane balance and lane drops are

crossing streams of traffic raises the problem of

discussed in Section 7.5 as matters of detailed

which to take over the top - the perennial Over

design.

versus Under debate. The choice of whether the crossing road should be taken over or under the freeway depends on a number of factors, not the least of which is the matter of terrain and

7.2

INTERCHANGE WARRANTS

7.2.1

Traffic volumes

construction costs. There are, however, a num-

With increasing traffic volumes, a point will be

ber of advantages in carrying the crossing road

reached where all the options of temporal sepa-

over the freeway. These are:

ration of conflicting movements at an at-grade

•

Exit ramps on up-grades assist deceler-

intersection have been exhausted. One of the

ation and entrance ramps on down-

possible solutions to the problem is to provide

grades assist acceleration and have a

an interchange.

•

•

•

•

Rising exit ramps are highly visible to

The elimination of bottlenecks by means of

drivers who may wish to exit from the

interchanges can be applied to any intersection

freeway.

at which demand exceeds capacity and is not

The structure has target value, i.e. it

necessarily limited to arterials. Under these cir-

provides advance warning of the possi-

cumstances, it is necessary to weigh up the

bility of an interchange ahead necessi-

economic benefits of increased safety, reduced

tating a decision from the driver whether

delay and reduced operating and maintenance

to stay on the freeway or perhaps to

cost of vehicles against the cost of provision of

change lanes with a view to the impend-

the interchange. The latter includes the cost of

ing departure from the freeway.

land acquisition, which could be high, and the

Dropping the freeway into cut reduces

cost of construction. As the construction site

noise levels to surrounding communi-

would be heavily constricted by the need to

ties and also reduces visual intrusion.

accommodate traffic flows that were sufficiently

For the long-distance driver on a rural

heavy to justify the interchange in the first

freeway, a crossing road on a structure

instance, the cost of construction could be sig-

may represent an interesting change of

nificantly higher than on the equivalent green

view.

field site. 7-3 Chapter 7: Interchanges


beneficial effect on truck noise.

7.2.2

Freeways

7.2.4

The topography may force a vertical separation

The outstanding feature of freeways is the limi-

between crossing roads at the logical intersec-

tation of access that is brought to bear on their

tion location. As an illustration, the through road

operation. Access is permitted only at designat-

may be on a crest curve in cut with the crossing

ed points and only to vehicles travelling at or near freeway speeds.

Topography

road at or above ground level. If it is not possi-

As such, access by

ble to relocate the intersection, a simple Jug-

means of intersections is precluded and the only

handle type of interchange as illustrated in

permitted access is by way of interchanges.

Figure 7.13 may be an adequate solution to the

Crossing roads are normally those that are high

problem.

in the functional road hierarchy, e.g. arterials, although, if these are very widely spaced, it may

7.3

be necessary to provide an interchange serving

WEAVING

a lower order road, for example a collector. The Highway Capacity Manual (2000) defines It follows that the connection between two free-

weaving as the crossing of two or more traffic

ways would also be by means of an inter-

streams travelling in the same general direction

change, in which case reference is to a systems

without the aid of traffic control devices but then

interchange as opposed to an access inter-

goes to address the merge-diverge as a sepa-

change.

rate issue. However, the merge-diverge operation, associated with successive single-lane on-

7.2.3

Safety

and off-ramps where there is no auxiliary lane, does have two streams that, in fact, are cross-

Some at-grade intersections exhibit high crash

ing. Reference to weaving should thus include

rates that cannot be lowered by improvements

the merge-diverge.

to the geometry of the intersections or through the application of control devices. Such situa-

Three types of weave are illustrated in Figures

tions are often found at heavily travelled urban

7.1, 7.2 and 7.3. A Type A weave requires all


intersections. Crash rates also tend to be high

weaving vehicles to execute one lane change.

at the intersections on heavily travelled rural

Type B weaving occurs when one of the weav-

arterials where there is a proliferation of ribbon

ing streams does not have to change lanes but

development.

the other has to undertake at most one lane change. Type C weaving allows one stream to

A third area of high crash rates is at intersec-

weave without making a lane change, whereas

tions on lightly travelled low volume rural loca-

the other stream has to undertake two or more

tions where speeds tend to be high. In these

lane changes.

cases, low-cost interchanges such as the Jughandle layout may be an adequate solution to

The Type B weave is, in essence, a Type A

the problem.

weave but with the auxiliary lane extending 7-4 Chapter 7: Interchanges

Figure 7.1: Type A weaves: (a) ramp-weave

Figure 7.2: Type B weaves:

(a) major weave with lane balance at exit (b) major weave with merging at entrance (c) major weave with merging at entrance and lane balance at exit 7-5 Chapter 7: Interchanges


(b) major weave with crown line

Figure 7.3: Type C weaves: (a) major weave without lane balance (b) two-sided weave either up- or downstream of the weaving area

Rural interchanges are typically spaced at dis-

and with an additional lane being provided

tances of eight kilometres apart or more. This

either to the on- or to the off-ramp. It follows that

distance is measured from centreline to centre-

a Type A weaving section can be easily convert-

line of crossing roads.

ed into a Type B weave. At any site at which a Type A weave appears, it would thus be prudent

The generous spacing applied to rural inter-

to check the operation at the site for both types

changes would not be able to serve intensively

of weave. Type C weaves rarely occur in South

developed urban areas adequately. As an illus-

Africa.

tration of context sensitive design, trip lengths are shorter and speeds lower on urban free-


7.4

LOCATION AND SPACING OF

ways than on rural freeways. As drivers are

INTERCHANGES

accustomed to taking a variety of alternative actions in rapid succession a spacing of closer than eight kilometres can be considered.

The location of interchanges is based primarily on service to adjacent land. On rural freeways

At spacings appropriate to the urban environ-

bypassing small communities, the provision of a

ment, reference to a centreline-to-centreline dis-

single interchange may be adequate, with larg-

tance is too coarse to be practical. The point at

er communities requiring more.

The precise

issue is that weaving takes place between inter-

location of interchanges would depend on the

changes and the available distance is a function

particular needs of the community but, as a gen-

of the layout of successive interchanges. For a

eral guide, would be on roads recognised as

common centreline-to-centreline spacing, the

being major components of the local system.

weaving length available between two diamond 7-6

Chapter 7: Interchanges

interchanges is significantly different from that

conditions on the freeway. The third criterion is

between a Par-Clo-A followed by a Par-Clo-B.

that of turbulence, which is applied to the

Weaving distance is defined in the Highway

merge-diverge situation.

Capacity Manual 2000 and other sources as the distance between the point at which the separa-

(1)

tion between the ramp and the adjacent lane is

Road Traffic Signs Manual to provide adequate

The distance required by the SADC

Figure 7.4: Weaving distance 0,5 metres to the point at the following off-ramp

sign posting which, in turn, influences the safe

at which the distance between ramp and lane is

operation of the freeway, is used to define the

3,7 m as illustrated in Figure 7.4.

minimum distance between ramps. The minimum distances to be used for detailed design

If this definition is adopted, the weaving length

purposes, as measured between Yellow Line

becomes a function of the rates of taper applied

Break Points, for different areas and inter-

to the on- and off-ramps.

change types should be not less than the values

Yellow Line Break Point (YLBP) distance is total-

stated in Table 7.1.

ly unambiguous and is the preferred option.

(2)

Three criteria for the spacing of interchanges

possible to meet the above requirements, relax-

can be considered. In the first instance, the dis-

In exceptional cases, where it is not

ation of these may be considered. It is, howev-

tance required for adequate signage should ide-

er, necessary to ensure that densities in the

ally dictate spacing of successive interchanges.

freeway left hand lane are not so high that the

If it is not possible to achieve these distances, consideration can be given to a relaxation

flow of traffic breaks down. Densities associat-

based on achieving Level Of Service (LOS) D

ed with LOS E would make it difficult, if not actu7-7



Reference to the

ally impossible, for drivers to be able to change

merge, stacking of vehicles will occur on the on-

lanes. Formulae according to which densities

ramp with the queue possibly backing up on to

can be estimated are provided in the Highway

the crossing road itself. Stacking can also occur

Capacity Manual (2000). Drivers need time to

on an off-ramp if the crossing road ramp termi-

locate a gap and then to position themselves

nal cannot accommodate the rate of flow arriv-

correctly in relation to the gap while simultane-

ing from the freeway. The queue could conceiv-

ously adjusting their speed to that required for

ably back up onto the freeway, which would cre-

the lane change. The actual process of chang-

ate an extremely hazardous situation.

ing lanes also requires time. In the case of the merge-diverge

It should be realised that relaxations below the

manoeuvre, turbulence caused on the left lane

distances recommended under (1) above will

of the freeway by a close succession of entering

result in an increase in the driver workload.

and exiting vehicles becomes an issue.

Failure to accommodate acceptable levels of

According to Roess and Ulerio this turbulence

driver workload in relation to reaction times can

manifests itself over a distance of roughly 450

be expected to result in higher than average

metres upstream of an off-ramp and down-

crash rates. Twomey et al demonstrate that, at

stream of an on-ramp. A spacing of 900 metres

spacings between noses of greater than 2 500

would suggest that the entire length of freeway

metres, the crash rate is fairly constant, i.e. the

between interchanges would be subject to tur-

presence of the following interchange is not a

bulent flow. The likelihood of breakdown in the

factor in the crash rate. At spacings of less than

traffic flow would thus be high and the designer

2500 m between noses, the crash rate increas-

should ensure that space is available for one

es until, at about 500 m between noses, the

area of turbulence to subside before onset of

crash rate is nearly double that of the 2500 m

the next.

spacing. This is illustrated in Figure 7.2 below

(3)


(4)

In off-peak periods, vehicles would be

moving between interchanges at the design

The question that must be addressed is the ben-

speed or higher. The geometry of the on- and

efit that the community can expect to derive in

off-ramps should be such that they can accom-

exchange for the cost of the higher accident

modate

speeds.

rate. By virtue of the fact that freeway speeds

Increasing the taper rates or reducing the length

tend to be high, there is a high probability that

of the speed change lanes purely to achieve

many of the accidents would be fatal. It is there-

some or other hypothetically acceptable Yellow

fore suggested that the decision to reduce the

Line Break Point distance does not constitute

interchange spacing below those listed in Table

good design.

7.1 should not be taken lightly.

(5)

manoeuvres

at

these

The spacing between successive inter-

changes will have an impact on traffic opera-

It would be necessary to undertake a full-scale

tions on the crossing roads and vice versa. If

engineering analysis of the situation that would

the crossing road can deliver vehicles to the

include:

freeway faster than they can carry out the

• 7-8


estimation of future traffic volumes at a

Figure 7.5: Relationship between interchange spacing and accident rate

• •

ten to twenty year time horizon, com-

are not achievable and an interchange is

prising weaving and through volumes in

absolutely vital for service to the community and

the design year;

adjacent land uses, relaxations may be consid-

calculation of traffic densities;

ered but then, to minimise the risk of crashes,

assessment of the local geometry in

the density calculated according to the above-

terms of sight distances, and horizontal

mentioned formulae should not exceed 22 vehi-

and vertical alignment;

• •

cles/kilometre/lane, which corresponds to LOS D.

development of a sign sequence; and a form of benefit/cost analysis relating

7.5

community benefits to the decrease in

BASIC LANES AND LANE BALANCE

traffic safety. Density offers some indication of the level of

an extended length of a route, irrespective of

exposure to risk and, for want of any better

local changes in traffic volumes and require-

measure, it is suggested that a density higher

ments for lane balance. Alternatively stated, the

than 22 vehicles/kilometre/lane, corresponding

basic number of lanes is a constant number of

to LOS D, would not result in acceptable design.

lanes assigned to a route, exclusive of auxiliary

It would be necessary to pay attention to reme-

lanes.

dial actions to prevent interchange constraints, such as inadequate ramp capacity, signalling or

The number of basic lanes changes only when

crossroad volumes, causing back up onto the

there is a significant change in the general level

freeway

of traffic volumes on the route. Short sections of the route may thus have insufficient capacity,

In summary: Spacings of interchanges in terms

which problem can be overcome by the use of

of their YLBP distances should desirably be in

auxiliary lanes. In the case of spare capacity,

accordance with Table 7.1. If these spacings

reduction in the number of lanes is not recom7-9



Basic lanes are those that are maintained over

mended because this area could, at some future

This can be used to drop a basic lane to match

time, become a bottleneck.

anticipated

Unusual traffic

flows

beyond

the

diverge.

demands, created by accidents, maintenance or

Alternatively, it can be an auxiliary lane that is

special events, could also result in these areas

dropped.

becoming bottlenecks. Basic lanes and lane balance are brought into harmony with each other by building on the

The basic number of lanes is derived from con-

basic lanes, adding or removing auxiliary lanes

sideration of the design traffic volumes and

as required.

capacity analyses. To promote the smooth flow

The principle of lane balance

should always be applied in the use of auxiliary

of traffic there should be a proper balance of

lanes. Operational problems on existing road-

lanes at points where merging or diverging

ways can be directly attributed to a lack of lane

manoeuvres occur. In essence, there should be

balance and failure to maintain route continuity.

one lane where the driver has the choice of a change of direction without the need to change

The application of lane balance and coordina-

lanes.

tion with basic number of lanes is illustrated in Figure 7.6

At merges, the number of lanes downstream of the merge should be one less than the number

7.6

of lanes upstream of the merge. This is typified

AUXILIARY LANES

by a one-lane ramp merging with a two-lane car-

As in the case of the two-lane two-way road

riageway that, after the merge, continues as a

cross-section with its climbing and passing

two-lane carriageway as is the case on a typical

lanes, and the intersection with its right- and left-

Diamond Interchange layout.

This rule pre-

turning lanes, the auxiliary lane also has its role

cludes a two-lane ramp immediately merging

to play in the freeway cross-section and the

with the carriageway without the addition of an

interchange. In a sense, the application of the

auxiliary lane.

auxiliary lane in the freeway environment is


identical to its application elsewhere. It is added At diverges, the number of lanes downstream of

to address a local operational issue and, as

the diverge should be one more than the num-

soon as the need for the auxiliary lane is past, it

ber upstream of the diverge. The only exception

is dropped.

to this rule is on short weaving sections, such as

Important features to consider in the application

at Cloverleaf Interchanges, where a condition of

and design of the auxiliary lane are thus:

this exception is that there is an auxiliary lane through the weaving section.

When two lanes diverge from the freeway, the above rule indicates that the number of freeway lanes beyond the diverge is reduced by one.

• • •

The need for an auxiliary lane;

7.6.1

The need for an auxiliary lane

The terminals; Driver information

Auxiliary lanes are normally required on free7-10


ways either as:

• • •

Ideally, maximum gradients on freeways are in

climbing lanes; or

the range of three to four per cent ensuring that

to support weaving; or

most vehicles can maintain a high and fairly

to support lane balance.

constant speed.

The climbing lane application is similar to that

However, in heavily rolling

country it is not always possible to achieve this

discussed in Chapter 4 in respect of two-lane

ideal without incurring excessive costs in terms

two-way roads whereas the weaving and lane

of earthworks construction.

balance applications are unique to the freeway

Because of the

heavy volumes of traffic that necessitate the

situation.

provision of a freeway, lane changing to overtake a slow-moving vehicle is not always easy

Climbing lanes

and, under peak flow conditions, may actually 7-11 Chapter 7: Interchanges


Figure 7.6: Coordination of lane balance with basic number of lanes

be impossible. Speed differentials in the traffic

ensure that the freeway is not unduly congested

stream are thus not only extremely disruptive

because of this practice, an auxiliary lane can

but may also be potentially dangerous. Both

be provided between adjacent interchanges

conditions, i.e. disruption and reduction in safe-

resulting in Type A weaving as described in

ty, require consideration.

Section 7.3.

If a gradient on a freeway is steeper than four

If a large number of vehicles are entering at the

per cent, an operational analysis should be car-

upstream interchange, it may be necessary to

ried out to establish the impact of the gradient

provide a two-lane entrance ramp.

on the Level of Service. A drop through one

these vehicles may exit at the following inter-

level, e.g from LOS B through LOS C to LOS D,

change but those wishing to travel further will

would normally suggest a need for a climbing

have to weave across traffic from still further

lane.

upstream that intends exiting at the following

Some of

interchange and then merge with through traffic As discussed in Chapter 4, crash rates increase

on the freeway.

The auxiliary lane is then

exponentially with increasing speed differential.

extended beyond the downstream interchange

For this reason, international warrants for climb-

to allow a separation between the two manoeu-

ing lanes normally include a speed differential in

vres. Similarly, a large volume of exiting vehi-

the range of 15 to 20 km/h. South Africa has

cles may necessitate a two-lane exit, in which

adopted a truck speed reduction of 20 km/h as

case the auxiliary lane should be extended

its speed-based warrant for climbing lanes. If,

upstream.

on an existing freeway, the measured truck

being. The desired length of the extension of

speed reduction in the outermost lane is thus 20

the auxiliary lane beyond the two interchanges

km/h or higher, the provision of a climbing lane

is normally assessed in terms of the probability

should be considered. In the case of a new

of merging vehicles locating an acceptable gap

design, it will be necessary to construct a speed

in the opposing traffic flow.

Type B weaving thus comes into


profile of the truck traffic to evaluate the need for a climbing lane.

Lane balance

Weaving

As discussed in Section 7.5, lane balance requires that:

•

In the urban environment, interchanges are fair-

In the case of an exit, the number of

ly closely spaced and local drivers are very

lanes downstream of the diverge should

inclined to use freeways as part of the local cir-

be one more than the number upstream; and

culation system - a form of rat-running in

•

reverse and as undesirable as the normal form

of lanes downstream of the merge

of rat-running where the higher order road is

should be one less than the number

bypassed through the use of local residential streets as long-distance urban routes.

In the case of an entrance, the number

upstream

To 7-12


7.6.2

This is illustrated in Figure 7.6. Single-lane on- and off-ramps do not require

Auxiliary lane terminals

An auxiliary lane is intended to match a particu-

auxiliary lanes to achieve lane balance in terms

lar situation such as, for example, an unaccept-

of the above definition. It should be noted that,

ably high speed differential in the traffic stream.

unless two-lane on- and off-ramps are provided,

It follows that the full width of auxiliary lane must

the Type A weave is actually a violation of the

be provided over the entire distance in which the

principles of lane balance.

situation prevails.

The terminals are thus

required to be provided outside the area of need To achieve lane balance at an exit, three lanes

and not as part of the length of the auxiliary

upstream of the diverge should be followed by a

lane.

two-lane off-ramp in combination with two basic lanes on the freeway. The continuity of basic

ntering and exiting from auxiliary lanes require a

lanes requires that the outermost of the three

reverse curve path to be followed. It is thus sug-

upstream lanes should be an auxiliary lane.

gested that the taper rates discussed in Chapter 4.4.3 be employed rather than those normally

If all three upstream lanes are basic lanes, it is

applied to on- and off-ramps.

possible that traffic volumes beyond the off-

The entrance

taper should thus be about 100 metres long and

ramp may have reduced to the point where

the exit taper about 200 metres long.

three basic lanes are no longer necessary. Provision of a two-lane exit would thus be a

7.6.3

Driver information

convenient device to achieve a lane drop while simultaneously maintaining lane balance. The

The informational needs of drivers relate specif-

alternative would be to provide a single-lane off-

ically to needs with regard to the exit from the

ramp, carrying the basic lanes through the inter-

auxiliary lane and include an indication of:

the additional construction costs involved, this

• • •

approach is not recommended.

These are fully discussed in Chapter 4.4.3 and

If a two-lane on-ramp is joining two basic lanes,

reference should be made to that chapter for

lane balance will require that there be three

further detail.

tance beyond the on-ramp terminal. In view of

the presence of a lane drop; the location of the lane drop; and the appropriate action to be undertaken

lanes beyond the merge. The outside lane of the three could become a new basic lane if the

7.7

INTERCHANGE TYPES

7.7.1

General

increase in traffic on the freeway merits it. On the other hand, it is possible that the flow on the ramp is essentially a local point of high density and that two basic lanes are all that are required downstream of the on-ramp. In this case, the

There is a wide variety of types of interchanges

outside lane could be dropped as soon as con-

that can be employed under the various circum-

venient.

stances that warrant the application of inter7-13 Chapter 7: Interchanges


change and dropping the outside lane some dis-

changes. The major determinant of the type of

between a major and a local road, as suggested

interchange to be employed at any particular

above in the case where local topography may

site is the classification and characteristics of

force a grade separation between the two roads.

the intersecting road.

Intersecting roads are

typically freeways or urban arterials but may

In addition to the classification and nature of the

also be collectors.

intersecting road, there are a number of controls guiding the selection of the most appropriate

In the case of freeways as intersecting roads,

interchange form for any particular situation. In

reference is made to systems interchanges.

the sense of context sensitive design, these

Systems interchanges exclusively serve vehi-

include;

cles that are already on the freeway system.

• • •

Access to the freeway system from the surrounding area is via interchanges on roads

Safety; Adjacent land use; Design speed of both the freeway and the intersecting road;

other than freeways, for which reason these

•

interchanges are known as access inter-

Traffic volumes of the through and turning movements;

changes. Service areas, providing opportunities

• • • • • • •

to buy fuel, or food or simply to relax for a while are typically accessed via an interchange. In some instances, the services are duplicated on either side of the freeway, in which access is via a left-in/left-out configuration. The requirements in terms of deflection angle, length of ramp and spacing that apply to interchange ramps apply

Traffic composition; Number of required legs; Road reserve and spatial requirements; Topography; Service to adjacent communities; Environmental considerations, and Economics.

equally to left-in/left-out ramps. In effect, this sitThe relative importance of these controls may

uation could be described as being an inter-

vary from interchange to interchange. For any

change without a crossing road.

particular site, each of the controls will have to

The primary difference between systems and

be examined and its relative importance


access/service interchanges is that the ramps

assessed. Only after this process will it be pos-

on systems interchanges have free-flowing ter-

sible to study alternative interchange types and

minals at both ends, whereas the intersecting

configurations to determine the most suitable in

road ramp terminals on an access interchange

terms of the more important controls.

are typically in the form of at-grade intersections.

While the selection of the most appropriate type Interchanges can also be between non-freeway

and configuration of interchange may vary

roads, for example between two heavily traf-

between sites, it is important to provide consis-

ficked arterials.

tent operating conditions in order to match driv-

In very rare instances there

may even be an application for an interchange

er expectations.


7.7.2

Systems interchanges

Partially directional interchanges allow the number of levels to be reduced. The Single Loop

As stated above, at-grade intersections are

Partially-directional Interchange, illustrated in

inappropriate to systems interchanges and their

Figure 7.7 (ii), and the Two Loop arrangement,

avoidance is mandatory. For this reason, hybrid

illustrated in Figure 7.7 (iii) and (iv), require

interchanges, in which an access interchange is

three levels. The difference between Figures

contained within a systems interchange, are to

7.7 (iii) and (iv) is that, in the former case, the

be avoided.

freeways cross and, in the latter, route continuity dictates a change in alignment. Loop ramps

Hybrid interchanges inevitably lead to an unsafe

are normally only used for lighter volumes of

mix of high and low speed traffic. Furthermore,

right-turning traffic. A three-loop arrangement

signposting anything up to six possible destina-

is, in effect, a cloverleaf configuration, with one

tions within a very short distance is, at best, dif-

of the loops being replaced by a directional

ficult. Selecting the appropriate response gen-

ramp and is not likely to occur in practice, large-

erates an enormous workload for the driver so

ly because of the problem of weaving discussed

that the probability of error is substantial. Past

below.

experience suggests that these interchange The principal benefit of the cloverleaf is that it

configurations are rarely successful.

requires only a simple one-level structure, in Directional interchanges provide high-speed

contrast to the complex and correspondingly

connections to left and to right provided that the

costly structures necessary for the directional

ramp exits and entrances are on the left of the

and partially directional configurations. The

through lanes. Where turning volumes are low

major weakness of the cloverleaf is that it

or space is limited, provision of loops for right

requires weaving over very short distances.

turning traffic can be considered.

Directional

Provided weaving volumes are not high and suf-

interchanges that include one or more loops are

ficient space is available to accommodate the

referred to as being partially-directional. If all

interchange, the cloverleaf can, however, be

right turns are required to take place on loops,

considered to be an option.

the cloverleaf configuration emerges. Various

required to take place on the main carriage-

forms of systems interchanges are illustrated

ways, the turbulence so created has a serious

below.

effect on the flow of traffic through the interchange area. The cloverleaf also has the characteristic of confronting the driver with two exits

Four-legged interchanges

from the freeway in quick succession.

Both

The fully directional interchange illustrated in

these problems can be resolved by providing

Figure 7.7 (i) provides single exits from all four

collector-distributor roads adjacent to the

directions and directional ramps for all eight

through carriageways.

turning movements.

The through roads and

ramps are separated vertically on four levels. 7-15 Chapter 7: Interchanges


If weaving is


Figure 7.7: Four-legged systems interchanges

Three-legged interchanges

It is also possible with this layout to slightly reduce the height through which vehicles have

Various fully-directional and partially-directional

to climb. Figure 7.8 (iii) illustrates a fully-direc-

three legged interchanges are illustrated in

tional interchange that requires only two but

Figure 7.8. In Figure 7.8 (i), one single structure

widely separated structures.

providing a three-level separation is required.

assumed as being at the top of the page, vehi-

Figure 7.8 (ii) also requires three levels of road-

cles turning from West to South have a slightly

way but spread across two structures hence

longer path imposed on them so that this

reducing the complexity of the structural design.

should, ideally be the lesser turning volume. 7-16


If North is

Figures 7.8 (iv) and (v) show semi-directional

vehicles entering from the crossing road may be

interchanges. Their names stem from the loop

doing so from a stopped condition, so that it is

ramp located within the directional ramp creat-

necessary to provide acceleration lanes to

ing the appearance of the bell of a trumpet. The

ensure that they enter the freeway at or near

letters "A" and "B" refer to the loop being in

freeway speeds.

Advance of the structure or Beyond it.

The

should be provided with deceleration lanes to

smaller of the turning movements should ideally

accommodate the possibility of a stop at the

be on the loop ramp but the availability of space

crossing road.

Similarly, exiting vehicles

may not always make this possible. As previously discussed, there is distinct merit

7.7.3

Access and service interchanges

in the crossing road being taken over the freeway as opposed to under it. One of the advan-

In the case of the systems interchange, all traf-

tages of the crossing road being over the free-

fic enters the interchange area at freeway

way is that the positive and negative gradients

speeds. At access and service interchanges,

respectively support the required deceleration 7-17



Figure 7.8: Three-legged systems interchanges

and acceleration to and from the crossing road.

crossing road, at the time of construction, stops

The final decision on the location of the crossing

immediately beyond the interchange.

road is, however, also dependent on other controls such as topography and cost.

Diamond Interchanges

Access interchanges normally provide for all

There are three basic forms of Diamond, being:

turning movements.

• • •

If, for any reason, it is

deemed necessary to eliminate some of the turning movements, the return movement, for

The Simple Diamond; The Split Diamond, and the Single Point Interchange.

any movement that is provided, should also be provided. Movements excluded from a particu-

The Simple Diamond is easy for the driver to

lar interchange should, desirably, be provided at

understand and is economical in its use of

the next interchange upstream or downstream

space. The major problem with this configura-

as, without this provision, the community served

tion is that the right turn on to the crossing road

loses amenity.

can cause queuing on the exit ramp. In extreme cases, these queues can extend back onto the

There are only two basic interchange types that

freeway, creating a hazardous situation. Where

are appropriate to access and service inter-

the traffic on the right turn is very heavy, it may

changes. These are the Diamond and the Par-

be necessary to consider placing it on a loop

Clo interchanges. Each has a variety of possi-

ramp. This is the reverse of the situation on sys-

ble configurations.

tems interchanges where it is the lesser vol-


umes that are located on loop ramps. It has the Trumpet interchanges used to be considered

advantage that the right turn is converted into a

suitable in cases where access was to provided

left-turn at the crossing road ramp terminal. By

to one side only, for example to a bypass of a

the provision of auxiliary lanes, this turn can

town or village. In practice, however, once a

operate continuously without being impeded by

bypass has been built it does not take long

traffic signals.

before development starts taking place on the

The Simple Diamond can take one of two con-

other side of the bypass.

figurations: the Narrow Diamond and the Wide

The three-legged

interchange then has to be converted into a

Diamond.

four-legged interchange. Conversion to a ParClo can be achieved at relatively low cost.

The Narrow Diamond is the form customarily

Other than in the case of the Par-Clo AB, one of

applied. In this configuration, the crossing road

the major movements is forced onto a loop

ramp terminals are very close in plan to the free-

ramp. The resulting configuration is thus not

way shoulders to the extent that, where space is

appropriate to the circumstances. In practice,

heavily constricted, retaining walls are located

the interchange should be planned as a

just outside the freeway shoulder breakpoints.

Diamond in the first instance, even though the

Apart from the problem of the right turn referred


to above, it can also suffer from a lack of inter-

The Split Diamond can also take one of two

section sight distance at the crossing road ramp

forms: the conventional Split and the transposed

terminals. This problem arises when the cross-

Split. This configuration is normally used when

ing road is taken over the freeway and is on a

the crossing road takes the form of a one-way

minimum value crest curve on the structure. In

pair.

addition, the bridge balustrades can also inhibit

queues backing up are not normally experi-

sight distance. In the case where the crossing

enced on Split Diamonds and the most signifi-

road ramp terminal is signalised, this is less of a

cant drawback is that right-turning vehicles have

problem, although a vehicle accidentally or by

to traverse three intersections before being

intent running the red signal could create a dan-

clear of the interchange. It is also necessary to

gerous situation.

construct frontage roads linking the two one-

The problems of sight distance and

way streets to provide a clear route for rightturning vehicles.

The Wide Diamond was originally intended as a form of stage construction, leading up to conversion to a full Cloverleaf Interchange. The

The transposed Split has the ramps between the

time span between construction of the Diamond

two structures. This results in a very short dis-

and the intended conversion was, however, usu-

tance between the entrance and succeeding

ally so great that, by the time the upgrade

exit ramps, with significant problems of weaving

became necessary, standards had increased to

on the freeway. Scissor ramps are the extreme

the level whereby the loop ramps could not be

example of the transposed Split. These require

accommodated in the available space.

The

either signalisation of the crossing of the two

decline in the popularity of the Cloverleaf has

ramps or a grade separation. The transposed

led to the Wide Diamond also falling out of

Split has little to recommend it and has fallen

favour.

into disuse, being discussed here only for completeness of the record.

The Wide Diamond has the problem of imposing The Single Point Interchange brings the four

a long travel distance on right-turning vehicles

interchange is required where space is at a pre-

road ramp terminals are located at the start of

mium or where the volume of right-turning traffic

the approach fill to the structure. To achieve this

is very high. The principal operating difference

condition, the ramps have to be fairly long so

between the Single Point and the Simple

that queues backing up onto the freeway are

Diamond is that, in the former case, the right

less likely than on the Narrow Diamond. The

turns take place outside each other and in the

crossing road ramp terminals are also at ground

latter they are "hooking" movements.

level, which is a safer alternative than having

capacity of the Single Point Interchange is thus

the intersections on a high fill. Finally, because

higher than that of the Simple Diamond. It does,

the ramp terminals are remote from the struc-

however, require a three-phase signal plan and

ture, intersection sight distance is usually not a

also presents pedestrians with wide unprotected

problem.

crossings. 7-19 Chapter 7: Interchanges

The


ramps together at a point over the freeway. This

but is not without its advantages. The crossing

The various configurations of the Diamond

Three configurations of Par-Clo Interchange are

Interchange are illustrated in Figure 7.9

possible: the Par-Clo A, the Par-Clo B and the Par-Clo AB.

Par-Clo interchanges

As in the case of the Trumpet

Interchange, the letters have the significance of


the loops being in advance of or beyond the Par-Clo interchanges derive their name as a

structure. The Par-Clo AB configurationhas the

contraction of PARtial CLOverleaf, mainly

loop in advance of the structure for the one

because of their appearance, but also because

direction of travel and beyond the structure for

they were frequently a first stage development

the other. In all cases, the loops are on oppo-

of a Cloverleaf Interchange. In practice, they

site sides of the freeway. Both the Par-Clo A

could perhaps be considered rather as a distort-

and the Par-Clo B have alternative configura-

ed form of Simple Diamond Interchange.

tions: the A2 and A4 and the B2 and B4. These

Figure 7.9: Diamond interchanges


configurations refer to two quadrants only being

from the intersections on the crossing road and

occupied or alternatively to all four quadrants

the only conflict is between right-turning vehi-

having ramps. The four-quadrant layout does

cles exiting from the freeway and through traffic

not enjoy much, if any, usage in South Africa.

on the crossing road. This makes two-phase signal control possible.

The various layouts are illustrated in Figures 7.10, 7.11 and 7.12.

The Par-Clo AB is particularly useful in the situation where there are property or environmental

Internationally, the Par-Clo A4 is generally

restrictions in two adjacent quadrants on the

regarded as being the preferred option for an

same side of the crossing road.

interchange between a freeway and a heavily

include a road running alongside a river or the

trafficked arterial. In the first instance, the loops

situation, frequently found in South Africa, of a

serve vehicles entering the freeway whereas, in

transportation corridor containing parallel road

the case of the Par-Clo B, the high-speed vehi-

and rail links in close proximity to each other.

Examples

loop. This tends to surprise many drivers and

The Rotary Interchange illustrated in Figure

loops carrying exiting traffic have higher acci-

7.12 has the benefit of eliminating intersections

dent rates than the alternative layout. Secondly,

on the crossing road, replacing them by short

the left turn from the crossing road is remote

weaving sections. Traffic exiting from the free-

Figure 7.10: Par-Clo A interchanges 7-21 Chapter 7: Interchanges


cles exiting the freeway are confronted by the

way may experience difficulty in adjusting speed

Kingdom as systems interchanges. In this con-

and merging with traffic on the rotary.

figuration, a two-level structure is employed. One freeway is located at ground level and the

Rotaries have also been used in the United

other freeway on the upper level of the structure


Figure 7.11: Par-Clo B interchanges

Figure 7.12: Par-Clo AB interchanges and rotaries 7-22 Chapter 7: Interchanges

with the rotary sandwiched between them. This

Where the need for the interchange derives

is the so-called "Island in the Sky" concept. The

purely from topographic restraints, i.e. where

Rotary is an interchange form that is unknown in

traffic

South Africa so that its application would com-

Interchange, illustrated in Figure 7.13, would be

promise consistency of design and thus be con-

adequate. This layout, also known as a Quarter

trary to drivers' expectations.

Link, provides a two-lane-two-way connection

volumes

are

low,

a

Jug

Handle

between the intersecting roads located in what-

7.7.4

Interchanges on non-freeway roads

ever quadrant entails the minimum construction and property acquisition cost.

Interchanges where a non-freeway is the major route are a rarity in South Africa. This applica-

Drivers would not expect to find an interchange

tion would arise where traffic flows are so heavy

on a two-lane two-way road and, in terms of

that a signalised intersection cannot provide suf-

driver expectancy, it may therefore be advisable

ficient capacity. In this case, the crossing road

to introduce a short section of dual carriageway

terminals would be provided on the road with

at the site of the interchange

the lower traffic volume. As a general rule, a simple and relatively low standard Simple

7.8

RAMP DESIGN

7.8.1

General

Diamond or a Par-Clo Interchange should suffice. An intersection with a particularly poor accident

A ramp is defined as a roadway, usually one-

history may also require upgrading to an inter-

way, connecting two grade-separated through

The accident history would provide

some indication of the required type of inter-

roads. It comprises an entrance terminal, a mid-

change.

section and an exit terminal.

Figure 7.13: Jug Handle interchange 7-23 Chapter 7: Interchanges


change.

between them;

The general configuration of a ramp is deter-

•

mined prior to the interchange type being selected.

The directional ramp also serving the right turn, with a curve only slightly in

The specifics of its configuration,

O

being the horizontal and vertical alignment and

excess of 90 degrees and free-flowing

cross-section, are influenced by a number of

terminals at either end, and

•

considerations such as traffic volume and com-

The collector-distributor road intended to remove the weaving manoeuvre from

position, the geometric and operational charac-

the freeway.

teristics of the roads which it connects, the local topography, traffic control devices and driver

The express-collector system discussed later is

expectations.

a transfer roadway and is not an interchange ramp.

A variety of ramp configurations can be used. These include:

•


•

•

7.8.2

The outer connector, which serves the left turn and has free-flowing terminals

Guideline values for ramp design speeds are

at either end;

given in Table 7.2. Strictly speaking, the design

The diamond ramp, serving both the left-

speed of a ramp could vary across its length

and right-turns with a free-flowing termi-

from that of the freeway to that of the at-grade

nal on the freeway and a stop-condition

intersection, with the design speed at any point

terminal on the crossing-road;

along the ramp matching the operating speed of

The Par-Clo ramp, which serves the

the vehicles accelerating to or decelerating from

right turn and has a free-flowing terminal

the design speed of the freeway. The design

on the freeway and a stop-condition ter-

speeds given in the table apply to the controlling

minal on the crossing road, with a 180

O

curve on the mid-section of the ramp. The ramp

loop between them;

•

Design speed

design speed is shown as a design domain

The loop ramp, serving the right turn

because of the wide variety of site conditions,

and which has free-flowing terminals at

terminal types and ramp shapes.

O

both ends and a 270 degree loop 7-24 Chapter 7: Interchanges

In the case of a directional ramp between free-

the substantial difference between the freeway

ways, vehicles must be able to operate safely at

design speed and that of the loop ramp, it is

the higher end of the ranges of speeds shown in

advisable not to have a loop on an exit ramp if

Table 7.2.

this can be avoided. Safety problems on ramp curves are most likely

Factors demanding a reduction in design speed

for vehicles travelling faster than the design

include site limitations, ramp configurations and

speed. This problem is more critical on curves

economic factors. Provided the reduction is not

with lower design speeds because drivers are

excessive, drivers are prepared to reduce speed

more likely to exceed these design speeds than

in negotiating a ramp so that the lower design

the higher ranges. Trucks can capsize when

speed is not in conflict with driver expectations.

travelling at speeds only marginally higher than the design speed. To minimise the possibility of

For directional ramps and outer connectors,

trucks exceeding the design speed it is suggest-

higher values in the speed domain are appropri-

ed that the lower limits of design speeds shown

ate and, in general, ramp designs should be

in Table 7.2 should not be used for ramps carry-

based on the upper limit of the domain. The

ing a substantial amount of truck traffic.

constraints of the site, traffic mix and form of interchange, however, may force a lower design

If site specific constraints preclude the use of

speed.

design speeds that more-or-less match anticipated operating speeds, the designer should

A Par-Clo or a loop ramp cannot be designed to

seek to incorporate effective speed controls,

a high design speed. A ramp design speed of

such as advisory speed signing, special pave-

70 km/h, being the low end of the domain for a

ment treatments, long deceleration lanes and

freeway design speed of 130 km/h, would

the use of express-collector systems in the

require a radius of between 150 metres and 200

design.

metres, depending on the rate of superelevation

Sight distance on ramps

area, that the space required to accommodate a loop with this radius would be available. Even if

It is necessary for the driver to be able to see

the space were available, the additional travel

the road markings defining the start of the taper

distance imposed by the greater radius would

on exit ramps and the end of the entrance taper.

nullify the advantages of the higher travel

At the crossing road ramp terminal, lanes are

speed. The added length of roadway also adds

often specifically allocated to the turning move-

the penalty of higher construction cost. This

ments with these lanes being developed in

penalty also applies to the ramp outside the loop

advance of the terminal. The driver has to posi-

because it has to be longer to contain the larger

tion the vehicle in the lane appropriate to the

loop. In general, loop ramps are designed for

desired turn. It is therefore desirable that deci-

speeds of between 40 and 50 km/h. Because of

sion sight distance be provided on the 7-25



7.8.3

selected. It is not likely, particularly in an urban

approaches to the ramp as well as across its

only commence at the nose.

The distance

length.

required to achieve the appropriate superelevation thus determines the earliest possible location of the first curve on the ramp.

Appropriate values of decision sight distance are given in Table 3.7.

7.8.4

Ramps are relatively short and the radii of

Horizontal alignment

curves on ramps often approach the minimum for the selected design speed. Furthermore, if

Minimum radii of horizontal curvature on ramps

there is more than one curve on a ramp, the dis-

are as shown in Table 4.1 for various values of

tance between the successive curves will be

emax. In general, the higher values of emax are

short. Under these restrictive conditions, transi-

used in freeway design and the selected value

tion curves should be considered.

should also be applied to the ramps.

Ramps are seldom, if ever, cambered and

Achieving the step down of radii from higher to

superelevation typically involves rotation around

lower design speeds on a loop may require the

one of the lane edges. Drivers tend to position

application of compound curves. In general, the

their vehicles relative to the inside edge of any

ratio between successive radii should be 1,5 : 1

curve being traversed, i.e. they steer towards

and, as a further refinement, they could be con-

the inside of the curve rather than away from the

nected by transition curves. The length of each

outside. For aesthetic reasons, the inside edge

arc is selected to allow for deceleration to the

should thus present a smoothly flowing three-

speed appropriate to the next radius at the entry

dimensional alignment with the outside edge ris-

to that arc. In the case of stepping up through

ing and falling to provide the superelevation.

successive radii, the same ratio applies but the

Where a ramp has an S- or reverse curve align-

design speed for the radius selected should

ment, it follows that first one edge and then the

match the desired speed at the far end of each

other will be the centre of rotation, with the

arc.

changeover taking place at the point of zero crossfall.


It is recommended that the designer develops a speed profile for the loop and bases the selection of radii and arc lengths on this speed profile.

In view of the restricted distances within which

As a rough rule of thumb, the length of each arc

superelevation has to be developed, the

should be approximately a third of its radius.

crossover crown line is a useful device towards rapid development. A crossover crown is a line

If a crossover crown line, discussed below, is

at which an instantaneous change of crossfall

not used, the crossfall on the exit or entrance

takes place and which runs diagonally across

taper between the Yellow Line Break Point and

the lane. The crossover crown could, for exam-

the nose is controlled by that on the through

ple, be located along the yellow line defining the

lanes.

edge of the left lane of the freeway, thus

Superelevation development can thus 7-26


enabling initiation of superelevation for the first

having as little as 120 to 360 metres between

curve on the ramp earlier than would otherwise

the nose and the crossing road. The effect of

be the case. The crossover crown should, how-

the midsection gradient, while possibly helpful,

ever, be used with caution as it may pose a

is thus restricted. However, a steep gradient (8

problem to the driver, particularly to the driver of

per cent) in conjunction with a high value of

a vehicle with a high load. This is because the

superelevation (10 per cent) would have a

vehicle will sway as it traverses the crossover

resultant of 12,8 per cent at an angle of 53 to

crown and, in extreme cases, may prove difficult

the centreline of the ramp. This would not con-

to control.

The algebraic difference in slope

tribute to drivers' sense of safety. In addition,

across the crossover crown should thus not

the drivers of slow-moving trucks would have to

exceed four to five per cent.

steer outwards to a marked extent to maintain

O

their path within the limits of the ramp width.

7.8.5

Vertical alignment

This could create some difficulty for them. It is suggested that designers seek a combination of

Gradients

superelevation and gradient such that the gradient of the resultant is less than ten per cent.

The profile of a ramp typically comprises a mid-

Table 7.3 provides an indication of the gradients

section with an appreciable gradient coupled

of the resultants of combinations of supereleva-

with terminals where the gradient is controlled

tion and longitudinal gradient.

over the freeway, the positive gradient on the

The combinations of gradient and supereleva-

off-ramps will assist a rapid but comfortable

tion shown shaded in Table 7.3 should be avoid-

deceleration and the negative gradient on the

ed.

on-ramp will support acceleration to freeway speeds. In theory, thus, the higher the value of

Curves

gradient, the better. Values of gradient up to eight per cent can be considered but, for prefer-

As suggested above, decision sight distance

ence, gradients should not exceed six per cent.

should be available at critical points on ramps. The K-values of crest curves required to meet

Diamond ramps are usually fairly short, possibly

this requirement are given in Table 7.4. 7-27



by the adjacent road. If the crossing road is

These values of K are based on the required

On sag curves, achieving decision sight dis-

sight distance being contained within the length

tance is not a problem so that the K-values

of the vertical curve. It is, however, unlikely that,

shown in Table 4.14 can be applied. If the inter-

within the confined length of a ramp, it would be

change area is illuminated, as would typically be

possible to accommodate the length of vertical

the case in an urban area, the K-values for com-

curve required for this condition to materialise.

fort can be used. Unilluminated, i.e. rural, inter-

The designer should therefore have recourse to

changes would require the application of K-val-

Equation 4.21 to calculate the K-value for the

ues appropriate to headlight sight distance.

condition of the curve being shorter than the

7.8.6 Cross-section

required sight distance. This equation is repeated below for convenience.

Horizontal radii on ramps are typically short and 4.21

hence are usually in need of curve widening.


The width of the ramps normally adopted is thus where K S

= =

Distance required for a

4 metres.

Because of the inconvenience of

1 % change of gradient (m)

changing the lane width of comparatively short


sections, this width is applied across the entire

for selected design

length of the ramp.

speed (m) h1

=


In the case of a loop ramp, the radius could be

h2

=

Object height (m)

as low as fifty metres. At this radius, a semi

A

=

Algebraic difference in

trailer would require a lane width of 5,07 metres.

gradient between the

It is necessary for the designer to consider the

approaching and

type of vehicle selected for design purposes and

departing grades (%)

to check whether the four metre nominal width is 7-28


adequate. If semi trailers are infrequent users

signalised or stop control at-grade intersection,

of the ramp, encroachment on the shoulders

the change in speed across the ramp is sub-

could be considered.

stantial.

Provision should thus be made for

acceleration and deceleration to take place The calculation of required lane width is

clear of the freeway so as to minimise interfer-

described in detail in Section 4.2.6. As sug-

ence with the through traffic and reduce the

gested above, regardless of the width required,

potential for crashes. The auxiliary lanes pro-

it should be applied across the entire length of

vided to accommodate this are referred to as

the ramp.

speed-change lanes or acceleration or deceleration lanes.

Ramp shoulders typically have a width of the

These terms describe the area

adjacent to the travelled way of the freeway,

order of 2 metres, with this width applying both

including that portion of the ramp taper where a

to the inner and to the outer shoulder. In con-

merging vehicle is still clear of the through lane,

junction with the nominal lane width of 4 metres,

and do not imply a definite lane of uniform width.

the total roadway width is thus 8 metres. This width would allow comfortably for a truck to pass

The speed change lane should have sufficient

a broken-down truck. In addition, it would pro-

length to allow the necessary adjustment in

vide drivers with some sense of security in the

speed to be made in a comfortable manner and,

cases where the ramp is on a high fill.

in the case of an acceleration lane, there should

7.8.7

Terminals

also be sufficient length for the driver to find and manoeuvre into a gap in the through traffic

The crossing road ramp terminals may be free-

stream before reaching the end of the accelera-

flowing, in which case their design is as dis-

tion lane. The length of the speed change lane

cussed below. Crossing road ramp terminals

is based on:

that are at-grade intersections should be

•

The design speed on the through lane,

designed according to the recommendations

i.e. the speed at which vehicles enter or

contained in Chapter 6. It is worth noting that,

exit from the through lanes;

•

from an operational point of view, what appears

est radius curve on the ramp, and

ing road ramp terminals on a Diamond

•

Interchange, is, in fact, two three-legged inter-

The tempo of acceleration or deceleration applied on the speed change lane.

Ideally, the crossing

road ramp terminal should be channelised to

In Chapter 3, reference was made to a deceler-

reduce the possibility of wrong-way driving.

ation rate of 3 m/s2 as the basis for the calculation of stopping sight distance. Research has

Vehicles entering or exiting from a freeway

shown that deceleration rates applied to off-

should be able to do so at approximately the

ramps are a function of the freeway design

operating speed of the freeway. Given the fact

speed and the ramp control speed. As both

that the crossing road terminal is invariably a

speeds increase, so does the deceleration rate, 7-29



tion, i.e. the design speed of the small-

to be a four-legged intersection, e.g. the cross-

sections back-to-back.

The control speed of the ramp midsec-

which varies between 1,0 m/s2 and 2,0 m/s2.

length of the entering vehicle is not relevant in

For convenience, the deceleration rate used to

determination of the taper length.

develop Table 7.5 has been set at 2,0 m/s . 2

The lengths of the deceleration and acceleration lanes shown in Tables 7.5 and 7.6 apply to gra-

Deceleration should only commence once the

dients of between - 3 per cent and + 3 per cent.

exiting vehicle is clear of the through lane.

Acceleration lanes will have to be longer on

Assuming that a vehicle with a width of 2,5

upgrades and may be made shorter on down-

metres is correctly positioned relative to the

grades, with the reverse applying to decelera-

ramp edge line to be centrally located within an

tion lanes.

ultimate ramp width of 4 metres, its tail end will clear the edge of the through lane at a distance,

The actual entrance or exit points between the

L, from the Yellow Line Break Point where with

L

=

3,2 / T

freeway and the ramp can take the form of a

T

=

Taper rate (as listed in

taper or a parallel lane. The parallel lane has

Table 7.7)

the problem of forcing a reverse curve path, possibly followed after the nose by a further curve to the left, whereas the taper involves only

The maximum legal length of any vehicle on

a single change of direction. As such, drivers

South African roads is 22 metres and this should

prefer the taper. In cases where an extended

be added to the distance, L, to establish the dis-

acceleration

tance from the Yellow Line Break Point, at which

or

deceleration

distance

is

required, for example where the crossing road

the deceleration can commence so that

passes under the freeway resulting in the onramp being on an upgrade and the off-ramp on

LT

=

3,2/T + 22

a downgrade, a straight taper would result in an


inordinately long ramp. The parallel lane allows Values of LT, are listed in Table 7.5. From this

the speed change to take place in an auxiliary

table it follows that, in the case of a diamond

lane immediately adjacent to the through lanes

ramp without curves, the distance from the

thus reducing the spatial demands of the inter-

Yellow Line Break Point to the crossing road

change.

ramp terminal should be not less than 437

Furthermore, in the case of the on-ramp, the

metres.

parallel configuration provides an extended distance to find a gap in the adjacent traffic stream.

The acceleration rate can, according to

Typical configurations of on- and off-ramp termi-

American literature, be taken as 0,7 m/s2. The

nals are illustrated in Figures 7.14 to 7.17.

length of the acceleration lane is thus as shown Taper

in Table 7.6. An important feature of the acceleration lane is the gap acceptance length, which should be a minimum of 100 metres to 150

Two different criteria apply to the selection of the

metres, depending on the nose width.

taper rate, depending on whether the ramp is an

The 7-30


rates corresponding to the condition of the wheel path having an offset of 150 millimetres

In the first case, the vehicle is merely required to

inside the Yellow Line Break Point.

achieve a change of direction from the freeway to the ramp. The taper rate should thus be suf-

These taper rates are higher than the rate of 1 :

ficiently flat to ensure that the vehicle path can

15 currently applied on South African freeways.

be accommodated within the lane width.

In

This rate derives from the application of an

Table 7.7, the radii of curvature corresponding

"operating speed" previously assumed to be 85

to a superelevation of 2,0 per cent for the vari-

per cent of the design speed and the further

ous design speeds are listed as are the taper

assumption that the wheel path could be



exit from or entrance to the freeway.

allowed to pass over the Yellow Line Break Point.

that, in the case of existing interchanges, it is

In practice, a 1: 15 rate requires that drivers

not always necessary to incur the expenditure of

accept a higher level of side friction in negotiat-

upgrading to a flatter taper. Trucks do, however,

ing the change of direction. Passenger cars can

sometimes roll over at the start of the ramp taper.

negotiate tapers of this magnitude with ease so


Figure 7.14: Single lane exit

Figure 7.15: Two-lane exit 7-32 Chapter 7: Interchanges

Figure 7:17: Two-lane entrance 7-33 Chapter 7: Interchanges


Figure 7.16: One lane entrance

Figure 7.18: Major fork If there is a high percentage of truck traffic and

rate does not have to be as flat as suggested

if the incidence of roll-overs is unacceptably

above. As in the case of the climbing lane, a

high, it may be necessary to consider upgrading

taper length of 100 metres would be adequate.

to the tapers suggested in Table 7.7. Parallel entrances and exits have a major In the case of the entrance ramp, the driver of

advantage over straight tapers in respect of the

the merging vehicle should be afforded suffi-

selection of curves on the ramps. In the case of

cient opportunity to locate a gap in the opposing

a design speed of 120 km/h, a straight taper

traffic and position the vehicle correctly to merge

provides a distance of just short of 300 metres between the Yellow Line Break Point and the

into this gap at the speed of the through traffic.

nose. A portion of this distance should ideally

Taper rates flatter than those proposed in Table

be traversed at the design speed of the freeway


7.7 should thus be applied to on-ramps. A taper

with deceleration commencing only after the

of 1:50 provides a travel time of about 13 sec-

vehicle is clear of the outside freeway lane.

onds between the nose and the point at which

About 220 metres is available for deceleration

the wheel paths of merging and through vehi-

and this suggests that a vehicle exiting at 120

cles would intersect. This has been found in

km/h is likely still to be travelling at a speed of

practice to provide sufficient opportunity to pre-

about 95 km/h when passing the nose. The first curve on the ramp could, allowing for superele-

pare to merge into gaps in the through flow

vation development to 10 per cent, be located

because drivers typically begin to locate usable

100 metres beyond the nose so that, at the start

gaps in the freeway flow prior to passing the

of the curve, the vehicle speed would be in the

nose.

range of 80 to 90 km/h. The first curve should thus have a radius of not less than 250 metres.

Parallel

In the case of the parallel ramp, the radius could obviously be significantly shorter, depending on

The parallel lane configuration is also initiated

the length of the deceleration lane preceding the

by means of a taper but, in this case, the taper

nose. 7-34


7.9

COLLECTOR - DISTRIBUTOR

7.10.1 Ramp metering

ROADS Ramp metering consists of traffic signals Collector-distributor roads are typically applied

installed on entrance ramps in advance of the

to the situation where weaving manoeuvres

entrance terminal to control the number of vehi-

would be disruptive if allowed to occur on the

cles entering the freeway. The traffic signals

freeway. Their most common application, there-

may be pretimed or traffic-actuated to release

fore, is at Cloverleaf interchanges. The exit and

the entering vehicles individually or in platoons.

entrance tapers are identical to those applied to

It is applied to restrict the number of vehicles

any other ramps. The major difference between

that are allowed to enter a freeway in order to

Cloverleaf interchange C-D roads and other

ensure an acceptable level of service on the

ramps is that they involve two exits and two

freeway or to ensure that the capacity of the

entrances in quick succession. The two exits

freeway is not exceeded. The need for ramp

are, firstly, from the freeway and, secondly, the

metering may arise owing to factors such as:

split between vehicles turning to the left and

•

Recurring congestion because traffic

those intending to turn to the right. The two

demand exceeds the provision of road

entrances are, firstly, the merge between the

infrastructure in an area;

•

two turning movements towards the freeway

Sporadic congestion on isolated sec-

and, secondly, the merge with the freeway

tions of a freeway because of short term

through traffic.

traffic loads from special events, normally of a recreational nature;

•

The distance between the successive exits

As part of an incident management sys-

should be based on signing requirements so as

tem to assist in situations where a acci-

to afford drivers adequate time to establish

dent downstream of the entrance ramp

whether they have to turn to the right or to the

causes a temporary drop in the capacity

left to reach their destination. Nine seconds is

of the freeway; and

•

generally considered adequate for this purpose

Optimising traffic flow on freeways

design speed of the freeway as they pass the

Ramp metering also supports local transporta-

nose of the first exit, the distance between the

tion management objectives such as:

•

noses should desirably be based on this speed.

Priority treatments with higher levels of service for High Occupancy Vehicles; and

The distance between successive entrances is

•

based on the length required for the accelera-

Redistribution of access demand to other on-ramps.

tion lane length quoted in Table 7.6. It is important to realise that ramp metering

7.10

OTHER INTERCHANGE DESIGN

should be considered a last resort rather than as

FEATURES

a first option in securing an adequate level of service on the freeway. Prior to its implementa-



and seeing that vehicles may be travelling at the

tion, all alternate means of improving the capac-

An express-collector system could, for example,

ity of the freeway or its operating characteristics

be started upstream of one interchange and run

or reducing the traffic demand on the freeway

through it and the following, possibly closely

should be explored. The application of ramp

spaced, interchange, terminating downstream

metering should be preceded by an engineering

of the second. The terminals at either end of the

analysis of the physical and traffic conditions on

express-collector system would have the same

the freeway facilities likely to be affected. These facilities include the ramps, the ramp terminals

standards as applied to conventional on- and

and the local streets likely to be affected by

off-ramps. The interchange ramps are connect-

metering as well as the freeway section

ed to the express-collector system and not

involved.

directly to the freeway mainline lanes.

The stopline should be placed sufficiently in

Traffic volumes and speeds on the express-col-

advance of the point at which ramp traffic will

lector roads are typically much lower than those

enter the freeway to allow vehicles to accelerate

found on the mainline lanes, allowing for lower

to approximately the operating speed of the

standards being applied to the ramp geometry

freeway, as would normally be required for the

of the intervening interchanges.

design of ramps. It will also be necessary to ensure that the ramp has sufficient storage to

The minimum configuration for an express-col-

accommodate the vehicles queuing upstream of

lector system is to have a two-lane C-D road on

the traffic signal.

either side of a freeway with two lanes in each direction. The usual configuration has more than two mainline lanes in each direction

The above requirement will almost certainly lead to a need for reconstruction of any ramp that is to be metered. The length of on-ramps is typi-

A similar configuration is known as a dual-divid-

cally determined by the distance required to

ed freeway, sometimes referred to as a dualdual freeway. In this case, the C-D roads are

enable a vehicle to accelerate to freeway

taken over a considerable distance and may

speeds. Without reconstruction, this could result

have more than two lanes. Furthermore, con-


in the ramp metering actually being installed at

nections are provided at intervals between the

the crossing-road ramp terminal.

outer and the core lanes along the length of the freeway. These, unfortunately, create the effect

7.10.2 Express-collector systems

of right-side entrances and exits to and from the outer lanes and are thus contrary to drivers'

Express-collector systems are used where traf-

expectations. The geometry of the situation is

fic volumes dictate a freeway width greater than

otherwise similar to that of the express-collector

four lanes in each direction. The purpose of the

system.

express-collector is to eliminate weaving on the mainline lanes by limiting the number of entrance and exit points while satisfying the demand for access to the freeway system.


TABLE OF CONTENTS 8

ROADSIDE SAFETY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1 8.1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1 8.1.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1 8.1.2 Safety objectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3 8.1.3 The "Forgiving Roadside" approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4 8.1.4 Design Focus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4 8.1.5 Roadside safety analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-5 8.1.6 Road safety audits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-6

8.2

ROADSIDE HAZARDS AND CLEAR ZONE CONCEPT . . . . . . . . . . . . . . . . . . . . . . . . . 8-6 8.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-6 8.2.2 Elements of the clear zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-8 8.2.3 Factors influencing the clear zone design domain . . . . . . . . . . . . . . . . . . . . . . . . . 8-9 8.2.4 Determining width of clear zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-10 8.2.5 Best practices in respect of roadside vegetation . . . . . . . . . . . . . . . . . . . . . . . . . 8-10

8.3

SIGN AND OTHER SUPPORTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-12 8.3.1 Basis for Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-12 8.3.2 Breakaway supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-13 8.3.3 Design and Location Criteria for Sign Supports . . . . . . . . . . . . . . . . . . . . . . . . . . 8-15 8.3.4 Design approach for lighting supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16

8.4

TRAFFIC SAFETY BARRIERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16 8.4.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16 8.4.2 Determining Need for Safety Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-17 8.4.3 Longitudinal roadside barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-18 8.4.4 Median barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-27

8.5

IMPACT ATTENUATION DEVICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-30 8.5.1 Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-30 8.5.2 Design/selection of impact attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-31 8.5.3 Functional considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-31 8.5.4 Sand-filled plastic barrel impact attenuators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-32

8.6

RUNAWAY VEHICLE FACILITIES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34 8.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34 8.6.2 Types of escape ramps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34 8.6.3 Criteria for provision of escape ramps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-35 8.6.4 Location of runaway-vehicle facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-36 8.6.5 Arrestor bed design features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-38

8.7

BRAKE CHECK AND BRAKE REST AREAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-39

LIST OF TABLES Table 8.1 Design elements that influence road safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3 Table 8.2 Clear zone distances (metres) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-11 Table 8.3: Roadside obstacles normally considered for shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-23 Table 8 4. Recommended minimum offset distances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-24 Table 8.5 Recommended maximum flare rates for barrier design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-25 Table 8.6 Recommended run-out lengths for barrier design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-26 Table 8.7 : Impact attenuator and end terminal application. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-31 Table 8.8 : Space requirements for plastic drum attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-32 Table 8.9: Length of Arrestor Bed (Over Uniform Gravel Depth) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-38

LIST OF FIGURES Figure 8.1: Roadside safety analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-7 Figure 8.2: Roadside recovery zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9 Figure 8.3: Adjustment for clear zones on curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-12 Figure 8.4: Breakaway supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-13 Figure 8.5: Classification of traffic barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-17 Figure 8.6: Classification of longitudinal barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-19 Figure 8.7: Warrants for use of roadside barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-22 Figure 8.8: Roadside barrier elements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-23 Figure 8.9: Length of need for adjacent traffic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-26 Figure 8.10: Length of need for opposing traffic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-26 Figure 8.11: Space requirement for plastic drum attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-32 Figure 8.12: Typical arrangement of sand-filled barrel attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-33 Figure 8.13: Typical arrestor beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34 Figure 8.14: Layout of arrestor bed adjacent to carriageway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-36 Figure 8.15: Layout of arrestor bed remote from carriageway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-37

Chapter 8 ROADSIDE SAFETY 8.1

INTRODUCTION

ing road safety and that, in the design of new roads or the upgrading of existing ones, particu-

8.1.1

General

lar attention should be given to safety as a prime design criterion.

Road crashes, to varying degrees, are caused A safe road should:

by defects attributable to the vehicle, the driver

•

or the road or by combination of these defects.

Warn and inform road users of changes in the approaching road environment;

In addition, a significant percentage of deaths

•

on roads in South Africa occur as a result of

Guide and control road users safely through the road environment;

•

pedestrians being on the road. The road, or more fully, the road environment, has been esti-

Provide a forgiving roadside environment;

•

mated to contribute to 28 per cent of all road

Provide a controlled release of information;

crashes in South Africa. Various studies have

•

indicated that up to 40 per cent or more of crash

Provide an aesthetically pleasing landscape;

reduction, which could reasonably be expected

•

on the road system, could accrue from the pro-

Maintain road user interest and concentration;

vision of safer roads. The cost of road crashes

• •

to society in South Africa exceeds the annual expenditure on roads, thus the expenditure of

Not surprise road users; Give consistent messages to road users; and

•

considerable sums of money can be justified in

Provide good visibility for all road users.

improved design appropriate and standards and

Numerous research projects have established

by catering for the presence of pedestrians on

relationships between crashes and geometric

roads. Crashes resulting from simply leaving

design elements (as well as operating speeds

the roadway regardless of the underlying cause,

and traffic volumes).

represent a substantial portion of the total road crash problem i.e. "run-off-the-road"

The various geometric design features of a

(ROR)

accidents account for 25 per cent of all road

road, shown in Table 8.1, affect safety by:

crashes in South Africa. They occur on both

•

Influencing the ability of the driver to

straight and curved sections of road and gener-

maintain vehicle control and identify

ally involve either rollover of the vehicle or colli-

hazards. Significant features include:

sion with fixed objects, such as trees, roadside structures etc.

Lane and shoulder width; Horizontal and vertical alignment;

It is thus obvious that the roadside environment and its design have a vital role to play in improv8-1

Chapter 8: Roadside Safety

Sight distance; Superelevation; and


reducing the crash rates on roads through

•

roadside design can mitigate the severity of a

Pavement surface and drainage.

Influencing the number and types of

crash. This interaction between road, driver and

opportunities that exist for conflict

vehicle characteristics unfortunately compli-

between vehicles. Significant features

cates attempts to estimate the accident reduc-

include:

•

o

Access control;

o

Intersection design;

o

Number of lanes;and

o

Medians.

tion potential of a particular safety improvement. The construction of a road is typically a trade-off between standards and the cost of providing

Affecting the consequences of an out-of-

them. High design standards might be expen-

control vehicle leaving the travel lanes.

sive to provide. However, the cost to society of

Significant features include:

road crashes and deaths often exceeds the total

o

Shoulder width and type;

annual expenditure on roads. Reducing initial

o

Edge drop;

o

Roadside conditions;

o

Side slopes; and

o

Traffic barriers.

construction (or capital) costs of road projects can result in increased life cycle costs if the cost of accidents, injuries and deaths is included in the economic calculations. It is the design engineer's responsibility to inform the client of the

In addition to geometric features, a variety of

consequences of inadequate expenditure on

other factors affect road safety, including other

safety.

elements of the overall road environment, such as:

• • • • • •


•

Pavement condition;

It is often extremely difficult, if not impossible, to

Weather;

correct safety defects at a later stage without

Lighting;

major reconstruction. For this reason, designing

Traffic flows;

for safety should occur at the outset, or be pro-

Traffic regulation;

vided for in stage construction drawings. Road

Presence of pedestrians; o

Intoxication and

o

Age;

safety audits on the design, carried out by an independent person or team should take place at various stages in the project, as provided for

Vehicle characteristics, such as: o

Size;

in Volume 4 of the South African Road Safety

o

Mass; and

Manual. Although this design guide focuses on

o

Braking capability.

road design features, the psychological aspects of driver behaviour are always present. An error

The effect of road design is somewhat obscured

in perception or judgement or a faulty action on

by the presence of these extraneous factors and

the part of the driver can easily lead to a crash.

most accidents result from a combination of factors interacting in ways that prevent a single fac-

Roads should be designed in such a manner

tor being identified as the cause of a crash.

that only one decision at a time is required from

However, even when a vehicle leaves the road

a driver, ensuring that he/she is never surprised

owing to driver error or mechanical failure, good

by an unexpected situation and that adequate 8-2


8.1.2

time is provided to make the decision.

Safety objectives

Research has shown that the number of accidents increases as the number of decisions required

Broadly, there are three avenues of action pos-

by the driver increases.

sible for mitigating road crashes and their con-

cussed in more detail in chapter 3 of this docu-

sequences;

ment, as well as in Volume 1 of the South

•

By reducing the possibility that run-off will occur;

African Road Safety Manual.

•

Once the run-off does occur, by providing opportunities for the driver of the

Standardisation in road design features and traf-

vehicle to recover and return to the road

fic control devices plays an important role in

without incident, and

•

reducing the number of required decisions, as

If a crash does occur, by providing design elements to reduce the severity

the driver becomes aware of what to expect on

of that collision.

a certain type of road. However it should be noted that standardization alone does not necessarily ensure a safe facility, hence the require-

The first of these areas of endeavour is dealt

ment for a safety audit of the design.

with by incorporating features in the overall 8-3 Chapter 8: Roadside Safety


This matter is dis-

design of the road which will reduce the possi-

ophy

and

approaches.

Transportation

bility that run-off will occur. These features are

Research Circular 435 states that:-

influenced to a large extent by the selected "Basically a forgiving roadside is one free of

design speed.

obstacles which could cause serious injuries to occupants of an errant vehicle. To the extent

This chapter deals with the latter two of these

possible, a relatively flat, unobstructed roadside

three possibilities. It provides the designer with

recovery area is desirable, and when these con-

guidance on the design of roadside environment

ditions cannot be provided, hazardous features

that includes elements to allow for recovery on

in the recovery area should be made breakaway

the part of the driver, as well as features which

or shielded with an appropriate barrier".

are intended to reduce the severity of such accidents as do occur.

When a vehicle leaves the traffic lane, the path of the vehicle and any object in or near that path

More specifically, it is recommended that the fol-

become contributing factors to the degree of

lowing safety objectives be adopted when a

severity of the crash. By designing a forgiving

road is designed:

• • •

Separate potential conflict points and

roadside the severity of crashes can be

reduce potential conflict areas;

reduced. The concept of designing a forgiving

Control the relative speeds of the con-

roadside should not be regarded as a by-prod-

flicting vehicles;

uct of the application of safety criteria to each

Guide the driver through unusual sec-

design element, but as an integral part of the

tions;

•

total engineering for the road.

Ensure that the needs of pedestrians and cyclists (if relevant) are also consid-

The need for a forgiving roadside is paramount

ered;


•

Provide a roadside environment that for-

on the outside of horizontal curves with radii of

gives a driver's errant or inappropriate

less than 1 000 metres, where the possibility of

behaviour, by attention to details such

an errant vehicle running off the road is great-

as the safe placement of roadside furni-

est.

ture and by the location and selection of

that horizontal curves with radii in excess of

types of traffic barrier.

However, this statement does not imply

1 000 metres are always safe since the phenomena of "risk adaptation", whereby drivers

8.1.3

The "Forgiving Roadside" approach

concentrate less and drive at higher speeds on sections of road they consider to be safe, should

The forgiving roadside concept, coined in the

be taken into account.

1960's, relates to the approach of making provision for errant vehicles leaving the roadway by

8.1.4

Design Focus

incorporating design elements that reduce the consequences of such departure. This concept

The focus of design measures outlined in this

is an integral part of modern road design philos-

chapter is primarily one of improving road safe8-4


ty through roadside hazard management by the

ments and other aspects of the facility design,

design and provision of appropriate recovery

and between the road itself, the driver and the

and protection measures. The effectiveness of

vehicle. As a result, information touching on

road safety features depends greatly on five

road design issues necessarily is available from

aspects;

many sources. Designers should not rely on

•

Knowledge of the safety characteristics

this Guide as the sole source of information on

and limitations of roadside features by

roadside design issues, particularly when deal-

the designer (and the maintenance per-

ing with unusual or local conditions that depart

sonnel);

from generally accepted situational norms.

The correct choice of appropriate treat-

Particular attention should be paid to the South

ment;

Africa Road Safety Manual produced by the

• •

The correct installation of the roadside

South African Committee of Land Transport

safety features;

•

Officials (COLTO).

The maintenance of the roadside safety features and roadside environment; and

•

8.1.5

Regular monitoring of installations to

Roadside safety analysis

ensure they perform adequately. The design of the roadside environment is a complex problem.

Evaluation of alternative

There are two needs that are the key to effective

designs and choosing between them are difficult

attention to safety in the roadside design

tasks, which involve

process.

•

the occurrence of crashes;

The need for explicit evaluation of

•

design trade-offs with an impact on road safety.

severity; and

In the traditional design process, attention to

•

safety has usually been implicit, not explicit.

the real costs of the property damage, injuries, and fatalities which can result.

The common myth among designers is that if current "standards" are met, then the road is

Nonetheless, such analysis which provides an

safe. The reality is that road design "standards"

explicit framework for considering design trade-

are often no more than a limit : one should not

offs is a much more desirable approach to road-

provide less than the standard stipulates but,

side safety design than meeting arbitrary "stan-

within limits, to provide more is often better.

dards" whose underpinnings may or may not be

Furthermore, just meeting the standard does not

appropriate to a given situation. Figure 8.1 illus-

mean that an appropriate amount of safety has

trates an algorithm for conducting a roadside

been provided. 2.

the outcome of crashes in terms of

safety analysis.

The need to recognize that the design

of the roadside environment is a highly complex and probabilistic process. There are many lev-

The process is generally based on two funda-

els of interaction between different roadside

mental models.

•

design components, between roadside ele-

Predictive models that provide a way of

8-5 Chapter 8: Roadside Safety


1.

degrees of uncertainty with respect to

•

estimating collision frequencies and

cally investigates the roadside safety of a partic-

severities under a wide variety of condi-

ular project, has a proven potential to improve

tions; and

the safety of both proposed and existing facili-

Cost-effectiveness models that provide

ties.

a way of quantifying the life-cycle costs (and benefits) associated with any given

Road safety audits, especially during the design

set of safety measures.

stage, create the opportunity to eliminate, as far Predictive models have been developed and

as possible, road safety problems in the provi-

deployed by a number of agencies in North

sion of new road projects. They should be seen,

America.

however, as part of the broader scope of the phi-

Although

the

latest AASHTO

Roadside Design Guide probably represents the

losophy of roadside hazard management.

most current and widely accepted effort in this regard, designers should be aware that the state

A road safety audit is a formal examination of

of the art in this area is continually developing

any road project which interacts with road users,

and should be monitored regularly for new mod-

in which a qualified and independent examiner

els and techniques which may have application

reports on the projects accident potential and

to their design challenges.

safety performance. The audit may be conducted at the project's:

The techniques of cost-effectiveness analysis

• • • • •

are well established and are applied for a variety of purposes in transportation and highway design agencies.

A number of alternative

approaches are available but, most commonly,

Feasibility stage; Draft design stage; Detailed design stage; Pre-opening stage; and On existing roads.

the tools used by transportation agencies are built on life-cycle costing models and use pres-

The earlier a road is audited within the design

ent worth or annualised cost techniques as their

and development process, the better.


underlying analysis methodology.

All these

approaches are built on fundamental assump-

The subject is dealt with more fully in Chapter

tions regarding parameters such as discount

2.5 of this document, and particularly in Volume

rates and unit crash costs. In order to enforce

6 of the South African Road Safety Manual.

consistent and comparable results across the

8.2

road authority, these basic assumptions are

ROADSIDE HAZARDS AND CLEAR ZONE CONCEPT

usually set as a matter of policy and represent a "given" for designers to use in their analyses.

8.2.1 8.1.6

Overview

Road safety audits Research has shown that in 50 per cent of all

First developed in the United Kingdom, Australia

run-off-the-road accidents the vehicle leaves the

and New Zealand, this process, which specifi-

road in a skidding manner. Roadside hazards 8-6


•

can significantly increase the severity of crashes and it is necessary to manage roadside haz-

signage;

•

ards in such a manner as to decrease the severity of these crashes.

Drainage structures such as culverts, drains, drop inlets;

Figure 8.1 illustrates this

• • • •

process. Common existing roadside hazards include:

•

Supports and poles (for lighting, utilities,

Trees;

Bridge abutments/piers; Side slopes such as embankments; Ends of traffic barriers, bridge railings; Incorrectly positioned traffic barriers, i.e. <3 metres off the roadway;



Figure 8.1: Roadside safety analysis

• • •

Obsolete roadside furniture;

The concept originated in the United States in

SOS call boxes; and

the early 1960's and has progressively been

Fire hydrants

refined and updated.

The clear zone width

varies between 4.0 and 10 metres with the Accidents involving roadside objects are signifi-

upper end of the scale being more appropriate

cant for both the urban and rural road environ-

for high-speed National Roads.

ments. In South Africa, approximately 25 per

studies have found that the first 4,0 - 5,0 metres

cent of all accidents involve vehicles running off

provide most of the potential benefit from clear

the road.

zones.

In 1996 alone, the accident costs

More recent

related to fixed object accidents amounted to R3,5 billion (1997 Rand).

8.2.2

It is not feasible to provide sufficient width adja-

Elements of the clear zone

The clear zone falls within an area called the

cent to the carriageway that will allow all errant

recovery zone. The recovery zone is the total

vehicles to recover. Therefore it is necessary to

unobstructed traversable area available along

reach a compromise or level of risk manage-

the edge of the road and, by convention, it is

ment. The most widely accepted form of risk

measured from the edge of the closest travel

management for roadside hazards is the 'clear

lane. The recovery zone may have recoverable

zone concept'. The clear zone is the horizontal

slopes, non-recoverable slopes and a clear run-

width (measured from the edge of the traffic

out area.

lane) that is kept free from hazards to allow an errant vehicle to recover. The clear zone is a compromise between the recovery area for

Figure 8.2 illustrates the clear zone concept in

every errant vehicle, the cost of providing that

the context of the roadside recovery zone.

area and the probability of an errant vehicle encountering a hazard. The clear zone should

Recoverable slopes are those on which a driver

be kept free from non-frangible hazards where

may, to a greater or lesser extent, retain or

economically possible; alternatively, hazards

regain control of a vehicle. A non-recoverable


within the clear zone should be protected. The

slope may be traversable, but a vehicle will con-

clear zone width is dependent on:

• • • •

tinue to the bottom. A clear run-out area is locat-

Speed;

ed at the toe of a non-recoverable slope, and is

Traffic volumes;

available for safe use by an errant vehicle.

Side slopes; and

There is also provision for a smooth transition

Horizontal geometry.

It should be noted that the clear zone width is

between slopes to allow for the safe passage of

not a magical number and, where possible, haz-

vehicles.

ards beyond the desirable clear zone should be minimized.

The clear zone is the total, fixed-object-free area available to the errant vehicle.

The design

Clear zone widths vary throughout the world

domain for the clear zone width has been found

depending on land availability and design policy.

to depend on traffic volume and speed, road 8-8


Figure 8.2: Roadside recovery zone geometry, embankment height, side slope and

80 per cent of the vehicles leaving a roadway

environmental conditions such as rain, snow,

out of control to recover….."

ice, and fog. The wider the clear zone, the less the frequency and severity of collisions with

The last portion of this statement requires

fixed objects. However, there is a point beyond

emphasis. Provision of the recommended clear

which any further expenditure to move or pro-

zone does not guarantee that vehicles will not

tect the fixed objects is not warranted because

encroach further than the recommended clear

the marginal risk reduction is too small.

zone distance.

Quite the contrary, the clear

zone principle embodies the explicit fact that

Factors influencing the clear zone

some portion of the vehicles that encroach will

design domain

go beyond the clear zone itself.

When originally introduced, the clear zone con-

Steeper embankment slopes tend to increase

cept dictated a single value of 9 metres and was

vehicle encroachment distances.

based on limited research. The concept was

on low-volume or low-speed facilities, the 9,0 m

formally introduced in the 1974 version of the

distance was found to be excessive and could

AASHTO report entitled Highway Design and

seldom be justified. As a result, as the concept

Operational Practices Related to Highway

evolved, design practice moved to a variable

Safety where the authors noted:

clear zone distance definition and a better

Conversely,

understanding of the wide range of factors that "…for adequate safety, it is desirable to provide

influence the limits of its design domain was

an unencumbered roadside recovery area that

gained.

is as wide as practical on a specific highway section.

Studies have indicated that on high-

The approach set out in paragraph 8.2.4 below,

speed highways, a width of 9 metres or more

and borrowed from Canadian practice reflects

from the edge of the travelled way permits about

the influence of: 8-9



8.2.3

• • • • •

Design speed;

For sections of road with horizontal curvature,

Traffic volumes;

these distances should be increased on the out-

The presence of cut or fill slopes;

side of curves by a factor that depends on the

The steepness of slopes; and

operating speed and the radius of the curve.

Horizontal curve adjustments.

Figure 8.3 provides guidelines on adjustment factors for clear zones on the outside of curves.

Designers should, however, recognize the limitations of the figures presented below. AASHTO

8.2.5

provides a caution to designers on the issue:

Best practices in respect of roadside vegetation

"…..the numbers obtained from these curves represent a reasonable measure of the degree

Single-vehicle collisions with trees account for a

of safety suggested for a particular roadside; but

sizeable proportion of all fixed object collisions.

they are neither absolute nor precise. In some

Unlike typical roadside hardware, with the

cases, it is reasonable to leave a fixed object

exception of landscaping, trees are not a design

within the clear zone; in other instances, an

element over which the designers have direct

object beyond the clear zone distance may

control. While policies and approaches vary by

require removal or shielding. Use of an appro-

agency, a number of best practices are present-

priate clear zone distance amounts to a com-

ed here to assist the designer in dealing with

promise between safety and construction

this complex and important issue.

costs." Depending on their size, trees within the clear

8.2.4

zone constitute a serious hazard. Generally, a

Determining width of clear zone

tree with a trunk diameter greater than 150 mm is considered a fixed object.

Table 8.2 provides an indication of the appropri-


ate width of a clear zone on a straight section of road, measured in metres from the edge of the

When trees or shrubs with multiple trunks, or

lane, according to design speed, traffic volumes

groups of small trees are close together,

and cut or fill slope values. The values in Table

because of their combined cross-sectional area,

8.2 are taken from the 1996 AASHTO Roadside

they may be considered as having the effect of

Design Guide, and suggest only the approxi-

a single tree

mate centre of a range to be considered and not a precise distance, since, in making their choice,

Typically, large trees should be removed from

designers should also consider specific site con-

within the selected clear zone for new construc-

ditions.

tion and reconstruction projects.

Segments of

a highway can be analysed to identify groups of Where side slopes are steeper than 1 : 4 (i.e.

trees or individual trees that are candidates for

non-trafficable) designers should give consider-

removal or shielding.

ation to the provision of a protection barrier. 8-10 Chapter 8: Roadside Safety

cle/tree collision risk although some isolated

public resistance, it will reduce the severity of

trees may be candidates for removal if they are

any crashes.

noticeably close to the roadway. If a tree or group of trees is in a vulnerable location but

Tree removal often has adverse environmental

cannot be removed, traffic barriers can be used

impacts. It is important that this measure only

to shield them.

be used when it is the only solution. For example, slopes of 1:3 or flatter may be traversable

Maintenance of the roadside plays an important

but a vehicle on a 1:3 slope will usually reach

role in helping to control vegetation and tree

the bottom. If there are numerous trees at the

problems by mowing within the clear zone and

toe of the slope, the removal of isolated trees on

eliminating seedlings before they create a haz-

the slope will not significantly reduce the vehi-

ard. 8-11



While tree removal generally generates some

Figure 8.3: Adjustment for clear zones on curves 8.3

SIGN AND OTHER SUPPORTS

poles located on the outside of horizontal curves and adjacent to pavements with low skid resist-

8.3.1

Basis for Design

ance pavements is greater than at other sites.

Although the objective of roadside design is to

These poles can be treated in a number of

provide an adequate clear zone to allow errant

ways, namely:

vehicles to recover without a crash, this is not

•

always possible. For various reasons, including

(this can include moving a lighting pole

traffic operation, certain obstacles may have to

to the inside of a horizontal curve rather

remain within the clear zone.

than on the outside)

•


These obstacles include:

• • • •

By relocating them to a safer location

Removal of some of the poles by:

Traffic signposts; Utility poles;

Increasing the pole spacing; The combining of a number of utilities or signs per pole; or

Roadway illumination features; and Structures, including headwalls of

•

drainage structures.

Installing underground cables;

Shielding the utility poles with an appropriate traffic barrier system and provision of a proper end-treatment;

Collisions with sign and lighting supports consti-

•

tute a significant portion of all vehicle crashes

If appropriate, installation of a break away device;

and thus merit serious attention. The roadside

•

hazard danger associated with utility and sign-

Providing a high skid resistance surfacing on curves; and

•

post poles increases with an increase in traffic

Attaching delineators to the device to

flow, pole density (poles per km of road) and the

increase its visibility if no other measure

offset from the edge of the road. The hazard of

can be implemented.


injured by the yielding support(s). It is therefore

Figure 8.1 illustrates the proposed methodology.

important that the designer should consider the

8.3.2

Breakaway supports

relative risks related to each location before a design is selected.

Definition The term "breakaway support", developed in the

A breakaway support is designed for loading in

late 1960's, refers to all types of signs, lumi-

shear and normally for impact at bumper height

naires and traffic signal supports that are safely

(typically 500 mm above ground level). It is crit-

displaced under vehicle impact, whether the

ical that the support be properly installed as to

release mechanism is a slip plane, plastic hinge,

ensure that loading takes place at the correct

fracture element or a combination of these.

height. Loading above the design height may cause the breakaway device to fail to activate

The AASHTO Standard Specifications for

because the bending moment in the breakaway

Structural

support may be sufficient to keep the support in

Supports

for

Highway

Signs,

Luminaires and Traffic Signals specifies that all

place.

supports located in the clear zone widths of

when the support is installed close to ditches or

high-speed facilities should be equipped with a

steep slopes, causing a vehicle to become air-

breakaway device unless they are protected

borne and hit the support at the wrong position.

Incorrect loading can also take place

of urban or low speed facilities, the use of break-

The soil type of the breakaway system is impor-

away devices is not advisable.

The AASHTO

tant as it may also affect the activation of the

Specifications seem to be inadequate as an

mechanism. In the case of fracture-type sup-

occupant can sustain serious injuries when

ports such as high carbon U-channel posts, tele-

striking the car interior during a vehicle impact at

scoping tubes and wood supports, the supports

40 km/h with a non-yielding object. The reason

may slip through saturated or loose soil during

for this guideline, however, is that there is a

impact, absorbing energy and changing the

probability of cyclists and pedestrians being

breakaway mechanism.

Figure 8.4: Breakaway supports 8-13 Chapter 8: Roadside Safety


with a suitable traffic barrier system. In the case

If a sign support is installed at a depth less than

Luminaires and Traffic Signals, should be used

1 metre, it will pull out of the soil during impact.

to determine whether a sign support conforms

Installations with anchor plates or those

to the criteria for a breakaway support.

installed deeper than 1 m are particularly sensiThe broad criteria which breakaway supports

tive to the foundation conditions. For small sign

should meet include:

supports using base-bending or yielding mecha-

•

nisms, the performance of the supports in strong

Dynamic performance criteria, i.e. implicit velocity breakaway thresholds;

soils is more critical.

•

Maximum remaining stub height of 100mm;

•

The maintenance requirements are critical in the

The need for the vehicle to remain

selection of a particular breakaway device. The

upright during and after the collision;

following maintenance requirements should be

and

•

considered:

No significant deformation of the vehicle or intrusion into the passenger compart-

(a)

ment during or after impact.

The availability of breakaway devices

will influence the costs associated with installa-

Non- breakaway sign supports

tion and maintenance or replacement after impact. An installation that can be reused can

The first requirement for sign supports is the

be more cost-effective than mechanisms that

need to structurally support the devices that are

have to be replaced.

mounted upon them. Signs and other devices should be carefully placed in order to minimize


(b)

The durability of a support is important

the hazard that they can represent to motorists.

as it will determine the life span of a support that

The following practices should be borne in mind

is not struck as compared to that of a non-break-

by designers when developing signing plans for

away support.

their projects:

(c)

(a)

A breakaway device yields when hit if it

Sign supports should not be placed in

is properly installed and maintained. The mech-

drainage ditches, where erosion might affect the

anism should then be replaced or repaired.

proper operation of breakaway supports.

Consequently, the availability of material, main(b)

tenance personnel and availability of personnel

Wherever possible, signs should be

placed behind existing roadside barriers

after an impact for each breakaway design

(beyond the deflection distance), on existing

should influence the selection thereof.

structures, or in non-accessible areas. If this cannot be achieved, then breakaway supports

Acceptance criteria for breakaway supports

should be used. (c)

The AASHTO guide, Standard Specification for Structural

Support

for

Highway

Only when the use of breakaway sup-

ports is not practicable should a traffic barrier or

Signs,

crash cushion be used to shield sign supports. 8-14


8.3.3

Design and Location Criteria for Sign

The design requirements for breakaway support

Supports

systems for roadside signs are documented in a number of publications.

The South African

Roadway signs fall into three primary classes:

Road Safety Manual should be used as the

overhead signs, large roadside signs, and small

basis for the design of these.

roadside signs.

Small road side signs

Overhead Signs

Small roadside signs are supported on one or more posts and have a sign panel area of less

Since overhead signs, including cantilevered

than 5,0 square metres. Although not perceived

signs, require massive support systems that

as significant obstacles, small signs can cause

cannot be made breakaway, they should be

serious damage to impacting automobiles, and

installed on or relocated to nearby overpasses

wooden posts should be used as far as possi-

or other structures, where possible.

ble.

All overhead sign supports located within the

The bottom of the sign panel should be a mini-

clear zone should be shielded with a crashwor-

mum 2100 mm above ground and the top of the

thy barrier. In such instances, the sign gantry

panel should be a minimum 2700 mm above

should be located beyond the design deflection

ground to minimize the possibility of the sign

distance of the barrier.

panel and post rotating on impact and striking the windshield of a vehicle.

Large Roadside Signs The requirements for breakaway support systems for roadside signs are documented in a

Large roadside signs are generally greater than

number of publications.

5,0 square metres in area. Typically, they have

The South African

Road Safety Manual should be used as the

two or more supports that are breakaway. The hinge for breakaway supports on large

Consideration to various factors should be given

roadside signs should be at least 2100 mm

when selecting, designing and locating break-

above ground, so that the likelihood of the sign

away and other supports. These include:

or upper section of the support penetrating the

• • •

windshield of an impacting vehicle is minimized. The required impact performance is shown in

Road environment : urban or rural; Terrain where device is installed; Proximity to drainage ditches or structures;

Figure 8.4.

•

Soil type used as a base for the breakaway support;

No supplementary signs should be attached

•

below the hinges if their placement is likely to

Maintenance requirements of the support (i.e., the simplicity of maintenance,

interfere with the breakaway action of the sup-

availability of material and the durability

port post or if the supplementary sign is likely to

of the support); and

•

strike the windscreen of an impacting vehicle. 8-15 Chapter 8: Roadside Safety

Expected impact frequency.


basis for design.

8.3.4

Design approach for lighting sup-

barrier and if it is within the design deflection

ports

distance of the barrier, it should be either a breakaway design, or the barrier should be

Lighting supports should be of the frangible

strengthened locally to minimize its deflection.

base, slip base or frangible coupling type. They are designed to release in shear when hit at a

Higher mounting heights can reduce the number

typical bumper height of about 500 mm.

of lights needed on a facility. High mast lighting - which requires far fewer supports located

As long as the side slopes between the roadway

much further from the roadway - can be benefi-

and the luminaire support are 6:1 or flatter, vehi-

cial. While consideration of this approach is rec-

cles should strike the support appropriately, and

ommended to designers, the massive nature of

breakaway action can be assured.

these high mast structures requires analysis and planning in the design and placement of the

Superelevation, side slope, rounding and vehi-

high mast supports.

cle departure angle and speed will influence the striking height of a typical bumper. Designers should consider this fact when developing illumination plans for their projects.

8.4

TRAFFIC SAFETY BARRIERS

8.4.1

Overview

Traffic safety barriers are systems utilized to As a general rule, a lighting support will fall near

shield road users from potential hazards along-

the line of the path of an impacting vehicle.

side the travelled way and should be able to

Designers should be aware that these falling

redirect or contain:

poles represent a threat to bystanders such as

•

An errant vehicle without imposing intolerable vehicle occupant forces;

pedestrians, bicyclists and uninvolved motorists.

•

Vehicles in range of sizes, weights and designs; or

•

Poles with breakaway features should not exceed 17 m in height - the current maximum

An errant vehicle over a range of impact speeds and impact angles.


height of accepted hardware. Traffic barriers are obstacles on the roadside and vehicles striking barriers can cause occu-

The mass of a breakaway lighting support

pant injury and/or vehicle damage. A traffic bar-

should not exceed 450 kg.

rier should be installed only if it is likely to reduce the severity of potential collisions. It is

Foundations for lighting supports should be

therefore of the utmost importance that, in

designed with consideration being given to the

selection of the traffic barrier, due cognisance

surrounding soil conditions that could influence

be taken of the characteristics of the particular

the effectiveness of the breakaway mechanism.

barrier system. Barrier systems differ not only in purpose but also in terms of deflection and redirecting properties.

When a lighting support is located near a traffic 8-16


Traffic barriers are either classified as being

Although this approach can be used, there are

impact attenuation devices or longitudinal barri-

often instances where the distinction between

ers.

the two conditions is not immediately obvious.

The purpose of an impact attenuation device is

In addition, this approach does not allow for

to cause a vehicle to decelerate and come to a

consideration of the cost-effectiveness of treat-

halt.

ment or non-treatment.

A longitudinal traffic barrier redirects a

vehicle parallel to the roadway. In recent years, techniques have been developed which allow warrants for barrier installation

Figure 8.5 shows a functional classification of

to be established on the basis of a benefit cost

traffic barriers.

analysis in which such factors as design speed,

8.4.2

traffic volume, installation and maintenance

Determining Need for Safety

costs, and collision costs are taken into consid-

Barriers

eration. Barriers are installed on the basis of warrant

Typically, such an approach is used to evaluate

analysis.

three options:

•

been based on a subjective analysis of certain

The removal or alteration of the area of

Figure 8.5: Classification of traffic barriers roadside elements or conditions within the clear

concern so that it no longer requires

zone. If the consequences of a vehicle running

shielding;

•

off the road and striking a barrier are believed to

The installation of an appropriate barrier; or

be less serious than the consequences if no

•

barrier existed, the barrier is considered war-

Leaving the area of concern unshielded (usually only considered on low-volume

ranted.

and/or low-speed facilities).



Traditionally, these warrants have

Once a barrier is found to be necessary for a

a fixed object within the clear zone that is con-

embankment, it should be provided over the

sidered to be a hazard and cannot be removed,

entire length of the embankment and not simply

relocated, made breakaway, or adequately

terminated when the embankment height

shielded by a longitudinal barrier.

becomes less than the warranted height. In considering the use of traffic barriers, designBarrier warrants for roadside obstacles are

ers should note that, even when these are prop-

based on their location within the clear zone and

erly designed and constructed, they might not

are a function of the nature of the obstacle, its

protect errant vehicles and their occupants com-

distance from the travelled portion of the road-

pletely. After installation of these, the severity of

way and the likelihood that it will be hit by an

collisions generally decreases but, as the num-

errant vehicle.

ber of installations increases, the frequency of minor collisions may also increase. For this rea-

Conventional criteria used for embankments

son, where cost-effective, the designer should

and roadside hazards are not usually applicable

make every effort to design without traffic barri-

to the pedestrian/bicyclist case, and these are

ers. This can be done by clearing the roadside

usually resolved through a careful individual

of obstacles, flattening embankment slopes and

evaluation of each potential project.

introducing greater median separation where possible.

As with roadside barriers, warrants for median

whilst a particular barrier system is chosen

barriers have been established on the basis that

based on the containment level required, regu-

a barrier should be installed only if the conse-

lar monitoring is essential to allow the system to

quences of striking the barrier are less severe

be replaced by a more adequate one if experi-

than the consequences that would result if no

ence indicates the need for this.

barrier existed. The primary purpose of a median barrier is to prevent an errant vehicle from

8.4.3

crossing a median on a divided highway and encountering oncoming traffic.

Longitudinal roadside barriers

As such, the Classification and performance characteristics

development of median barrier warrants has


It should be noted, however, that,

been based on an evaluation of median crossover collisions and related research stud-

Figure 8.6 shows the classification of longitudi-

ies.

nal barriers based on their deflection character-

In determining the need for barriers on

medians, median width and average daily traffic

istics.

It should be noted that the deflection

volumes are the basic factors generally used in

characteristics of a barrier system are not an

the analysis. However the incidence of illegal

indication of its effectiveness or safety.

cross-median movements may also justify the Misconceptions exist regarding the advantages

use of median barriers.

of the different longitudinal barrier types. Some Warrants for implementing impact attenuation

engineers firmly believe that one system is bet-

divides (crash cushions) are based on shielding

ter than another based on its deflection charac8-18


Figure 8.6: Classification of longitudinal barriers teristics. The deflection characteristics of a par-

2)

ticular system are not a measure of its effective-

lateral barrier deflections, but higher vehicle

ness. The mechanisms by which a vehicle is

deceleration rates. These barrier systems have

restrained after impacting a traffic barrier differ

application in areas where lateral restrictions

completely depending on the type of barrier

exist and where anticipated deflections have to

selected. The reaction of a vehicle on impact

be limited.

with different types of barriers is thus also differ-

post-and-beam system and have design deflec-

ent.

tions ranging from 0,5 to 1,7 metres.

They usually consist of a strong

Rigid systems, usually taking the form

In accomplishing their task of guiding and redi-

of a continuous concrete barrier. These tech-

recting impinging vehicles, a longitudinal barrier

nologies result in no lateral deflection, but

should balance the need to prevent penetration

impose the highest vehicle deceleration rates.

of the barrier with the need to protect the occu-

They are usually applied in areas where there is

pants of the vehicle. Various barrier technolo-

very little room for deflection or where the penal-

gies achieve this in various ways and can be

ty for penetrating the barrier is very high.

grouped into three distinct types:

Numerous shapes are available, including a

1)

high version for use where there is a high per-

Flexible systems, resulting in large lat-

eral barrier deflections, but the lowest vehicle

centage of trucks.

deceleration rates. Such systems have application in places where a substantial area behind

Designers should familiarize themselves with,

the barrier is free of obstructions and/or other

and design to, the specific performance charac-

hazards within the zone of anticipated lateral

teristics of their selected or candidate technolo-

deflection. These barriers usually consist of a

gies.

weak post-and-beam system, and their design Selection guidelines

deflections are typically in the range of 3,2 metres to 3,7 metres.but can be as low as1,7 metres.

Roadside barriers may be subjected to a wide 8-19 Chapter 8: Roadside Safety


3)

Semi-rigid systems, providing reduced

range of impacts by errant vehicles and provide

The requirements of NHCRP Report 350 should

a wide range of protection to the occupants of

be regarded as the minimum.

such vehicles.

It is therefore necessary to

determine the level of protection that they will

Performance capability

provide. The "design conditions" for a particular barrier The procedures and criteria for assessing the

need to be assessed carefully because areas

safety performance of traffic barriers and other

with poor geometrics, high traffic volumes, high

features have been standardized in the USA

speeds and a large proportion of heavy vehicles

through the publication of various National Co-

might not be consistent with the "conditions"

operative

Program

assumed when the barrier had been tested.

(NCHRP) Reports. The current test battery is

Such sites might require barriers with a higher

described

than normal performance level.

Highway in

Research

NCHRP

Report

350

"Recommended Procedures for the Safety Evaluation Performance of Highway Features"

Site conditions

(and includes various test levels defined by the size of the test (design) vehicle, impact speed

Site conditions play a major role in the selection

and angle). This provides the designer with the

of appropriate barriers. The slope approaching

opportunity to match test conditions with the

a flexible barrier should, for example, not

anticipated operational conditions on the road.

exceed 10 per cent and rigid barriers should not be used where the expected impact angle is

In selecting an appropriate traffic barrier it is

large. Narrow fill sections could result in condi-

essential that designers have a good under-

tions where post spacing and post support might

standing of the protection level expected from

be inadequate to allow them to perform as

the barrier and they should note that, if they

intended.

choose a particular system that is inadequate, they might make themselves, or their agency,

A number of site-specific aspects will have a


liable for damages.

major influence on the selection of a particular type of barrier to meet the performance require-

The following factors should therefore be seri-

ments at that location. These aspects include:

ously considered before a particular barrier is

•

Compatibility.

selected:

All barriers are subject to damage and

• •

Performance capability;

require intermittent maintenance.

Site conditions;

Keeping the number of different barrier

o

Compatibility;

types to a minimum therefore simplifies

o

Life cycle costs;

maintenance. Special barrier designs

o

Maintenance;

should only be considered when site or

o

Aesthetics; and

operational conditions cannot be satis-

o

Field experience.

fied with the standard barrier.


•

Warrants for use

Life cycle costs. It is prudent to realize that any barrier system accrues costs throughout its

Roadside hazards that warrant shielding by bar-

life. High initial costs could mean low

riers include embankments and roadside obsta-

maintenance costs, whilst low initial

cles.

costs could mean higher maintenance

embankments generally use embankment

costs. In addition, expected accident

height and side slope as the parameters in the

costs should be considered in the cal-

analysis and essentially compare the collision

culation of life cycle costs.

•

Warrants for the use of barriers on

severity of hitting a barrier with the severity of

Maintenance.

going down the embankment.

Most systems require very little routine

Figure 8.7,

When the barrier has

adapted from Australian guidelines, provides

been involved in a crash, the subse-

guidance for the installation of such barriers on

quent rehabilitation costs may be signif-

embankments.

maintenance.

icant, to the point of being excessive in the case of a high accident location. It

However such warrant procedures are regarded

should be noted that only material

as less than adequate because they do not take

specifically designed for that particular

into account the probability of a crash occurring

system should be used for mainte-

•

nance, and the tendency to "mix and

against the barrier or the cost of installing a bar-

match" should be avoided.

rier versus leaving the slope unprotected. The

Aesthetics.

development of cost analysis techniques pro-

It is important to realize that all traffic

vides the designer with an approach to

barriers are visual obstructions. Should

analysing the need for roadside embankment

this become a particular concern it is

protection barriers. In South Africa, however,

necessary to ensure that alternative

there is a lack of reliable data to carry out such

systems that may be considered are

analyses and it is necessary for the designer to

able to meet the performance require-

make site-specific analyses, using Figure 8.7 as

ments. Aesthetics should, under no circumstances,

be

given

a guide.

preference

•

Field experience.

The significance of this figure is that it provides

Site personnel's' experience of the per-

a range of values of fill slope for which, at cer-

formance,

maintenance

tain heights of fill, a barrier may be more or less

requirements of installed systems as

hazardous than the embankment it protects.

cost

and

well as the traffic police services' expe-

For example, at a fill height of 6 metres, a fill

rience of the performance of particular

slope steeper than 1:3 would warrant the use of

barrier systems under impact condi-

a barrier while a fill slope flatter than 1:4 would

tions, should not be under-estimated by the designer.

not require protection.

Early identification of

On the intervening

potential problems can ensure that

slopes, the designer should use his or her dis-

future installations operate effectively.

cretion in determining the need for a barrier.



before safety considerations.

Figure 8.7: Warrants for use of roadside barriers

contact;

In respect of roadside obstacles that need pro-

•

tection from errant vehicles (or vice versa) warrants for shielding or otherwise can be devel-

near the right of way in locations where

oped using a quantitative cost-effective analy-

there is a history of run-off-the road

sis, which takes the characteristics of the obsta-

crashes; or

•

cle and its likelihood of being hit into account.


Shielding businesses or residences

Separating pedestrians and/or cyclists

However, once again the designer must exam-

from vehicle flows in circumstances

ine each site specifically to determine the

where high-speed vehicle intrusions

necessity or otherwise for shielding. Table 8.3

onto boulevards or sidewalk areas

provides an overview of the types of non-tra-

might occur.

versable terrain and fixed objects that are norIn all these cases, conventional criteria will not

mally considered for shielding.

serve to provide warrants for barriers, and the In some situations, a measure of physical pro-

designer should be aware of the needs and cir-

tection may be required for pedestrians or bicy-

cumstances of the individual situation when

clists using, or in close proximity to, a major

deciding on appropriate action.

street or highway.

Examples of such cases

could include;

•

Longitudinal barrier placement

A barrier adjacent to a school boundary or property to minimize potential vehicle

A typical longitudinal roadside barrier installa8-22


way;

• • • •

two-way road, is illustrated in Figure 8.8. The length of need as illustrated in this figure is illustrated in more detail in Figure 8.10.

Rail deflection distance; Terrain effects; Flare rate; and Length of need.

The factors to be considered in barrier installa-

Barriers should ideally be set as far away from

tion are the following:

the travelled way as possible.

•

that:

Offset of the barrier from the travelled

Figure 8.8: Roadside barrier elements 8-23 Chapter 8: Roadside Safety

This ensures


tion, with its associated elements for a two-lane,

Barrier deflection

Barriers should ideally be set as far away from the travelled way as possible.

This ensures The expected deflection of a barrier should not

that:

• • •

There is more recovery area to regain

exceed the available space between rail and the

control of the vehicle;

object being shielded. If the available space

There is better sight distance;

between the rail and the obstacle is not ade-

Less barrier is required to shield the

quate for non-rigid barrier systems then the bar-

hazard; and

•

rier can be stiffened in advance of, and along-

Adverse driver reaction to the barrier is

side, the fixed object. This can be achieved

reduced.

through reducing the post spacing, increasing However placing the barrier away from the road-

post sizes or increasing the rail stiffness by nest-

way and closer to the hazard may have disad-

ing rail elements. However care should be exer-

vantages. These are:

cised when considering this step since the total

•

system characteristics might be altered.

The possible impact angle increases, leading to higher risk of the vehicle penetrating the rail as well as increased col-

Other areas of concern include the possibility of

lision severity. The South African Road

•

Safety Manual provides a detailed dis-

rolling over when vehicles with a high centre of

cussion on this issue.

gravity impact a barrier or of vehicles dropping

The roadside area in front of the barrier

over the edge when a barrier, positioned too

has to be traversable and as flat as pos-

close to the edge, deflects on impact.


sible.

Recommended offset distances measured from

A minimum distance of 600 mm behind flexible

the edge of the travelled way are shown in Table

guardrails to the edge of an embankment would

8.4. Barriers are typically placed at a distance

generally provide enough resistance to lateral

of 0,3 metres beyond the edge of the usable

movement of the posts to resist the rail tension.

shoulder so that the greater of the distance in Table 8.4 or the width of the shoulder plus 0,3 metres should be used. 8-24 Chapter 8: Roadside Safety

vehicle trajectory to stabilize.

Terrain effects

Installation of

guardrails on slopes steeper than 1:6 is not recRoadside features such as kerbs and drainage

ommended because inadequate lateral support

inlets affect the bumper height and suspension

for the guardrail posts would result. If this loca-

and may cause errant vehicles to snag or vault

tion is unavoidable, consideration should be

the barrier.

given to deeper postholes.

Kerbs should preferably be sited behind the

Flare rate

guardrail face. Barrier offsets less than 230 mm behind the kerb would still be acceptable. The

A barrier flare may be used to increase the bar-

height of the rail should be carefully considered

rier offset from the edge of the roadway. This is

to limit the possibility of the bumper or a wheel

normally used to position the barrier terminal

under-riding the rail. This may be achieved by

further from the roadway, to adjust the existing

setting the rail height relative to the road surface

roadside features, to reduce the total length of

in front of the kerb.

rail and to reduce driver reaction to the close proximity of the barrier rail next to the road.

Slopes Flared barriers can, however, also lead to Roadside barriers perform best when installed

increased impact angles causing higher impact

on slopes of 1:10 or flatter. Slope changes may

severity, as well as to larger rebound angles

cause vehicles to impact higher on the barrier

causing greater conflicts with other vehicles.

ing. Should barriers be installed beyond a slope

The maximum recommended flare rates are

change, they should be set back at least 3,5

shown in Table 8.5. Flatter rates may be used

metres from the slope break line to allow the

particularly where extensive grading would be



than normal, increasing the possibility of vault-

Figure 8.9: Length of need for adjacent traffic

Figure 8.10: Length of need for opposing traffic required to provide a 1:10 approach slope to the

Run-out length is the theoretical distance

barrier.

required for a vehicle leaving the roadway to come to a stop prior to impacting a hazard. The

Length of need

design of a traffic barrier requires provision to be

The variables to be considered in the design

made for sufficient length to restrict such a vehi-

process of barriers are shown in Figure 8.9 for

cle from reaching the hazard. The recommend-

the approach side towards a hazard and in

ed run-out lengths are shown in Table 8.6.


Figure 8.10 for the trailing side beyond the hazard, providing for the shielding of the hazard for

The run-out length is measured along the edge

opposing traffic.

of the road.


A control line is established

between the end of the run-out length and the

median barriers are generally justified. These

far side of the hazard to be shielded. The length

figures presuppose that the particular section of

of need for a standard barrier would then be the

roadway under consideration does not suffer

length between the near side of the hazard and

from an adverse cross-median collision history

the position where the barrier intersects the con-

and that unauthorized cross-median U-turns do

trol line. If the barrier is designed for a continu-

not take place.

ous hazard such as a river or a critical fill embankment, then the control line would be

Once the need for a median barrier is estab-

between the end of the run-out length and the

lished, the designer should consider several fac-

end of the desirable clear zone. The same prin-

tors in developing the barrier layout.

ciple is adopted to determine the length of need

include:

• • •

for opposing traffic. The standard guardrail ends at the end of the

These

Terrain effects; Flare rate of the barrier; Treatment of rigid objects in the median; and

length of need. An acceptable end-treatment

•

should be added to this length to determine the

Openings in the median as a result of underpasses.

total length of installation. Terrain effects An application of the length of need principles to practical design problems is given in Volume 6

For a median barrier to be effective, it is essen-

of the South Africa Road Safety Manual.

tial that, at the time of impact, the vehicle has all its wheels on the ground and that its suspension

8.4.4

Median barriers

system is neither compressed nor extended. Kerbs and sloped medians are of particular con-

Most of the principles with respect to longitudi-

cern, since a vehicle, which traverses one of

nal barriers also apply to median barriers.

these features prior to impact, may go over or

Regarding warrants for their use, median barri-

under the barrier or snag on its support posts.

that would result if they did not exist are more

Kerbs offer no safety benefits on high-speed

severe than the consequences of striking them.

roads and are not recommended where median

However, excessive incidence of illegal cross-

barriers are present.

median movements might justify the use of median barriers.

Medians should be relatively flat (slopes of 1:10 or less) and free of rigid objects. Where this is

For median widths of 15 metres or greater,

not the case, carefully considered placement of

median barriers are generally not required,

the median barrier is needed. AASHTO notes

whilst, for median widths of 10 metres and less

three conditions where specific guidelines for

with ADT's in excess of 30 000 vpd, and 8

median barrier placement should be followed:

metres and less with ADTs below 30 000 vpd, 8-27 Chapter 8: Roadside Safety


ers should only be installed if the consequences

•

In depressed medians or medians with a

roadside barriers should be used near the

ditch section the slopes and ditch sec-

shoulder adjacent to each of the travelled ways.

tion should first be checked to determine whether a roadside barrier is warranted.

Flare rates

If both slopes require shielding, a roadside barrier should be placed near the

If a median barrier has to be flared at a rigid

shoulder on each side of the median. If

object in the median, the flare rates for roadside

only one slope requires shielding, a

barriers should be used for the median barrier

median barrier should be placed near

flare as well.

the shoulder of the adjacent travelled way.

Rigid objects

•

•

If neither slope requires shielding but both are steeper than 1:10, a median

A special case may result in circumstances

barrier should be placed on the side with

where a median barrier is not warranted but

the steeper slope, when warranted.

where a rigid object warrants shielding. Typical examples are bridge piers, overhead sign sup-

If both slopes are relatively flat, then a

port structures, and high mast lighting installa-

median barrier may be placed at or near

tions. If shielding is necessary for one direction

the centre of the median if vehicle over-

of travel only, or if the object is in a depressed

ride is not likely.

median and shielding from either or both direcFor stepped medians that separate travelled

tions of travel is necessary, the criteria for road-

ways with significant differences in elevation, a

side barriers should be used.

median barrier should be placed near the shoulder adjacent to each travelled way if the

If shielding for both directions of travel is neces-

embankment slope is steeper than 1:10. If the

sary and if the median side slopes are steeper

cross-slope is flatter than 1:10, a barrier could

than 1:10 the designer may investigate the pos-

be placed at or near the centre of the median.

sibility of a crash cushion (or an earth berm) to


shield the object. A second possibility involves Placement criteria are not clearly defined for

the use of semi-rigid barriers with crash cush-

raised medians or median berms.

ions or earth berms to shield the barrier ends.

Research

suggests that the cross section of a median berm itself, if high and wide enough, can redi-

Median openings as a result of underpasses

rect vehicles impacting at relatively shallow angles.

In certain instances, the cost implications of providing underpasses have the result that an

As a general rule, if the cross section is inade-

opening in the median occurs.

quate for redirecting errant vehicles, a semi-rigid

instances the use of transverse barriers (or con-

barrier should be placed at the apex of the

crete balustrades) shielded by impact attenua-

cross-section. If the slopes are not traversable,

tion devices should be considered.


In such

Such a terminal must not spear, vault, or roll a

End treatments

vehicle in either head-on or angled hits. Traffic barriers (both roadside and median

2.

Barrier end treatments should gradually

types) themselves represent fixed objects.

stop or redirect an impacting vehicle when a

Impact with their untreated terminal sections

barrier is hit end on. The end treatment should

can have severe consequences, primarily

also be capable of redirecting a vehicle impact-

because of the very high deceleration rates

ing the side of the terminal.

experienced by vehicle occupant under such cir-

3.

cumstances, but also often because penetration

same redirectional characteristics as the barrier

of the passenger compartment by the barrier

to which it is attached for impacts at or near the

itself is a distinct possibility. There are a number

end of the terminal and within the length of

of different end treatments available for the var-

need. The end should be properly anchored

ious types of barriers.

and capable of developing the full tensile

The end treatment should have the

strength of the barrier elements. A proper end terminal has two functions:

•

4.

In any non-rigid barrier system, the end

sometimes be introduced far enough from

terminal should act as an anchor to

approaching traffic so that the end can be con-

allow the full tensile strength of the sys-

•

Where space is available, a barrier can

tem to be developed during downstream

sidered non-hazardous and no additional end

angled impacts on the barrier.

treatment is required. Flare rates, in this case,

Regardless of the type of barrier, the

should be in accordance with those mentioned

end terminal should be crashworthy, i.e.

above. Positive end anchorage is required in

it must keep the vehicle stable and it

semi-flexible systems in order to preclude pene-

must keep the vehicle occupants away

tration of the barrier within the length of need.

from rigid points creating high decelera-

Care should be taken, however, to ensure that

tion resulting in serious injuries or death

this flaring back does not create a hazard for

during impact.

traffic in the opposing direction. 5.

End treatments involving turned down

tems often result in penetration of the passenger

terminals parallel to the direction of travel may

compartment, and that high-speed impacts with

cause impacting vehicles to vault and roll over

concrete barriers result in intolerable decelera-

or ride up the terminal and hit the object the bar-

tion forces. In designing crashworthy end treat-

rier is intended to protect. Consequently, turned

ments, designers must create treatments that

down terminals should not be used on the

provide vehicle deceleration rates that are with-

approach ends of roadside or median barriers

in recommended limits for survivability.

on high-speed, high-volume roads unless they are also flared.

A number of principles relevant to barrier end

6.

treatments are offered:

eliminates the danger of an untreated barrier

1.

end and reduces the opportunity for errant vehi-

Crashworthy end treatments are essen-

Termination of a barrier in a back slope

cles to penetrate the end of the barrier.

tial if a barrier terminates within the clear zone. 8-29



Experience has shown that metal beam sys-

A number of end treatments have been

thumb approach. Road designers should still

developed for metal beam barriers that utilize a

investigate physical site restrictions such as lon-

combination of a breakaway mechanism and a

gitudinal space, hazard width, slopes and sur-

cable with a flared configuration to address the

face types. At locations with a high likelihood of

spearing and roll-over potential and to develop

collisions, the costs of accidents and repair

the full tensile strength of the rail for down-

should be factored into the decision matrix in

stream impacts.

addition to the initial installation costs.

7.

8.

Where an end treatment is designed as

a "gating" device, i.e., to allow for controlled

Designers should note that new technologies

penetration of a vehicle when impacted, through

are continually being developed and tested.

a breakaway mechanism, care should be taken

Nothing in this Guide relieves the designer of

to provide an adequate run-out area behind the

the responsibility of keeping abreast of these

end treatment.

new technologies and their potential application

9.

to the roadside barrier end treatment problem.

The concrete safety shape barrier can

be terminated by tapering the end. However, this treatment should only be used where

8.5

IMPACT ATTENUATION DEVICES

8.5.1

Function

speeds are low (60 km/h or less) and space is limited.

Flaring the barrier beyond the clear


zone should be considered on higher speed facilities where space is available.

Impact attenuators, sometimes called crash

10.

Proprietary mechanical end treatments

cushions, are best suited for use in places

are often suitable only for limited types of barri-

where fixed objects cannot be removed, relocat-

er applications. When adopting such technolo-

ed or made breakaway, and cannot be ade-

gies, designers should ensure not only the effi-

quately shielded by a longitudinal barrier. They

cacy of the technology of their choice but also its

have proven to be an effective and safe means

compatibility with the barrier technology being

of shielding particular types of roadside obsta-

used. In addition to information generally avail-

cles, and accomplish their task by absorbing

able from the manufacturers and suppliers of

energy at a controlled rate, thereby causing the

these treatments, road agencies and others

vehicle to decelerate so as to reduce the poten-

compile and provide appropriate guidance in

tial for serious injury to its occupants.

respect of crash testing results and system

operational impact attenuation devices have

compatibility recommendations.

been designed and tested by their manufactur-

11.

ers and acceptable units can usually be select-

All systems should be installed with a

level surface leading to the treatment. The use

Most

ed directly from design charts.

of kerb and gutter is discouraged, but if they are needed, only the mountable type should be

Typical objects and areas that can benefit from

specified.

the use of impact attenuators include:

• The principles noted above provide a rule of

A freeway exit ramp gore area in an elevated or depressed structure where a


the preliminary design of the impact attenuator:

ing;

•

The ends of roadside or median barriers;

height;

•

Rigid objects like cantilever sign gantries within the clear zone;

• •

Hazard characteristics - type width and Site geometry - including space available for installation;

•

Construction work zones; and Toll booths.

Traffic pattern - bi-directional or unidirectional traffic;

•

Slopes - preferably on flat surface, but

It is difficult to develop an easy selection

with a slope of no more than 1:50 over

process for determining the most appropriate

the length of the attenuator;

• •

impact attenuator for a specific situation. This is owing to the large number of factors influencing

Design speed; Kerb and roadway elevation; preferably no kerb within 16 metres of attenuator;

the choice. The choice could therefore be nar-

• • •

rowed down to the use of impact attenuators that have been installed in South Africa and for which a track record (however small at this

Probable angle of impact; Base type and base features; Site features - are there any unique site

nance attenuators.

•

features; Orientation - an attenuator should be oriented to maximize likelihood of head-on impact, though a maximum angle of up to 10 degrees between roadway centre line and attenuation device is acceptable; and Placement area.

8.5.2

8.5.3

Functional considerations

•

stage) is being built up. Another major consideration is the ease with which these impact attenuators can be routinely maintained or reinstated after an impact. Certain impact attenuators are marketed specifically as low mainte-

Design/selection of impact attenuators

Attenuators as well as barrier end-treatments The detail design of impact attenuators should

can be installed as bi-directional or uni-direc-

be done in conjunction with the manufacturer of

tional as well as with redirective or non-redirec-

a specific attenuator and will be dependent on

tive capabilities. As a general rule the chosen

the actual attenuator chosen for installation.

system should be able to redirect an errant vehi-

The following factors should be considered for

cle if the hazard being shielded is less than 3 m 8-31



• •

bridge rail end or a pier requires shield-

from the edge of the travelled way. Typical func-

tum and therefore do not need a backdrop in

tional considerations for attenuators and barrier

front of the hazard being shielded.

end terminals are given in Table 8.7. Recommendation pertaining to these devices The designer should allow for enough space to

are as follows:

install an attenuator in the most effective way

•

Single rows of barrels should not be allowed for permanent installation;

and to ensure that its performance will not be

•

compromised by insufficient placement areas.

Barrels should be spaced some 150 mm apart and stop 300 mm to 600 mm short

The particular system's requirements in terms of

of the hazard being shielded;

installation should also be met.

•

Barrels should be positioned in such a way that rigid corners of the hazard are

Figure 8.11 and Table 8.8 show the space to be

overlapped by barrels by some 760 mm

reserved for sand-filled plastic drum attenuators

(300 mm minimum) to reduce the sever-

under different design speed conditions.

ity of angled impacts near to the rear of the attenuator. Where such attenuators

8.5.4

are subject to bi-directional traffic flow,

Sand-filled plastic barrel impact

the array of barrels should be flush with

attenuators

the edge of the hazard so as to ensure


that reverse direction traffic does not Sand-filled plastic barrel impact attenuators

inadvertently impact the rear end of the

work on the principle of conservation of momen-

barrel arrangement.

Figure 8.11: Space requirement for plastic drum attenuators 8-32 Chapter 8: Roadside Safety

If speeds higher than 95 km/h are antic-

A number of typical arrangements of sand-filled

ipated, barriers can be lengthened.

barrel attenuators are shown in Figure 8.12.

Since most serious accidents occur at

The legend illustrates the mass of sand con-

excessive speeds, an "over-design" is

tained in each barrel.

acceptable where space permits.

Figure 8.12: Typical arrangement of sand-filled barrel attenuators 8-33 Chapter 8: Roadside Safety


•

8.6

RUNAWAY VEHICLE FACILITIES

The purpose of escape ramps and arrestor beds is to stop, without serious injury or serious dam-

8.6.1

Introduction

age to vehicles, to adjacent property or to other road users, those vehicles that become out-of-

Runaway-vehicle escape ramps and associated

control on long downhill gradients. Deceleration

arrestor beds are specifically designed to

rates of between 5 m/s2 and 6 m/s2 are obtained

reduce the safety hazard associated with out-of-

by a full width level arrestor bed without the use

control heavy vehicles where long steep grades

of vehicle brakes. (A 10 per cent down gradient

occur.

on the bed surface can reduce the deceleration

The following factors are generally associated

by about 1 m/s2). It should be noted that, under

with such incidents:

high deceleration, inadequately restrained vehi-

• •

Gradient;

cle occupants, or insecurely attached cargo may

Driver error such as failure to downshift

not be safely contained.

gears;

•

Equipment failure such as defective

8.6.2

brakes;

• • • •

Types of escape ramps

Inexperience with the vehicle; Unfamiliarity with the specific location;

There are six different types of truck escape

Driver impairment due to fatigue or alco-

ramp:

hol; and

•

Sand pile;

Inadequate signing of downgrade.

+%

-% Road

A. Sandpile -% -%

Road


B. Descending grade -% Road

0,0% C. Horizontal grade

-% +% Road D. Ascending grade

Figure 8.13: Typical arrestor beds 8-34 Chapter 8: Roadside Safety

• • • •

Ascending grade arrestor bed;

bed increases the rolling resistance, as in the

Horizontal grade arrestor bed;

other types of ramps, while the force of gravity

Descending grade arrestor bed; and

acts in the opposite direction to the movement

Roadside arrestor bed.

of the vehicle. The loose bedding material also serves to hold the vehicle in place on the ramp

Figure 8.13 illustrates four types in general use.

grade after it has come to a safe stop. The sand pile types are composed of loose, dry

Ascending grade ramps without an arrestor bed

sand and are usually no more than 130 metres

are not encouraged in areas with moderate to

in length. The influence of gravity is dependent

high commercial vehicle usage as heavy vehi-

on the slope of the surface of the sand pile. The

cles may roll back and jack-knife upon coming

increase in rolling resistance to reduce overall

to rest.

lengths is supplied by the loose sand.

The

deceleration characteristics of the sand pile are

Each one of the ramp types is applicable to a

severe and the sand can be affected by weath-

particular situation where an emergency escape

er. Because of these characteristics, sand piles

ramp is desirable and should be compatible with

are less desirable than arrestor beds. They may

the location and topography. The most effective

be suitable where space is limited and the com-

escape ramp is an ascending ramp with an

pacting properties of the sand pile are appropri-

arrestor bed. On low volume roads with less

ate.

than approximately 1000 vehicles per day, clear run-off areas without arrestor beds are accept-

Descending grade ramps are constructed paral-

able.

lel and adjacent to the through lanes of the highway. They require the use of loose aggre-

8.6.3

gate in an arrestor bed to increase rolling resist-

Criteria for provision of escape ramps

ing-grade ramps can be rather lengthy because

On hills where there is a history of accidents

the gravitational effect is not acting to help

involving runaway vehicles, consideration

reduce the speed of the vehicle.

should always be given to the provision of escape ramps with arrestor beds. A measure of

In a horizontal-grade ramp, the effect of the

effectiveness can be assessed by an analysis of

force of gravity is zero and the increase in rolling

personal injury and "damage only" accidents

resistance has to be supplied by an arrestor bed

plus the incidence of damage to property, based

composed of loose aggregate.

on local records.

This type of

ramp will be longer than those using gravitational forces to help stop the vehicle.

On new construction, where long steep gradi-

An ascending-grade ramp uses both the arrest-

ents are unavoidable, and where the probability

ing bed and the effect of gravity, in general

of damage caused by runaway vehicles is

reducing the length of ramp necessary to stop

greater than normal, the provision of arrestor

the vehicle. The loose material in the arresting

beds should be considered as an integral part of 8-35



ance and thus slow the vehicle. The descend-

• •

the project design. As a guide for provision, where gradients are over 5 per cent, an arrestor

The conditions at the bottom of the grade;

bed should be considered if the gradient (in per

• • •

cent) squared, multiplied by the approach length from the crest, in kilometres, is over 60.

The percentage of heavy vehicles; Horizontal alignment; Topography (i.e. effect on cost of earth works); and

On long, straight stretches of down grade where

•

a sufficiently steep or long up-grade occurs before any bend is met, observations of heavy

Toll plazas at the bottom of steep grades

Runaway-vehicle facilities should not be con-

vehicle driver choice indicate that arrestor beds

structed where an out-of-control vehicle would

are unlikely to be used.

8.6.4

The length of downgrade;

need to cross oncoming traffic.

Location of runaway-vehicle facilOn divided carriageways, safety ramps may

ities


also be located in the median if sufficient space On both new and existing roads, engineering

is available. This would be in conflict with driv-

judgement is required to determine the location

er expectations and prominent advance warning

of arrestor beds. Relevant factors to be consid-

signs prior to the safety ramp exit would have to

ered include:

be provided. Because of the conflict with driver

•

expectations, providing arrestor beds in the

The location of previous accidents;

Figure 8.14: Layout of arrestor bed adjacent to carriageway


Figure 8.15: Layout of arrestor bed remote from carriageway

median is not a recommended practice but, in

local schools should be informed that these are

the case of a sharp curve to the left at the bot-

dangerous locations so that children are dis-

tom of a steep downgrade this location may

couraged from playing in them.

prove to be necessary. Lack of suitable sites for the installation of

For safety ramps to be effective, their location is

ascending type ramps may necessitate the

critical. They should be located prior to, or at

installation of horizontal or descending arrestor

the start of, the smaller radius curves along the

beds. Suitable sites for horizontal or descend-

alignment. For example, an escape ramp after

ing arrestor beds can also be limited, particular-

the tightest curve will be of little benefit if trucks

fill side of the roadway formation. The entrance

it. As vehicle brake temperature is a function of

to the bed should be clearly signed for drivers of

the length of the grade, escape ramps are gen-

runaway vehicles and clearway restrictions

erally located within the bottom half of the steep-

should be applied so that the entrance is kept

er section of the alignment.

freely accessible.

Adequate visibility to the

Where possible, arrestor beds should not be

entrance must be available so as to give drivers

located in verges on the outside of right-hand

time to manoeuvre their vehicles into the bed

curves, or at any other location where there is a

and, as a minimum requirement, visibility equal

likelihood of vehicles mistakenly entering the

to the desirable stopping sight distance appro-

bed during the hours of darkness.

priate to the anticipated maximum speed of run-

Where

arrestor beds are constructed in built-up areas,

away vehicles should be provided. 8-37



ly if the downward direction is on the outside or

are unable to negotiate the curves leading up to

8.6.5

Arrestor bed design features

increased by 3 per cent for each one degree of slope.

Suggested layouts for an arrestor bed immediately adjacent to the carriageway, are shown in

Width of bed

Figure 8.14 and, in Figure 8.15, for a bed separated from the road. Where space permits, the

As a general guide a constant bed width of 4,0 -

arrangement separated from the road should be

5,0 metres is adequate. Barrier kerbing with a

used.

300 mm up stand should be installed at the side of the bed remote from the carriageway to assist

Length of bed

in restricting sideways movement. The use of safety fencing may also be desirable. The bed

The length of bed required to halt runaway vehi-

should be separated from the main carriageway,

cles is dependent on the predominant vehicle

where possible, by at least 2,0 metres, and flush

type, the likely speed of entry into the bed, the

kerbing may be required locally where the road

type and depth of aggregate used and the slope

is provided with 1,0 metre wide hard strips.

of the arrestor bed. The bed length for all-purpose roads should cope with the critical design

Depth of bed material

vehicle, generally a large articulated vehicle with multiple axle groups. This vehicle is likely to

Beds should have depths between 300 mm and

have the highest entry velocity and the lowest

450 mm with the depth gradually increasing

average deceleration rate.

over an initial length in order to provide for


smooth vehicle entry.

Where entry velocities

Table 8.9 gives suggested lengths for horizontal

are less than 75 km/h, vehicle deceleration is

grade arrestor beds (excluding the initial depth

significantly higher for beds which contain

transition zone).

Where the bed surface is

greater depths of bed material, whereas, at

aligned on a downgrade its length should be

speeds above 75 km/h, decelerations tend to be

* L as shown on Figure 8.13 (bed length to be 25 per cent greater)


8.7

independent of bed depth. The greater (450

BRAKE CHECK AND BRAKE REST AREAS

mm) depth gives around 50 per cent greater stopping ability than the minimum (350 mm) provision and should be considered where bed

A brake check area or a compulsory truck stop

length is restricted.

is an area set aside before the steep descent as distinct from a brake rest area which is an area

Type of material

set aside for commercial vehicles part way down or at the bottom of the descent.

To achieve a high deceleration rate it is necessary that vehicle tyres sink into the bed material. Rounded uncrushed gravel and single size

These facilities should be provided on routes

cubic aggregate or similar artificial lightweight

that have long steep downgrades and commer-

aggregate has performed satisfactorily in tests

cial vehicle numbers of around 500 per day,

and should be used in preference to angular

especially on National Roads and principal traf-

gravel (i.e. crushed rock) or sand, which tend to

fic routes. These areas, when used, will ensure

restrict wheel penetration and compact with time

that drivers begin the descent at zero speed and

and usage.

in a low gear that may make the difference between controlled and out-of-control operation

Arrestor bed material should be free draining

on the downgrade. They would also provide an

and adequate drainage should be provided so

opportunity to display information about the

that in freezing or saturated conditions it still

grade ahead, escape ramp locations and maxi-

retains its function of wheel penetration, thereby

mum safe recommended descent speeds.

bringing vehicles to a standstill. A suitable specification for the bed material is given below as a

These areas may need to be large enough to

guide. This may need to be modified to allow

store several semi-trailers, the actual numbers

locally available suitable materials to be used, in

depending on volume and predicted arrival rate.

the light of further experience and testing. Their location would need good visibility with Adequate signage should be provided to advise The material should be clean, uncrushed, hard

drivers in advance of the facilities.

durable natural gravel consisting primarily of

signs, specific to the site, may need to be

smooth round particles. Alternatively, an appro-

designed for these areas.

priate artificial lightweight aggregate may be used. The following particle size distribution is suitable: BS Sieve Size 10 mm 5 mm

Percentage by mass passing 100% 0%


Special


acceleration and deceleration tapers provided.

Typical specification for arrestor bed material

TABLE OF CONTENTS 9

ROAD BETTERMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1 9.1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1 9.1.1 Balanced context-sensitive design of 3R projects . . . . . . . . . . . . . . . . . . . . . . . . . 9-1 9.1.2 The myth of the sub-standard road . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2 9.1.3 Possible methodologies for determination of road improvements . . . . . . . . . . . . . . 9-2

9.2

COST-EFFECTIVENESS OF GEOMETRIC IMPROVEMENTS . . . . . . . . . . . . . . . . . . . . 9-4

9.3

CRASH PREDICTION MODELS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-5 9.3.1 Averages from historical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-5 9.3.2 Regression analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-6 9.3.3 Before-and-after studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-6 9.3.4 Expert judgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-6

9.4

A NEW APPROACH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-6 9.4.1 Base model for roadway segments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-9 9.4.2 Base model for intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-10 9.4.3 Crash modification factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-11

9.5

ECONOMIC ANALYSIS OF GEOMETRIC IMPROVEMENTS . . . . . . . . . . . . . . . . . . . . 9-14 9.5.1 Crash reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-14 9.5.2 Time savings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-15 9.5.3 The speed profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-16 9.5.4 Evaluation of geometric improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-16

LIST OF TABLES Table 9.1: Table 9.2: Table 9.3: Table 9.4:

Cost of crashes by severity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-4 Crash modification factors for various lane widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-11 Crash modification factors for various shoulder types and widths. . . . . . . . . . . . . . . . . . . . . . . . 9-12 Crash modification factors for right-turn lanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-14

LIST OF FIGURES Figure 9.1: Fatal Crash rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-7 Figure 9.2: Fatality rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-8 Figure 9.3: Crash rates for all types of crashes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-8

Chapter 9 ROAD BETTERMENT 9.1

Introduction

ments at these locations may be more safety cost-effective

than

routine

cross-section

improvements.

3R projects are essentially aimed at extending the life of a road through maintenance of the pavement at the levels of:

Modification may simply entail a modest

•

Resurfacing, which involves the applica-

upgrade of the cross-section over the length of

tion of a new wearing course to the

the section to be rehabilitated.

existing surface, with this wearing

include changes to selected portions of the hor-

course being anything from a fog spray

izontal or vertical alignments.

It may also

to an asphalt overlay;

•

9.1.1

Restoration, typically consisting of

Balanced context-sensitive design of 3R projects

patching of potholes and repair of failed sections that may extend for several

•

metres;

The objective of geometric modification is to

Rehabilitation, where the entire base

enhance the safety of the road. For this reason,

course layer - and perhaps the lower

a decision to simply increase the widths of the

layers as well - are scarified, reshaped

lanes or shoulders or both may not be appropri-

and recompacted.

ate. Realignment of a markedly substandard

Reference is sometimes made to 4R, with the

horizontal curve may have an impact on safety

fourth R referring to reconstruction.

that is greater than that of merely adding 300

In the case of rehabilitation or reconstruction

should thus study the entire length of the road

projects, the possibility of modifying the geome-

section in depth and develop a package of

try of the road where it is currently sub-standard

improvements that will have the maximum

also merits consideration. Unfortunately, many

impact on safety in terms of the funds available.

opportunities for low-cost safety improvements are lost because of the overriding focus on

A road with a design speed of 120 km/h and a

pavement improvements.

Safety needs are

cross-section comprising 3,7 metre lanes and

often not addressed until little time remains to

2,5 metre shoulders may conceivably be the

accommodate the design time for geometric

ideal in a given situation. It may also be unaf-

improvements and the available funding is allo-

fordable. The road authority then has to con-

cated totally to the pavement.

Furthermore,

sider the relative merits of improving safety over

there is a tendency to focus on widening of

only a short section of the road, leaving the rest

lanes or shoulders rather than on the recon-

as is, or of making more modest safety improve-

struction of sharp curves or replacement of nar-

ments over the whole length of the road. The

row culverts and bridges. Very often, improve-

latter is generally the preferred option in terms 9-1

Chapter 9: Road Betterment


mm to the width of the lanes. The designer

of consistency of design and matching driver

ing short tangents and low-radius curves on a

expectations because the high-standard road

road traversing an essentially flat landscape.

section may encourage speeds that are inapWhere, for either of the reasons listed above, a

propriate to the balance of the road.

road is described as being sub-standard, it may An important feature of geometric improve-

be argued that any improvement is better than

ments carried out as part of a 3R project is that,

none even if the result is still modestly sub-stan-

after upgrading, there should still be balance

dard. The counter argument could be that driv-

between the various elements of the road. A

ers will proceed cautiously on a road which is

high-standard cross-section generally creates

"obviously dangerous" and the accident rate will

the impression in the driver's mind of a high

be correspondingly low. If the design speed of

design speed, which may be at variance with

the road is, however, improved so as to be clos-

the reality of the situation as defined by the hor-

er to the speed drivers wish to select - but with-

izontal alignment.

out actually achieving it - they may be unaware of the shortfall and thus tend to overdrive the

9.1.2

The myth of the sub-standard road

alignment with a consequent increase in the accident rate. The designer should be sensitive

In reality, there is no such thing as a sub-stan-

to the merits of both arguments and, within the

dard road. Two possible situations can, howev-

flexibility offered by the adoption of guidelines

er, give rise to the belief that a road is not to

as opposed to rigid standards, achieve the best

standard. These are:

possible compromise within the context of the

•

A mismatch between the various dimen-

financial and physical restraints attached to the

sions, for example a low design speed

specific project.


vertical alignment superimposed on a

•

high design speed horizontal alignment.

It is suggested that, as a general rule, improve-

Similarly, a narrow cross-section applied

ments to the alignment and cross-section of a

to a road that, in all other respects, sup-

road be in balance to a single design speed and

ports high-speed travel.

that, if this speed is low for the circumstances,

A low design speed of the road in rela-

advisory speeds be posted.

tion to the environment being traversed.

9.1.3

Possible methodologies for

With regard to the first point, if there is no mis-

determination of road improve-

match, i.e. if the various dimensions of the road

ments

are in balance with one another, a design speed can be established that will match the geometry

The most obvious reason for improving a road is

of the road. The second point suggests that this

a mismatch in dimensions. This is because a

design speed may be inappropriate to the cir-

design speed that is too low may require full

cumstances. An example would be where prop-

reconstruction at almost every horizontal and

erty boundaries impose an alignment compris-

vertical curve. This may be outside the budget 9-2


The

is dictated by the horizontal rather than the ver-

designer should thus establish precisely where

tical alignment so that a low design speed on a

dimensional mismatches occur along the road

crest curve would tend to be overdriven, i.e.

and the extent of the mismatch.

drivers would have less stopping sight distance

typically available for betterment works.

than they need for safety. The speed of trucks, The mismatch referred to above is that between

on the other hand, is dictated largely by gradi-

the horizontal and the vertical alignments and

ents and it is unlikely that they would exceed the

the cross-section.

design speed of a crest vertical curve.

A further mismatch is

described in detail in Section 4.2.2, being differ-

The coordination of the vertical and horizontal

ences between successive elements of the horizontal alignment.

alignments as described in Chapter 5 should

These differences are

also be checked, although financial constraints

between the design and operating speeds on

may make it difficult to devote funding purely to

individual curves and between operating speeds on successive curves.

improvement of the aesthetics of the road.

Design speeds and

However, if it is necessary to rebuild a horizon-

operating speeds along the route should thus be

tal or a vertical curve for whatever reason, the

calculated and compared.

aesthetics of the situation should be considered at the same time.

In terms of road safety, differences between design and operating speeds along the road are

Having achieved an acceptable level of internal

possibly more likely to lead to crashes than

coherence of the horizontal alignment and an

would mismatches between road dimensions.

appropriate design speed for the vertical align-

The designer should, therefore, in the first

ment, the designer can then consider the cross-

instance, tabulate the design and operating

section.

greater than those allowable in terms of Section

With time, "desirable" lane widths have

4.2.2.

The magnitude of these differences

increased from 2,7 metres to 3, 7 metres (9 feet

offers a yardstick for prioritising the required

to 12 feet) and shoulder widths from 0,3 metres

improvements. In addition, the tabulation would

to 3,0 metres, with the latter including a 0,5

suggest a range of curve radii that could be con-

metre allowance for rounding. The upper limit of

sidered at each point along the road.

design speeds has increased from 100 km/h to 130 km/h. There is thus a distinct likelihood that

After evaluation of the horizontal alignment, the

a road that is at or beyond the end of its design

vertical alignment can be checked, specifically

life may have a horizontal and vertical alignment

for the K-values of the crest curves, as these

matching an acceptable design speed, but still

dictate the availability of stopping sight distance.

be narrower than is considered desirable.

This design speed should be equal to, or prefer-

Under these circumstances, improving the

ably greater than, the design speed of the hori-

cross-section over the full extent of the better-

zontal alignment. The speed of passenger cars

ment project should be considered. 9-3



speeds along the road, highlighting differences

The principle of balanced design should not

guide at best. Costing should thus be on a proj-

however be ignored.

ect-by-project basis.

Even if the funds for

improvements were unlimited, it would not be

Central to any system whereby geometric

wise to develop a cross-section appropriate to a

improvements can be prioritised, is knowledge

high design speed when the alignment is, in

of the value of the benefits that are likely to

fact, appropriate to a lower design speed. As

accrue from any improvement.

stated above, a mismatch between the dimen-

the form of:

sions of the road constitutes poor design.

•

Benefits take

reduction in the number and/or severity of crashes; and

•

When a table defining and prioritising all the

reduction in travel time across the improved road section.

geometric improvements that should be made as part of the 3R project has been prepared, the

The cost of crashes in 2001 Rands is listed in

final step in the process would be to locate

Table 9.1. For convenience, a weighted aver-

these improvements relative to the identified

age of R 30 k has been adopted.

pavement improvements.

It is likely that the

best return on investment would be achieved if

Crash prediction models are used in conjunction

the investment were located at a point where

with average crash costs to derive the annual

both the pavement and the geometry have to be

savings derived from a given geometric

improved.

improvement over the design life of the improvement. These models are normally structured to

9.2

COST-EFFECTIVENESS OF GEO-

predict the number of crashes that are likely to

METRIC IMPROVEMENTS

occur given a number of preconditions such as lane width, shoulder width and radius of curvature.

The construction costs involved in geometric


improvements cannot be quantified on a countrywide basis. The availability of materials, the

Where road improvements are contemplated,

remoteness of sites, the presence of site-specif-

the focus is on the marginal reduction in crash-

ic complications and the variation in skills levels

es that is likely to be achieved by a deliberate

all contribute to variations in cost that would

change in these preconditions. The lanes or the

tend to make a national average a very rough

shoulders may be widened or the radius of a

9-4 Chapter 9: Road Betterment

•

horizontal curve increased. A vertical curve may be flattened or an intersection relocated. It is

Predictions from statistical models based on regression analysis;

• •

difficult to assign a reduction in crash rate to any one specific geometric element and even more

Results of before-and-after studies; or Expert judgments by experienced engi neers.

difficult to evaluate the consequences of more than one variation in the road geometry. For

When used alone, each of these approaches

example, it is reasonable to assume that an

has weaknesses.

increase in lane width would result in a reduction in the crash rate and an increase in shoul-

9.3.1

der width would also reduce the crash rate. The

Averages from historical data

question then arises whether the reduction in South Africa does not have useful crash data-

crashes because of a combination of lane and

bases. In countries where these do exist, it has

shoulder width increases would equal the sum

been found that historical data are highly vari-

of the reductions caused by the individual increases.

able. Consequently, estimates of the expected

Similarly, an unchanged width

accident rate based on a short-duration data set

between shoulder breakpoints would result in a

are likely to be unreliable.

reduction in shoulder width if the lane width were increased.

Would there then be a net A form of historical data that has been and still

increase or a net decrease in the crash rate? In

is used in South Africa refers to "red spot loca-

the sections that follow, these matters are dis-

tion". A location with a high frequency of crash-

cussed in more depth.

es is identified on the basis of its experiencing

9.3

more than a specified number of crashes of

CRASH PREDICTION MODELS

varying severity within a period typically of one to three years. However, statistical theory sug-

In North America and Europe, various road

gests that, because of the random nature of

agencies have developed and maintain crash

crashes, a location with a high short-term crash

recording systems to monitor the safety of the

future. This phenomenon, known as regression

Useful though these are as a point of departure,

to the mean, has been verified experimentally

they provide historical or retrospective informa-

through the conducting of before-and-after stud-

tion whereas effective management requires a

ies with no intervening remedial action being

prospective viewpoint. It is necessary to know

undertaken. The after studies often showed an

what the current safety performance of a road is

improvement in the safety of the location.

and what it is likely to become if certain remedial actions are undertaken.

Historical crash data can thus lead to an incorrect - specifically a pesimistic - interpretation of

Estimates of the future safety performance of a

the situation.

road were based on one of four approaches:

•

Averages from historical accident data; 9-5 Chapter 9: Road Betterment


rate is likely to experience fewer accidents in the

road networks for which they are responsible.

9.3.2

Regression analysis

would be a powerful tool in the assessment of the safety effects of geometric and traffic control

Statistical models are developed on the basis of

features.

a database of crash and roadway characteristics. An appropriate functional form is selected

9.3.4

Expert judgment

for the model and its parameters are then determined by means of multiple regression analysis.

Expert judgment developed after many years of

Regression models are accurate in their predic-

experience in the highway safety field plays an

tion of the total crash experience for a location

important role in the development of reliable

or class of locations but are less effective in iso-

safety estimates. Although experts have diffi-

lating the effects of individual geometric or con-

culty in providing quantitative i.e. absolute esti-

trol features.

mates, they are very capable of developing relative or judgments of the form: A is likely to be

Regression models assume a statistical correla-

more or less than B or C would not be more than

tion between roadway features and crashes,

20 per cent of D. They thus need a frame of ref-

although these correlations could be spurious or

erence based on historical data, statistical mod-

not representative of a cause-and-effect rela-

els or before-and-after studies to make useful

tionship. For example, the fact that crash rates

judgments.

tend to decrease with time could suggest that

9.4

the passage of time is sufficient to cause the reduction.

A NEW APPROACH

Furthermore, a strong correlation

between independent variables would make it

Having discussed the weaknesses of the four

impossible to isolate their individual effects.

basic forms of assessment of safety, it is suggested that combining all four into a single form

9.3.3

of assessment would lead to a more reliable

Before-and-after studies

estimate of safety than could be achieved if each one were used individually.

The principal weakness of before-and-after


studies has already been discussed. Because

The Interactive Highway Safety Design Model

of regression to the mean, the user of such a

(IHSDM), briefly introduced in Chapter 2 of this

study cannot be sure that it represents the true

Guideline, includes a crash prediction module

effect of the improvement on the safety of the crash location being studied.

based on a combination of all four forms of

These studies

assessment.

tend to provide an over-optimistic interpretation

The algorithms in this module

have been developed in the United States for

of the value of the improvement.

rural two-lane highways. Separate algorithms have been developed for roadway segments

However, safety experts are of the opinion that,

and at-grade intersections. The two algorithms

if the bias resulting from regression to the mean

can be used together to predict crash experi-

could be eliminated, the before-and-after study

ence for an entire highway section or improve9-6


ment project. The roadway segment algorithm

The effect of average daily traffic (ADT) on pre-

predicts all non-intersection-related crashes.

dicted crash frequency is incorporated as part of

For example, a ran-off-road crash within 15

the base condition and the effects of geometric

metres of an intersection could be considered

design and traffic control measures are incorpo-

by the investigating officer to be unrelated to the

rated through the CMFs.

intersection. For modeling purposes, crashes occurring within 76 metres (250 feet) of an inter-

South African fatality and crash rates are higher

section because of the presence of the intersec-

than those of the United States for which the

tion are considered to be intersection-related

relationships were derived.

crashes.

between South African and American road safe-

The differences

ty indicators are illustrated in Figures 9.1, 9.2 and 9.3.

Each of these algorithms is composed of two components, being the base model and crash modification factors (CMF). They take the form;

Although Figures 9.1 and 9.2 display an encour-

9.1

aging trend in the South African rates, the ratio

Nbr=

predicted number of total road

between South African and American crash

way segment crashes per year

rates for all types of crashes has stayed fairly

after application of accident

constant at a factor of 2,5 over the period shown

modification factors

in the graphs.

predicted number of total road way segment crashes per year

In

for nominal or base condition

allowance is made for a calibration procedure to

CMF= crash modification factors for

modify the national relationship to the accident

roadway segments

the American

application

of

IHSDM,

history of the individual States or local areas

Figure 9.1: Fatal Crash rates 9-7 Chapter 9: Road Betterment


where Nrs =


Figure 9.2: Fatility rates

Figure 9.3: Crash rates for all types of crashes within each State. A calibration factor of 2,5

involves developing an inventory of the road

should, on the basis of Figure 9.3, be applied to

network stratified in terms of ADT in order to

Equation 9.1 to match South African conditions.

extract:

•

Number of kilometres of tangent road way;

The individual States of the United States gen-

•

erally have crash and road information superior

Number of kilometres of roadway on horizontal curves;

to that available in South Africa so that their cal-

•

ibration procedure can be more precise than

Average degree of curvature for hori zontal curves;

that proposed above. The calibration procedure

•

Number of kilometres of level roadway;


• •

Number of kilometres of roadway on

The roadside hazard rating is that devised by

grade; and

Zegeer et al and has the following structure:

Average percentage of gradient for

Rating =

•

roadway on grade.

1 Wide clear slopes not less than

Values for the various geometric parameters,

9 metres from the pavement

i.e.:

edgeline;

• • •

• •

Average lane and shoulder width; Shoulder type (paved, gravel or turf); Driveway density (number per kilome-

Rating =

•

tre); and

• •

Average roadside hazard rating No spiral transition present;

o

Super elevation not deficient

Recoverable. 2 Clear zone for 6 to 7,5 metres from pavement edgeline;

• •

For horizontal curves; o

Side slopes flatter than 1:4;

Rating =

•

also have to be derived.

Side slope ofabout 1:4; Recoverable. 3 Clear zone for 3 metres from pavement edgeline;

• •

The above information is input into the crash prediction module and the predicted number of crashes calculated.

Rating =

The actual number of

•

crashes recorded in the database, divided by

Side slope of about 1: 3 to 1:4; Marginally recoverable. 4 Clear zone for 1,5 to 3 metres from pavement edgeline;

the predicted number of crashes, provides the

• •

calibration factor.

Side slope of about 1:3 to 1:4; May have guardrail about 1,5 to 2 metres from pavement edgeline;

•

Base model for roadway seg-

May have exposed trees, poles or other objects about 3 metres

ments

from pavement edgeline;

•

Marginally forgiving, but increased

The predicted number of crashes for the base

chance of a reportable roadside

condition is given as

crash. 9.2

where Nbr

=

EXPO =

Predicted number of crashes for base condition Exposure (106 veh km/year)

=

ADT x 365 x L/106

LW

=

Lane width (metres)

SW

=

Shoulder width (metres)

RHR

=

Roadside hazard rating (integer value between 1 and 7)

DD

=

Driveway density (number /km)

W

=

Weighting factor for specific road segment



9.4.1

Rating =

• • •

5

9.4.2

Clear zone of 1,5 to 3 metres

Base models have been developed for:


•

Side slope of about 1:3;

Three-legged STOP-controlled inter sections;

•

May have guardrail 0 to 1,5 metres from pavement edgeline;

•

Base model for intersections

Four-legged STOP-controlled intersec tions; and

•

May have rigid obstacles or

Four-legged signalised intersections.

embankment within 2 to 3 metres of pavement edgeline;

• Rating =

• • • •

Virtually non-recoverable.

These predict crash frequency per year for inter-

6

section-related crashes that occurred within 76

Clear zone of 1,5 metres or less

metres of an intersection. They are limited to


intersections of two-lane two-way roads without

Side slope of about 1:2;

auxiliary lanes at the intersections.

No guardrail; Exposed rigid obstacles within

For three-legged intersections:

0 to 2 metres of the pavement

Nbi

edgeline;

• Rating =

•

=

0,49 ln ADT2)

Non-recoverable.

9.4

For four-legged intersections:

7

Nbi

Clear zone of 1,5 metres or less

=

exp ( -9,34 + 0,60 ln ADT1 + 0,61 ln ADT2)


•

exp (-10,79 + 0,79 ln ADT1 +

Side slope of 1:2 or steeper;

9.5

For four-legged signalised intersections:

The weighting applied in Equation 9.2 is calculated as 9.3 =

weight factor for the ith horizontal curve

=

proportion of segment length contained within Curve i

Ri

=

radius of Curve i (metres)

WVj

=

weight factor for the jth vertical curve

=

proportion of segment length contained within Curve j

Aj

=

algebraic difference in gradient across the jth vertical curve (per cent)

WGk

=

weight factor for kth grade

=

proportion of segment length contained in kth grade

Gk

=

absolute value of gradient for the kth grade (per cent).

• • •

Cliff or vertical rock cut;


where WHi

Nbi

=

exp ( -5,73 + 0,60 ln ADT1 + 0,20 ln ADT2)

No guardrail;

9.6

Non-recoverable with high like-

Where ADT1 and ADT2 refer to the average

lihood of severe injuries from

daily traffic on the major and minor roads

roadside collision.

respectively and Nbi is the predicted number of 9-10 Chapter 9: Road Betterment

o

crashes at intersections for the nominal or base

Intersection sight distance

The CMF's in respect of lane widths and shoul-

condition.

der types and widths are shown in Tables 9.2

9.4.3

As

and 9.3.

Crash modification factors

shown

in

Equation

9.1,

the

The CMF in respect of horizontal curvature is

Crash

shown in Equation 9.7.

Modification Factors (CMF) are multipliers

9.7

applied to the base condition. The CMFs developed for the American IHSDM model were based on a panel's best judgment of the relative where LC

merits of available research findings with the

=

Curve length (km)

credibility of the model being supported by a

R

=

Radius of curve (m)

sensitivity analysis. The CMFs incorporated in

S

=

1 if transition curve present

the model include:

•

o

Lane width;

o

Shoulder type and width;

is present If transition curves are present, the length vari-

o

Length;

able, Lc, represents the length of the circular

o

Radius;

portion of the curve.

o

Presence or absence of transition curves;

The CMF for superelevation deficiency is

Superelevation;

expressed as

Gradients;

9.8

Driveway density; Passing lanes/short four-lane sections;

• •

0 if no transition curve

Horizontal curves;

o

• • •

=

Roadway segments:

where ed

Roadside design;

=

1,0 for ed < 1%

=

superelevation deficiency (per cent)

Intersections: o

Angle of skew

It should be noted that, as discussed in Chapter

o

Traffic control form

4, a superelevation deficiency of four per cent or

o

Exclusive right-turn lanes

more constitutes poor design. 9-11



•


The CMF for gradient is given in Equation 9.9.

CMF

The equation is based on the absolute value of

where G

=

1 + 0,035 G

=

Gradient (%)

9.9

gradient simply because an upgrade in one direction is a downgrade in the opposite direc-

The crash rate is strongly correlated with the

tion. The gradient factor is applied to the entire

number of accesses or driveways along the

grade, i.e. from one Vertical Point of Intersection

road. Expressed in terms of a driveway density

(VPI) to the next.

in driveways per kilometre, the CMF is shown in


and four-legged STOP-controlled intersections

Equation 9.10 as

respectively. 9.10 where DD

=

CMF

=

exp (0,0040 θ) 9.12(a)

Driveway density CMF

(Driveways/km) ADT

= =

exp (0,0054 θ) 9.12(b)

Average daily traffic Where θ

=

intersection skew angle

The CMF for passing lanes or climbing lanes is

expressed as the absolute value of the differ-

taken as 0,75 for total crashes in both directions

ence between 90O and the actual intersection

of travel over the length of the passing or climb-

angle.

ing lane. Where auxiliary lanes are provided on both sides of the road over a short length of the

Signal control separates the movements from

road segment, the CFM improves to 0,65 for the

conflicting approaches so that the CMF for skew

length of the auxiliary lanes.

at signalized four-legged intersections is 1,00 for all angles.

The quality of roadside design is represented by the roadside hazard rating described in Section

The safety differences between STOP-con-

9.4.1. The calculated CMF applies to the total

trolled and signalized intersections are account-

roadway segment over which the roadside haz-

ed for by separate base models rather than by a

ard rating applies and is thus independent of the

CMF. The nominal case for STOP-control has

length of the segment. The CMF is calculated

STOP-control on the minor approaches only.

as shown in Equation 9.11.

Minor-road YIELD-control is treated identically to STOP-control. A CMF of 0,53 is applied to

CMF Where RHR

= =

0,7915 + 0,0718.RHR

the CMF for minor-road STOP control to provide

9.11

for conversion to all-way STOP control. This

Roadside hazard rating

suggests that all-way STOP control has a crash

The number of legs in an intersection has a sig-

leg STOP control. This should not, however be

nificant effect on the crash rate. This is attribut-

interpreted as an argument in favour of arbitrar-

able to the difference in the number of conflict

ily replacing minor-leg control by all-way control.

points in four- as opposed to three-legged inter-

The CMF applies only when all-way STOP con-

sections. These differences are accommodated

trol is, in fact, warranted.

in the base models described in Section 9.4.2 and thus do not generate a CMF. It is, however,

The nominal or base condition for provision of

widely accepted that departures from an angle

right-turn lanes at intersections is the absence

of skew of

90O,

whether positive or negative, is

of turning lanes. No data are available to quan-

detrimental to safety. This impact is captured in

tify the effect of right-turn lanes on the minor

Equations 9.12(a) and 9.12(b) for three-legged

legs of intersections. The CMFs for right-turn 9-13



rate that is 47 per cent lower than that for minor-

lanes on the major legs are presented in Table

In the case of all-way STOP and signal control,

9.4. The CMFs for installation of right-turn lanes

a CMF of 1 applies.

on both major legs of a four-legged intersection

9.5

are simply the square of the value for a rightturn lane on a single approach.

ECONOMIC ANALYSIS OF GEOMETRIC IMPROVEMENTS

The CMFs

apply to total intersection-related crashes.

9.5.1

Crash reduction

The nominal or base condition for intersection sight distance is the availability of adequate

In Section 9.4, a method for predicting the num-

sight distance in all quadrants of the intersec-

ber of crashes on a specified stretch of roadway

tion. Where, for reasons of road alignment and

was offered.

terrain, sight distance is less than that specified

derivation of the probable difference in accident

for a design speed of 20 km/h less than the

rate resulting from a specific geometric improve-

design speed of the major road, it is considered

ment. The procedure involved would be to cal-

to be limited. Restrictions of sight distance by

culate the number of crashes that could be

specific obstructions such as trees and bushes

expected to occur:

do not qualify for consideration of CMFs, it being

•


•

removed.

and/or shoulder width across the length of the road or improvement of a specific horizontal or

1,05 for sight distance restriction in one

vertical curve or any combination of these

quadrant of the intersection;

options.

1,10 for sight distance restriction in two quadrants of the intersection;

• •

if the geometry was upgraded.

Upgrading could be an increase in the lane

The CMFs for restricted sight distance are:

•

if the geometry of the road was unaltered; and

assumed that these obstructions would be

•

This methodology supports the

Benefits would accrue from these upgrades

1,15 for sight distance restriction in three quadrants of the intersection; and

throughout the design life of the road. As ADT

1,20 for sight distance restriction in four

is an important component of the accident pre-

quadrants of the intersection.

diction


model,

an

academically

correct

9.5.2

approach would involve calculating the predict-

Time savings

ed number of crashes with and without the proposed upgrades for each successive year on

Improvements in geometry can lead to increas-

the basis of an annually increasing ADT, attach-

es in travel speed on a road and hence to reduc-

ing a cash value to the reduction in crashes,

tions in travel time. The travel speed will vary

thereafter discounting benefits to the current

along the length of the road because of changes

year and summing them for comparison with the

in the geometry as well as changes in traffic-

anticipated construction cost.

related variables such as flow, directional split and traffic composition, i.e. the percentage of

In view of the reliance placed on an external

various vehicle types in the traffic stream. The

model and the coarseness of the calibration fac-

traffic-related variables have to be eliminated

tor, this level of refinement is deemed to be

from the comparison between the two condi-

unnecessary.

It is suggested that the Net

tions (with and without geometric upgrades) and

Present Worth of an annual series with a dura-

this is done by determining the speed profiles of

tion equal to the design life of the proposed

vehicles with headways typically longer than 10

upgrade would provide an adequate indication

seconds.

of the merits or otherwise of the upgrade. In effect, the assumption is made that the ADT will

Calculation of the benefit, being the value of the

remain constant for the life of the upgrade.

reduction in travel time arising from improvements to the geometry of the road segment,

The Net Present Worth is expressed as

requires that a speed profile be employed to determine the journey times between the termi9.7

nals of the road segment under consideration. The development of the speed profile is dis-

AS

=

Net present worth

=

Annual saving arising

cussed in the following section. For one year, the benefit is expressed as:

from improvements in safety i

=

Interest rate

n

=

Design life of upgrade

=

ADT (T1 - T2)V

where AT

=

Annual saving arising

9.8

from reduction in travel time

(years) In Section 9.2, reference was made to the aver-

ADT

=

Average daily traffic

T1

=

Travel time with upgrades

age cost of a crash as being of the order of R 30 000.

AT

T2

The annual saving to be inserted in

=

Travel time without upgrades

Equation 9.7 is thus the product of the reduction

V

in the predicted number of crashes annually and

=

Cash value of one hour saved

R 30 000.



where P

The benefit, AT, is inserted into Equation 9.7 to

all of them. Summing the benefits resulting from

derive the Net Present Worth of the time savings

safety improvements and time saving improve-

accruing over the design life of the geometric

ments is thus pointless.

upgrades.

against summing all benefits is that the prime

A further argument

objective of a 3R project is to extend the life of

9.5.3

The speed profile

the pavement. Savings arising from geometric improvements should be seen as a bonus and not as a prime objective.

Development of the speed profile requires an ability to predict speed on the basis of the geometric elements that present themselves. Much

The designer should create a strip map listing all

research on speed prediction has been carried

the planned pavement enhancement activities

out worldwide and some of this work is dis-

by stake value along the road segment under

cussed in Chapter 4. In general it has been

consideration. Brief descriptions of the nature

found that lane and shoulder widths are not sta-

of proposed geometric improvements and their

tistically significant as descriptors of speed and

Net Present Worth should then be added to the

they do not thus appear in the prediction mod-

strip map. With this strategic overview of the 3R

els.

project, it should be possible to select the geometric improvements that could form part of the project.

For convenience, Equation 4.1 is repeated below V85 where V85

=

105,31 + 1,62 x 10-5 x

It is not possible to lay down hard and fast rules

B2 - 0,064 x B

concerning the further definition of the project.

85th

(4.1)

A geometric improvement may have such a high

percentile speed

(km/h)

Net Present Worth that it changes the prioritiza-

=

Bendiness

tion of the pavement remedial works. On the

=

57 300/R (degrees/km)

other hand, an improvement, even with a high

=

57 300 θ/L

Net Present Worth, may fall in an area where no

R

=

Radius (metres)

remedial works are intended and, because of

θ

=

Deviation angle

this, is abandoned. Clearly, a high level of engi-

(radians)

neering judgment would have to brought to bear

Total length of curve

on a 3R project to determine the best possible

(metres).

combination of pavement and geometric

B


=

L

=

improvements.

9.5.4

Evaluation of geometric improvements

Assuming that improvements could be made at several points along the road, it is highly unlikely that sufficient funding would be available to build 9-16 Chapter 6: Intersection Design


GRADE SEPARATION STRUCTURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-1 10.1

Design consistency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-1

10.2

Future capacity requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-1

10.3

Underpasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-2 10.3.1 National road structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-2 10.3.2 Cross roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-2 10.3.3 Agricultural underpasses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-3 10.3.4 Cattle and equestrian underpasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-3 10.3.5 Pedestrian underpasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-3

10.4

Overpasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-3 10.4.1 Cross roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-3 10.4.2 Agricultural overpasses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-6 10.4.3 Footbridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-6 10.4.4 Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-7 10.4.5 Foot walks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-7 10.4.6 Balustrades and parapets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-7

LIST OF TABLES Table 10.1: Standard underpass widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-3 Table 10.2: Standard widths on overpasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-4

LIST OF FIGURES Figure 10.1: Figure 10.2: Figure 10.3: Figure 10.4: Figure 10.5: Figure 10.6: Figure 10.7:

National Road underpasses - Two-lane single carriageway bridges . . . . . . . . . . . . . . . . . . . . 10-2 National Road underpasses- Dual carriageways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-2 Two-lane cross road . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-4 Four-lane cross roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-5 Four- and six-lane cross roads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-5 Six-lane cross-roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-6 Bridge width for agricultural overpass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-6

Chapter 10 GRADE SEPARATION STRUCTURES 10.1

DESIGN CONSISTENCY

The provisions contained in this chapter should

In this guideline, the words “underpass” and

be used in conjunction with the SANRAL Code

“overpass” refer to the position of the minor road

of Procedure for the Planning and Design of

relative to the major road.

Structures. In the event of inconsistencies, the

10.2

Code of Procedure for the Planning and Design

FUTURE CAPACITY REQUIREMENTS

of Structures shall be the governing document. This chapter relates primarily to clearances for

For capacity analysis in respect of structures, a

structures for geometric design purposes and

design life of 50 years should be used, and any

the clearances given should be regarded as

structure over or under a national road should

minimum requirements. Any variations to these

provide for the eventual development of the

clearances should be agreed with the client

national road to its full standard during this life-

prior to the incorporation into the design.

time. At the time of construction of a structure

Nevertheless, the onus rests on the designer to

over a single-carriageway national road on per-

ascertain their applicability to given conditions

manent alignment, the matter of the construc-

and the designer should ensure that the clear-

tion of either one or both spans of the eventual

ances provided do not impede the required sight

two-span structure should be discussed with the

distance. Due cognisance should be taken of

Road Agency's Regional Manager.

accommodating the specific clearances and no

In the case where a double carriageway nation-

part of the structure should encroach on the

al road crosses over a secondary road, the deci-

envelope defining the clearance requirements.

sion on whether or not to fill in the gap in the

However, where the specific horizontal clear-

median caused by not "decking" the two sepa-

ance cannot be achieved, a longitudinal protec-

rate sections of the underpass under the

tive barrier should be provided.

National road carriageway, should be carefully considered with adequate weight being given to

Where structures either over or under railway

safety as a result of the creation of a hazardous

lines are required, the regulations of the Code of

situation for vehicles on the national road.

Procedure issued by the relevant railway authority should be applied to the geometric

In general on divided carriageways a bridge

design. In cases where these regulations differ

median with a width of 10 metres or less should

from the suggestions contained in these guide-

be decked.

lines, the matter should be discussed with the Roads Agency's Regional Manager. 10-1 Chapter 10: Grade Seperation Structures


possible future carriageway or road widening in


Figure 10.1: National Road underpasses - Two-lane single carriageway bridges

Figure 10.2: National Road underpasses- Dual carriageways 10.3

UNDERPASSES

10.3.2 Cross roads

10.3.1 National road structures

Table 10.1 gives standard underpass widths in metres.

Where a National Road is carried over a travelled way, the full width of the National Road

The normal vertical clearance of an underpass

carriageway and shoulders should be provided

is 5,2 metres. However, the requirements of the

between kerbs on all structures as shown in

Provincial or other authority should also be

Figures 10.1 and 10.2.

taken into account. 10-2 Chapter 10: Grade Seperation Structures

Note: For four- and six-lane underpass cross roads, a five metre median is employed.

Bridge designers will normally make use of the

be avoided and crossings should be straight so

median to accommodate a centre pier. The

that the entire length can be seen from each

median would normally accommodate an ade-

end.

quate length of right turn slot 3,7 metres wide,

10.3.5 Pedestrian underpasses

except where the first intersection on the crossroad is very close to the structure.

Pedestrian underpasses should only be consid-

10.3.3 Agricultural underpasses

ered in cases where an overhead structure is impractical. The minimum vertical and horizon-

The normal horizontal clearance between abut-

tal clearances for pedestrian underpasses will

ments for an agricultural underpass is 4,0

be 2,5 metres x 2,5 metres.

metres. If local conditions allow, this dimension may be reduced.

Proposals for wider clear-

In urban areas, lighting should be provided for

ances should be motivated and referred to the

pedestrian underpasses, as well as for pedestri-

National Road Agency's Regional Manager. If

an ramps.

access to the farm, it should have a minimum

The maximum longitudinal slope on ramps

vertical clearance of 4,3 metres. Any other agri-

should not exceed 9 per cent.

cultural underpasses provided would normally

10.4

have a vertical clearance of 4 metres, or 2,5

OVERPASSES

metres if this is acceptable to the farmer.

10.4.1 Cross roads 10.3.4 Cattle and equestrian underpasses Table 10.2 gives the standard widths in metres The vertical and horizontal clearance for cattle

between guardrails on overpasses for roads

and equestrian underpasses should normally be

crossing the national road. These overpasses are

3 metres x 3 metres. Skewed crossings should

dimensioned in more detail in Figures 10.3 to 10.6. 10-3

Chapter 10: Grade Seperation Structures


the agricultural underpass provides the only

Notes

structure. The geometry of each individual case must be evaluated in terms of the local gradient

1.

2.

In this type of road there is no median

and superelevation, if any, as well as the shape

but an extra lane is added for back-to-

of the structure, which may also be carrying a

back right hand turn storage.

road with its own gradient and superelevation.

All roads having 4 or more lanes shall be

Due to the vulnerability of the individual beams

constructed as two separate carriage

of precast prestressed beam and slab decks to

ways with, in the interests of safety, a

impact, a minimum vertical clearance of 5,6

median of adequate width.

metres should be used for this type of deck. Alternatively, the beam bottom flanges should

The vertical clearance above any point of the

be joined together to form a continuous solid

road surface which is under a structure should,

soffit.


for new bridges, be not less than 5,2 metres. This allows for future resurfacing under the

In the case of existing bridges, a relaxation to

bridge. The critical point is not necessarily on

not less than 4,9 metres can be considered to

the centreline of the road passing under the

make provision for rehabilitation of the pave-

Figure 10.3: Two-lane cross road 10-4 Chapter 10: Grade Seperation Structures

Figure 10.4: Four-lane cross roads

Figure 10.5: Four- and six-lane cross roads ment without having to raise the bridge deck. It

case of interchanges, this is normally

should be noted that the maximum permissible

made up of 1,0 metre of shoulder and a

height of a double-decker bus is 4,6 metres and,

1,5 metre sidewalk or a 2,5 metre shoul-

for any other vehicle, 4,3 metres.

der and no sidewalk. Where there is no

On routes that serve as Superload routes, additional clearances may be necessary.

ity may use the 2,5 metres as it sees fit.

The

b.

designer should check whether a route is

In the case of minor roads, the clear

intended for super loads and, if so, what require-

width between guardrails or handrails

ments exist for it..

may be reduced to 9,4 metres.

c.

General Notes

The requirements of the provincial or other authority should also be taken into

a.

In each case, an allowance of 2,5 metre

account when the dimensions of over-

between the edge of the carriageway

pass structures are determined.

and the handrail has been made. In the 10-5 Chapter 10: Grade Seperation Structures


interchange, the controlling local author-

Figure 10.6: Six-lane cross-roads


Figure 10.7: Bridge width for agricultural overpass 10.4.2 Agricultural overpasses

should be referred to the Regional Manager of the NRA.

Shoulders and foot walks are not necessary on agricultural

overpasses.

The

10.4.3 Footbridges

clearance

between guardrails should be either 4,0 metres, as shown in Figure 10.7 where adequate stop-

Where possible, overhead crossings for pedes-

ping sight distance is obtainable, or 5,5 metres

trians should have a minimum width of 2,0

where stopping sight distance is not obtainable

metres, and should have handrails normally

with a structure serving more than one property.

1,10 metres in height. The handrails should be

Any proposed deviation from this standard

designed to be vandal-proof and durable. 10-6

Chapter 10: Grade Seperation Structures

Because of the relative lightness of pedestrian

provided on bridges carrying National Road

footbridges, a vehicle impacting the structure is

freeways.

likely to cause considerable damage, including

10.4.6 Balustrades and parapets

the possibility of the deck dropping onto the roadway below.

A vertical clearance of 5,9

metres is thus normally required for footbridges.

Details for balustrades and parapets should be as specified in the Code of Procedure for the

Ramp approaches should be designed to

Planning of Design Structures.

encourage pedestrian usage by following established routes. Where appropriate, these may be provided with steps on the steeper slopes near the ends of the bridge to encourage usage of the bridge. Vandalism

includes

deliberately

dropping

objects onto vehicles passing under footbridges. Where this is likely to occur or has been known to occur, the footbridge should be enclosed by a cage over its full length.

10.4.4 Services In cases where permission is granted by the NRA for the carrying of services across the national road reserve, these services should be situated in such a way that they will not be visible to traffic on either the National Road or the the foot walks (where these exist) or under the balustrade.

10.4.5 Foot walks Where provided on structures carrying cross roads, foot walks should be have a minimum width of 1,25 metres. Pedestrians on the foot walks should be protected from traffic by means of a barrier kerb on the side of the foot walks. For obvious reasons, no foot walks are to be 10-7 Chapter 10: Grade Seperation Structures


cross road. The services should be located in

TABLE OF CONTENTS 11

TOLL PLAZAS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-1 11.1

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-1

11.2

Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-1 11.2.1 Positioning of the Plaza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-1 11.2.2 Road User Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-1 11.2.3 Land required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-1 11.2.4 Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-2 11.2.5 Operational Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-2 11.2.6 Security. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-2

11.3

Design Norms and Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-2

LIST OF FIGURES Figure 11.1: Typical layout of toll plaza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-3 Figure 11.2: Cross-section of toll plaza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-4 Figure 11.3: Island layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-5 Figure 11.4: Section through islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-6

Chapter 11 TOLL PLAZAS 11.1

INTRODUCTION

11.2.2 Road User Safety

Considerable experience in the planning, design

The road geometry through the plaza and on its

and operation of toll plazas has been gained in

approaches is the major determinant of road

South Africa on our National Highways. This

user safety.

section summarises the important factors as

located on a tangent and meet the following

they impact on the planning and design of major

measures of sight distance.

The plaza should preferably be

roads. The purpose is to familiarize designers with the concepts and constraints of toll plaza

Stopping sight distance must be continuously

design and provide information for use in layout

available along the road. At the approach to the

design and basic planning.

plaza court, vehicles, and heavy vehicles in particular, require sufficient distance to stop at the

11.2

PLANNING

rear of a 5-vehicle queue. Because of speed restrictions in the plaza area, an approach

11.2.1 Positioning of the Plaza

speed of 100km/hr can be assumed for design. See Sections 3.5.4 to 3.5.6.

A network analysis of travel volumes, origins and destination and the relative costs of using

Decision sight distance must be available to the

the tolled road is a prerequisite. Once the finan-

start of the approach tapers as discussed in

cial viability is established i.e. the income

Section 3.5.8. The column "Interchanges: Sight

stream is in balance with the overall project

distance to nose" in Table 3.7 should be used.

costs, it is then necessary to position the plaza so as to maximize the defined catchment. This

11.2.3 Land required

would normally require the evaluation of a num-

• • • • •

Road user safety

As a general guide, the road reserve for a main

Land required

line plaza with 8 lanes would require widening

Cost

on one side by 15 metres and, on the side con-

Operational efficiency

taining the control building, by approximately 30

Security

metres. The control building itself could occupy a fenced site of approximately 60 metres by 25

There are a number of measures of effective-

metres. These dimensions are average and are

ness that can be applied against these broad

heavily influenced by the landform and the sur-

objectives.

rounding land use.

11-1 Chapter 11: Toll Plazas


ber of alternatives as they impact on;

11.2.4 Cost

11.2.6 Security

The construction cost and operational cost

Location factors that influence toll plaza securi-

determine the economy of the toll plaza loca-

ty and which should be evaluated are;

tion. The measure of effectiveness is the pres-

• • •

Land use in the vicinity

11.3

Design Norms and Dimensions

ent value of future expenditure. The following are the important items that should be consid-

Other access routes to the toll plaza Vegetation

ered.

• • • • • • • • • •

Land costs Mass earthworks Geotechnical conditions

The toll plaza layout will ultimately be governed

Cost of services

by the number of lanes required. The standard

Cost of relocating services

toll booth module is 5,0 m wide. The lane width

Stormwater drainage

is 3,0 metres and the toll island is 2,0 metres

Pavement widening

wide.

Plaza construction and running costs

arrangement and island details are shown in

Accommodation of traffic

Figures 11.1 to 11.4. An extra 3,0 m shoulder is

Maintenance costs.

recommended at the toll lanes on the periphery

The general layout of a plaza, lane

to accommodate abnormal vehicles.

11.2.5 Operational Efficiency For planning purposes, the average queuing The geometry of the road at the approaches and

and processing time per vehicle can be taken as

within the plaza court influences the operational

30 seconds and the maximum queue length

efficiency. Tapers should be such that vehicles

should not exceed 5 vehicles.

can spread comfortably and merge without undue constraint. Approach tapers of 1:8 and merge tapers of 1:5 are acceptable to and from


a point 50 metres from the centre line across the plaza. A maximum longitudinal grade of 1,5% should be maintained within the 60 metre queuing length on either side of the plaza. A grade of 3% should not be exceeded at the approach to the taper areas.

11-2 Chapter 11: Toll Plazas

11-3 Chapter11: Toll Plazas


Figure 11.1: Typical layout of toll plaza


Figure 11.2: Cross-section of toll plaza 11-4 Chapter11: Toll Plazas

11-5 Chapter11: Toll Plazas


Figure 11.3: Island layout


Figure 11.4: Section through islands 11-6 Chapter11: Toll Plazas

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South African National Roads Agency Limited

Historically Geometric Design was predicated on the capabilities of the design vehicle. Comparatively recently, Geometric Design has undergone a paradigm shift of note. It is now accepted that a road designed to “standards” is not necessarily safe and, furthermore, that human factors play a greater role in the determination of geometric design standards than do the limitations of the various design vehicles. This Guideline document, compiled by CSIR on behalf of the South African National Road Agency, replaces the previous G2 Manual and is based on the new design philosophy.

The Guideline discusses the: • Design philosophy and techniques • Design controls • Road elements being: • The horizontal and vertical alignments; and • The cross-section • Aesthetics • Intersections • Interchanges and grade separation structures • Safety features • Road betterment • Toll plazas

SANRAL Geometric Design Guide

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