F'OITEWORD Road Engineering Association of Malaysia (REAM), througlr the cooperation and support of various road authorities and engineering institutions in Malaysia, publishes a series of official
documents on STANDARD SPECIFICATIONS, GUIDELINES, MANUALS and TECHNICAL NOTES which are related to road engineering. The aim of such publications is to achieve quality and consistency in road and highway construction. The cooperating bodies are:-
Public Works Department, Malaysia Malaysian Highway Authority Institution of Engineers, Malaysia Institution of Highways & Transportation (Malaysian Branch)
The production of such documents are carried out through several stages. The documents are initially compiled/drafted by the relevant Technical Committee and subsequently scrutinised by the relevant Standing Committee of REAM. They are finally endorsed by road authorities and practitioners of road engineering at a conference/workshop before publication. REAM welcomes feedback and suggestions which can update and improve these documents.
This GUIDE ON GEOMETRIC DESIGN OF ROADS is the revision of the existing Arahan Teknik (Jalan) 8/86 "A Guide on Geometric Design of Roads" published by Jabatan Kerja Raya.
The revision was undertaken by the Standing Committee on Technology and Road Management of the Road Engineering Association of Malaysia (REAM) with the mandate given by the Director-General of Public Works Department, Malaysia.
The draft of this Arahan Teknik was presented and discussed at the Forum on Technology and Road Management in November 1997 and the 4th Maiaysian Road Conference in November 2000.
It is hoped that this Guide will will
address the shortcomings of Arahan Teknik (Jalan) 8/86 and be accepted by the highway engineering fraternity in the country.
ROAD ENGINEERING ASSOCIATION OF MALAYSIA 46-4, Jalan Bola Tarnpar 13114, Section 13,40100 Shah Alam, Selangor, Malaysia. Tel:603-55136521 Fax:603-55136523 e-mail:
[email protected]
ROAD ENGINEERING ASSOCIATION OF MALAYSIA A GUIDE ON GEOMETRIC DESIGN OF ROADS TABLE OF CONTENTS PAGE NO.
INDEX
I.
INTRODUCTION
I
2.
ROAD CLASSIFICATIONS AND DESIGN STANDARDS
z
2.1
ROAD CLASSIFICATION / HIERARCHY
2
2.2
DESIGN STANDARDS FOR ROADS
5
2.3
ACCESS CONTROL
6
)4
SELECTION OF DESIGN STANDARDS
8
DESIGN CONTROL AND CRITERIA
A
11
J.l
TOPOGRAPHY AND LAND USE
ll
J.Z
TRAFFIC
12
3.3
DESIGN VEHICLES CHARACTERISTICS
l.+
J-+
SPEED
18
3.5
CAPACITY
20
ELEMENTS OF DESIGN
25
4.1
SIGHT DISTANCE
25
A'
HORIZONTAL ALIGNMENT
a1 J+
+.J
VERTICAL ALIGNMENT
44
4.4
COMBINATION OF HORIZONTAL AND VERTICAL
52
ALIGNMENT
5.
CROSS SECTION ELEMENTS
61
5.1
PAVEMENT
6l
5.2
LANE WIDTH & MARGINAL STRIPS
62
5.3
SHOULDERS
62
5.4
KERBS
66
5.5
SIDEWALKS
69
5.6
TRAFFIC BARRIERS
6q
5.7
MEDIANS
69
TABLE OF CONTENTS (Cont'd) INDEX
PAGE NO. 5.8
SERVICE ROADS
10
5.9
U-TURN
73
BRIDGE AND STRUCTURE CROSS SECTION
16
BUS LAYBYES
to
5.
l0
5.t
6.
I
OTHER ELEMENTS AFFECTING GEOMETRIC
DESIGN
6.I ROAD SAFETY 6.2 DRAINAGE 6.3 LIGHTING 6.4 UTILITIES 6.5 SIGNING AND MARKINGS 6.6 TRAFFIC SIGNALS 6.1 EROSION CONTROL, LANDSCAPE DEVELOPMENT AND ENVIRONMENTAL IMPACTS
80 80 80
8I 81
8I 8I 82
TABLE OF CONTENTS (Cont'd) TABLES
PAGE NO.
TABLE 2-I
CHARACTERISTICS OF ROAD CATEGORIES
A T
TABLE2-2A
SELECTION OF ACCESS CONTROL (RURAL)
8
TABLE2-28
SELECTION OF ACCESS CONTROL (URBAN)
8
TABLE 2-3
SELECTION OF DESIGN STANDARD
9
TABLE
DIMENSION OF DESIGN VEHICLES
14
TABLE 3-2A
DESIGN SPEED FOR RURAL ROADS
19
TABLE 3-2B
DESIGN SPEED FOR URBAN ROADS
1q
TABLE 3-3
CONVERSION FACTORS TO P.C.U.
2l
TABLE 3-4
LEVELS OF SERVICE
22
TABLE 3-5A
DESIGN LEVEL OF SERVICE AND VOLUME/
aA
3.1
CAPACITY RATIO (RURAL) TABLE 3-58
DESIGN LEVEL OF SERVICE AND VOLUME/
24
CAPACITY RATIO (URBAN) TABLE
4-1
TABLE4-2
MINIMUM STOPPING DISTANCE
zo
EFFECTS OF GRADES IN STOPPING SIGHT
2'7
DISTANCE _ WET CONDITIONS TABLE 4-3
DECISION SIGHT DISTANCE
28
TABLE 4-4
MINIMUM PASSING SIGHT DISTANCES
30
(2LANE-2WAY) TABLE 4-5
MINIMUM RADIUS
3s
TABLE 4-6
RELATIONSHIP OF DESIGN SPEED TO MAXIMUM
5l
RELATIVE PROFILE GRADIENTS
TABLE4.]A TABLE 4-78
TABLE 4-8 TABLE 4-9A
(RURAL) 38 DESIGN SUPERELEVATION TABLE (URBAN) 39 PAVEMENT WIDENING ON OPEN ROAD CURVES 43 MAXIMUM GRADES FOR RI. R2. Ul & U2 45 DESIGN SUPERELEVATION TABLE
STANDARD ROADS
TABLE 4-98
MAXIMUM GRADES FOR R3 AND R4 STANDARD 45 ROADS
TABLE 4-9C
MAXIMUM GRADES FOR U3 AND U4 STANDARD 45 ROADS
TABLE 4-9D
MAXIMUM GRADES FOR R5 STANDARD ROADS
TABLE 4-9E
MAXIMUM GRADES FOR U5 STANDARD ROADS
TABLE 4-9F
MAXIMUM GRADES FOR U6 AND R6 STANDARD ROADS
ul
A<
46
TABLE OF CONTENTS (Cont'd) TABLES
PAGE NO.
TABLE 4-1OA
CREST VERTICAL CURVE (K VALUES)
50
TABLE 4-IOB
SAG VERTICAL CURVE (K VALUES)
51
TABLE
TYPICAL PAVEMENT SURFACE TYPE
6l
TABLE 5-2
LANE & MARGINAL STRIP WIDTHS
6l
TABLE 5-3A
USABLE SHOULDER WiDTH (RURAL)
64
TABLE 5-3B
PAVED SHOULDER WIDTH (URBAN)
65
TABLE 5-4
PAVED SHOULDER WIDTH (RURAL)
65
TABLE 5-5A
MEDIAN WIDTH (RURAL)
70
TABLE 5-58
MEDIAN WrDTH (URBAN)
70
TABLE 5-6
DESIRABLE DISTANCE BETWEEN U-TURNS
74
5-1
TABLE OF CONTENTS (Cont'd) PAGES NO
FIGUR.ES
FIGURE 2-I
FLOW CHART FOR SELECION OFDESIGN STANDARDS IO
FIGURE
P DESIGN
FIGURE 3-3
VEHICLE SU DESIGN VEHICLE WB-15 DESIGN VEHICLE
FIGURE 3-4
RELATIONSHIP OF LEVEL OF SERVICE TO OPERATING 23
3-1
FIGURE 3-2
15
16
I1
SPEED AND VOLUME / CAPACITY RATIO FIGUR.E 4-1
MEASURING SIGHT DISTANCE ON PLAN
32
FIGURE 4-2
MEASURING SIGHT DISTANCE ON PROFILE
JJ
FIGURE 4-3
COMPARISON OF SIDE FRICTION FACTORS ASSUMED
35
FOR DESIGN OF DIFFERENT TYPES OF ROADS FIGURE 4-4
4]
DIAGRAMMATIC PROFILES SHOWING METHODS OF ATTAINiNG SUPERELEVATION
FIGURE 4-5
4't
CRITICAL LENGTHS OF GRADE FOR DESIGN FOR TYPICAL HEAVY TRUCK OF 180kg/kw.
FIGURE 4-6
A&B C&D E&F G,H&I J,K&L
FIGURE 4.6
M
59
FIGURE 4-6
N
60
FIGURE 5-1
TYPICAL CROSS SECTION OF SHOULDER AND VERGE
63
FIGURE 5-2A
ROAD KERB DETAILS
67
FIGURE 5-28
ROAD KERB DETAILS
68
FIGURE 5-3
TWO-WAY SERVICE ROAD
72
FIGURE 5-4
MINIMUM DESIGN FOR DIRECTU-TURNS
'75
FIGURE 5-5
TYPICAL BRIDGE CROSS SECTIONS
77
FIGURE 5-6
CLEARANCES AT UNDERPASS
18
FIGURE 5-7
LAYOUT OF BUS LAYBYES
'79
FIGURE 4-6 FIGURE 4-6 FIGURE 4-6 FIGURE 4-6
.*1i
55
56 57 58
'i{F,:i'. Ei:.
"'r:' iisif-.:"
;l
::5;;r.?;
A Guide on Geometric Design of Roads
CHAPTER 1 - INTRODUCTION This Guide is limited to the geometric features of road design as distinguished from structural design. It is intended as a manual on the geometric design of road, inclusive both in rural as well as in urban conditions. The geometric design of road is only applicable to Rural or Urban areas as specifically indicated in this Guide.
This Guide is to be applied to all new construction and improvements of roads for vehicular traffic. Comments from users will be most welcomed as this Guide will continue to be updated from time to time.
This Guide is to be used in conjunction with other Arahan Tekniks that have been or will be produced by Jabatan Keria Raya or other Agencies.
A Guide on Geometrtc Design
of
Roads
CHAPTER 2 - ROAD CLASSIFICATIONS AND DESIGN STANDARDS
2.r
ROAp CLASSTFTCATTON / HrERARelgy
2.1.1 The Importance
of Road Classification
The importance of defining a hierarchy of roads is that it can help clarify policies concerning the highway aspects of individual planning decisions on properties served by the road concerned. Further'more specific planning criteria could be developed and applied according to a road's designation in the hierarchy: for example design speed, width of carriageway, control over pedestrian, intersections, frontage access etc. In this way the planning objectives would be clear for each level of road in the hierarchy and policies on development control and traffic management would reinforce one another.
2.1.2 Functions of Road Each road has its function according to its role either in the National Network, Regional Network, State Network or City/Town Network. The most basic function of a road is transportation. This can be further divided into two sub-functions; namely mobility and accessibility. However, these two sub-functions are in trade off. To enhance one, the other must be limited. In rural areas, roads are divided into five categories, narnely, EXPRESSWAY, HIGHWAY, PRIMARY ROAD, SECONDARY ROAD ANd
MINOR ROAD. In urban areas, roads are divided into four categories, namely EXPRESSWAY, ARTERIAL, COLLECTOR and LOCAL STREET. They are in ascending order of accessibility and thus in descending order of mobility.
2.1.3
Road Category and Their Application Categories of Road 2 GROUPS
URBAN (4 Categories)
1. Expressway
2. Arterial 3. Collector 4. Local Street
RURAL (5 Categories)
l.
Expressway
2. Highway 3. Primary Road
4. Secondary Road 5. Minor Road
A Guid.e on Geometric Design af Roads
Categories of roads in Malaysia are defined by their general functions as follows: (a)
Expresswav
An Expressway is a divided highway for through traffic with full control of access and always
with grade separations at all intersections'
In rural areas, they apply to the interstate highways for through traffic and form the basic framework of National road transportation for fast travelling. They serve long trips and provide'higher speed of travelling and comfort. To maintaiu this, they are fully access controlled and are designed to the highest standards.In urban areas, they form the basic framework of road transportation system in urbanised area for through traffic. They also serve relatively long trips and smooth traffic flow and with full access control and complements the Rural Expressway. (b)
Highway They constitute the interstate national network for intermediate traffic volumes and complements the expressway network. They usually link up directly or indirectly the Federal Capital, State capitals, large urban centres and points of entry/exit to the country. They serve long to intermediate trip lengths. Speed of travel is not as important as in an Expressway but relatively high to medium speed is necessary. Smooth traffic is provided wrth partial access control.
(c)
Primary Roads They constitute the major roads forming the basic network of the road transportation system within a state. They serve intermediate trip lengths and medium travelling speeds. Smooth traffic is provided with partial access control.They usually link the State Capitals and district Capitals or other Major Towns.
(d)
Secondary Roads
They constitute the major roads forming the basic network of the road transportation system within a District or Regional Development Areas. They serve intermediate trip lengths with partial access control. They usually link up the major towns within the District or Regional Development Area. (e)
Minor Roads They apply to all roads other than those described above in the rural areas. They form the basic road network within a Land Scherne or other sparsely populated rural area. They also include roads with special functions such as holiday resort roads, security roads or access roads to microwave stations. They serve mainly local traffic with short trip lengths with no access control.
A Guide on Geometric Design of Roads
(0
Arterials An arterial is a continuous road within partial access control for through traffic within urban areas. Basically it conveys traffic from residential areas to the vicinity of the central business district or from one part of a city to another which does not intend to penetrate the city centre. Anerials do not penetrate identifiable neighbourhoods. Smooth traffic flow is essential since it carries larse traffic volumes.
(e)
Collectors A collector road is a road with partial access control designed to serve as a collector or distributor of traffic between the arterial and the local road systems. Collectors are the major roads which penetrate and serve identifiable neighbourhoods, commercial areas and industrial areas.
(h)
Local Streets The local street system is the basic road network within a neighbourhood and serves primarily to offer direct access to abutting land. They are links to the collector road and thus serve short trip lengths. Through traffic should be discouraged. The characteristics for road categories are summarised as in Table 2-r.
TABLE 2.1
:
CHARACTERISTICS OF ROAD CATEGORIES
ROAD AREA
Trip Length Long
RURAL
Med
Expressway
I
NETWORK
Speed
Highway
I
Primary Road
-
I
Short
I
Local Street
Low
High
Low
Med
National network
t
I
I
I
I
!E
I
E
State network
E
r
!
T E
* IT
E
I
National network
I E
I
Expressway
Collector
Med
-I
I
Minor Road
Arterial
Hieh I
Secondary Road
URBAN
Design Volume
CATEGORIES
-
District network Supporting network National network
Major links to
I
Urban centres
E
E E
-
I
Major streets within urban centres
E
Minor network.
streets/town
A Guide on Geometric Design of Roads
2.2
DESIGN STANDARDS FOR ROADS
2.2.1 The Importance
of Standardisation
Technically, the geometric design of all roads need to be standardised for the following reasons:-
(a)
to provide uniformity in the design of roads according to their performance requirements.
(b)
to provide consistent, safe and reliable road facilities for movement of traffic.
(c)
to provide a guide for less subjective decision on road design.
2.2.2 Rural and Urban
Areas
a
gazetted Municipality limits or township having a population of at least 10,000 where buildings and houses are gathered and business activity is prevalent. Any roads outside the Municipality limits is considered rural, including roads connecting Municipalities that are more than 5 kilometres apart.
Urban areas are defined as roads within
There is no fundamental difference in the principles of design for rural and urban roads. Roads in urban areas, however, are characterised by busy pedestrian activities and frequent stopping of vehicles owing to short intersection spacings and congested built-up areas. Lower design speeds are usually adopted for urban roads and different cross sectional elements are applied to take into account the nature of traffic and adjoining land use. It is for these reasons that variations in certain aspects of geometric design are incorporated for these two broad groups of roads.
2.2.3 Application
of Design Standards for Roads
The design standard is classified into six (6) groups (R6, R5, R4, R3, R2 & R1) for rural (R) areas and into six (6) groups (U6, U5, U4,U3,U2 & Ul) for urban (U) areas. Each of these standards are listed below with descending order of hierarchy. (a)
Standard R6/U6:
Provides the highest geometric design standards for rural or urban areas.They usually serve long trips with high travelling speed (90 kph or higher)of travelling,comfort and safety. It is always designed with dividedcarriageways and with fuli access control. The Rural and Urban Expressway falls under this standard.
A Guide on Geomelric Design of Roads
(b)
Standard R5/U5:
Provides high geometric standards and usually serve long to intermediate trip lengths with high ro medium travelling speeds (80 kph or higher). It is usually with partial access control. The Highway, Primary Road and Arterial fall under this standard. It is sometimes designed as divided carriageways with partial access control.
(c)
Standard R4N4:
Provides medium geometric standards and serve intermediate trip lengths with medium travelling speeds (70 kph or higher). It is also usually with partial access control. The Primary Road, Secondary Road, Minor Arterial and Major Collector fall under this standard.
(d)
Standard R3/U3:
Provides low geometric standard and serves mainly local traffic. There is partial or no access control. The Secondary Road, Collector or Major Local Streets are
within this standard. The travelling speed is usuallv 60 kph.
(e)
Standard
R2N2:
Provides low geometric standards for two way flow. It is applied only to local traffic with low volumes of commercial traffic. The Minor Roads and Local Streets fall under this standard. The travelling speed is usually 50 kph.
(0
Standard R1/U1:
Provides the lowest geometric standards and is applied to road where the volumes of commercial vehicles are very
low in comparison to passenger traffic. The travelling speed is 40 kph or less. In cases where commercial traffic
is not envisaged such as private access road in low cost housing area, the geometry standards could be lowered especially the lane width and gradient.
2.3
ACCESS CONTROL
2.3.1
Degree of Control Access control is the condition where the right of owners or occupants of abutting land or other persons to access, in connection with a road is fully or partially controlled by the
public authority.
A Guide on Geometric Design of Roads
Control of access is usually classified into three types for its degree of control, namely full control, partial control and non-control of access.
Full Control of Access means that preference is given to through traffic by providing with selected public roads only and by prohibiting crossings at grade or direct private driveway connections. The access connections with public roads varies from 2 km in the highly developed central business area to 8 km or more in the sparsely access connecting
developed urban fringes.
Partial Control of Access means that preference is given to through traffic to a degree that in addition to access connection with selected public roads, there may be some crossings trafficked roads. At-grade intersections should be limited and only allowed at selected locations. The spacin g of at-grade intersections preferably signalised may vary from 0.4 km to 1.0 km.
To compensate for the limited access to fully or partially
access controlled roads,
frontage or service roads are sometimes provided along side of the main roads.
In Non-Control Access, there is basically no limitation of access.
2.3.2
Selection of Access Control The selection of the degree of control required is imporlant so as to preserve the as-built capacity of the road as well as improve safety to all road users. Two aspects pertaining to the degree of control is to be noted.
(a)
during the time of design in the consideration of accesses to existing develooments.
(b)
after the completion of the road in the controi of accesses to future develonments.
The selection of degree of access control depends on traffic volumes, function of the road and the road network around the areas. Tables 2-2A and2-2F_ are general guides for the selection of desree of access control.
A Guide on Geometric Design of Roads
TABLE 2-ZA: SELECTION OF ACCESS CONTROL (RURAL)
Xt'tn Road
Standard _____
R5
R6
R4
R3
R2
R1
N
N
categorr---'--
Expressway Highway Primary Road
F P P
Secondary Road Minor Road
P P
P
TABLE 2-28: SELECTION OF ACCESS CONTROL (URBAN) -----=pesign Standard \\\ __r\\_
Road
Category \-r'-\
Expressway
U6
U5
U4
P
P
P
P
P
N
N
:
F = P = N -
U2
U1
N
N
F
Arterial Collector Local Street Note
U3
Full Control of Access, Partial Control of Access, No Control of Access
2.4
SELECTION OF DESIGN STANDARDS
2.4.1
Selection of Design Standard The selection of the required design standard should begin with the assessment of the function of the proposed road and the area it traverses. If there is an overlapping of functions, the ultimate function of the road shall be used for the selection criteria. The projected average daily traffic (ADT) at the end of the design life (15 years after completion of the road) should then be calculated and from Table2-3, the required design standard can be obtained. From the capacity analysis the required nurnber of lanes can then be calculated.
A Guide on Geometric Design of Roads
The selection of design standards used for various categories of roads is as shown in Table 2-3 while the process of selecting design standard are as shown in Figure 2-I. TABLE 2.3: SELECTION OF DESIGN STANDARD
Area
\
Projected
\ Road \
Category
ADT
Traffic
10,000
>10,000
Volume
to 3,000
3,000 to
1,000
1,000
150
Highway Primary Road
p<
R4 R4
Arterials Collector
Local Street
150
R3
Minor Road
Expressway
<
R6 R5
Secondary Road
URBAN
to
\
Expressway
RURAL
All
D'
RI
U2
U1
U6 U5 U5
U4 U4 U4
U3 U3
A Guide on Geometric Design of Roads
FIGURE 2-1 : FLOW CHART FOR SELECTION OF DESIGN STANDARDS
Project Identification
Designate Road Group According to Nature of Area Road Traverses Select Road Category based on function e.g. Primary Road
Select Road
Category based on function e.g. Arterial
Estimate ADT at end of desisn life Table 2.3 Determine design standard e.g. R6,U3 etc.
Route Location
Survey and Design
r0
A Guide on Geometric Design of Roads
CHAPTER 3 - DESIGN CONTROL AND CRITERIA This chapter discusses those characteristics that govern the various elements in the design of highway. These include the topography and land use through which the route is to be located, the traffic that will use the route, design vehicles characteristics, design speed of the route and the capacity to be provided along the route. 3.1
TOPOGRAPHY AND LAND USE The location of a road and its design are considerably influenced by the topography, physical features, and land use of the area traversed. Geometric design elements such as alignment, gradients, sight distance and cross-section are directly affected by the topography, and must be selected so that the road designed will reasonably fit into those natural and man-made features and economise on construction and maintenance.
The topography through which the road passes can generally be divided into three groups, namely, FLAT, ROLLING and MOUNTAINOUS; where FLAT terrain means:
The topographical condition where highway sight distances, as governed by both horizontal and vertical restrictions, are generally long or could be made to be so without construction difficulty or expertise. The natural ground cross slopes (i.e., perpendicular to natural ground contours) in a flat terrain are generally below 37o.
ROLLING terrain means:
The topographical condition where the natural slopes consistently rise above and fall below the road or street grade and where occasional steep slopes offer some restrictions to normal horizontal and vertical roadway alignment. The natural ground cross slopes in a rolling terrain are generally between 37o-25%o.
MOUNTAINOUS terTaln means:
The topogaphical condition where longitudinal and transverse changes in the elevation of the ground
with respect to the road or street are abrupt and where benching and side hill excavation are frequently required to obtain acceptable horizontal and vertical alignment. The natural ground crossslopes in a mountainous terrain are generally above 25Vo.
11 ll
A Guide on Geometric Design of Roads
The topography of a route may affect the operation characteristic of the roadway and this may affect the type of road to be built. For example, steep gradient will reduce the capacity of a2-Iane road (single carriageway) and lower the operation speed of this link. However, if a dual carriageway or a wider road is used, the effects will be much less and hence the operation speed and characteristics of the road will be enhanced. Consequently, the nature of the terrain sometimes determines the type of road to be built.
Topographical conditions may also affect the cross-sectional alrangement of divided roads. In some cases a facility of a single road formation may be appropriate; in others it may be more fitting to locate the road within two separate road formation due to economic or environmental considerations. In urban areas, land development for residential, commercial and industrial purposes will restrict choice of road location, lower running speed, generate more turning movements, and require more frequent intersections than in open rural areas. Geologic and climatic conditions must also be considered for the location and geometric design of a road.
3.2
TRAFFIC The design of a road should be based on traffic data, which serve to establish the "loads" for geometric design. Traffic data for existing roads or section of road are available from the annual "Road Traffic Volume, Malaysia" published by Highway Planning Unit (HPU) of the Ministry of Works, Malaysia.
3.2.1 Average Daily Traffic (ADT) ADT represents the total traffic for the year divided by 365, or the average traffic volume per day. ADT is imporlant for purposes such as determining annual usage as justification for proposed expenditures, or for design of structural elements of a road. The projected ADT is also used to designate the standard of road as shown in Table 2.3 - Selection of Design Standards. However, the direct use of ADT in geometric design is not appropriate because it does not indicate the significant fluctuation in the traffic volume occurring during various months of the year, days of the week or hours of the day. A more appropriate measurement is the hourly volume which is used to determine the capacity requirement of the road.
3.2.2
Design Hourly Volume (DHV) The traffic pattern on any road shows considerable fluctuation in traffic volumes during the different hours of the day and in hourly volumes throughout the year. It is difficult to determine which of these hourly traffic volumes should be used for design. It would be wasteful to base the design on the maximum peak hour traffic of the year, yet the use of the average hourly traffic would result in an inadequate design.
TZ
A Guide on Geometric Design of Roads
To determine the hourly traffic best fitted for design, a curve showing the variation in hourly traffic volume during the year is used. The hourly traffic used in design is the 30th. highest hourly volume of the year, abbreviated as 30 HV. The design hourly volume, abbreviated as DHV is the 30HV of the future year chosen for the design. The above criteria is applicable to most rural and urban roads. However, for roads on which there is unusual or highly seasonal fluctuation in traffic flow such as holiday resort roads, the 30HV may not be appropriate. It may be desirable, to choose an hourly volume for design (about 50 percent of the hourly volumes) expected to occur during a very few maximum hours (15 to 20) of the design year whether or not it is equal to 30HV.
3.2.3
Design Hourly Volume Ratio (K)
K is the traffic ratio of DHV to the designed ADT. K's value ranges froml%o to 20Vo. The actual value should be obtained from traffic data or census. As a guide, K=12.0Vo for urban roads and K=15%o for rural roads may be used. Where the design of the road facility is sensitive to the value of K, the K value should be established from traffic survey/study. The directional distribution of traffic (D) during the design hour should be determined by field measurements on the facilities. Generally D value of 60Vo can be used in urban areas and 65Va in rural areas. Traffic distribution by directions is generally consistent from year to year and from day to day except on some roads serving holiday resort area.
3.2.4 Traffic Composition This refers to the percentage of various classes of vehicles in the DHV. Vehicle of different sizes and weights have different operating characteristics which must be considered in geometric design. Commercial vehicles generally are heavier, slower and occupy more roadway space, thus imposing greater traffic effect on the road than passenger vehicles"
Vehicles of various sizes and weights are classified into the following six (6) groups under the annual National Traffic Census conducted by HPU: a. motor cycles b. cars and taxis c. light vans and utility vehicles d. medium lorries (2 axle) e. heavy lorries (3 or more axles)
f.
buses
On existing road network, the traffic composition at peak hour can be established from "Road Traffic Volume Malaysia" published by the Highway Planning Unit, Ministry of Works.
3.2.5 Projection of Traffic New roads or improvements of existing roads should be based on future traffic expected
A Guide on Geometric Design of Roads
to use the facilities. Desirably, a road should be designed to accommodate the traffic that might occur within the Iife of the facility under reasonable maintenance. l'he projection of traffic for use in the design should be based on period a of l5 years after completion of the road
3.3 The geometric design of roads is affected by the physical characteristics of vehicles and the proportions of various sizes vehicles using the roads. A design vehicle is a selected motor vehicle, the weight, dimensions and operating characteristics of which are used to establish highway design controls to accommodate vehicles of a designated type. For purposes of geometric design, the design vehicle should be one with dimensions and minimum turning radius larger than thoie of almost all vehicles in its class. Since roads are designed for future traffic, the sizes of vehicles used in the design should be determined by analysing the trends in vehicle dimensions and characteristics.
3.3.1
Design Vehicles The design vehicles to be used for geometric
design follows that used by AASHTo as in chapter II of AASHTo "Design vehicle" - -A policy of Geometric Design of
Highways and streets (1994). Figure 3-1,3-2,and 3-3 show the dimensions and turning characteristics for the p, SU and wB-r5 design vehicres.
3.3.2
Summary of Dimension of Design Vehicles Table 3-1 below summarises the design vehicle dimensions and characteristics.
TABLE 3-I-DIMENSION OF DESIGN VBHICLES Design Vehicles
Equivalent Type in
Type
AASHTO
Dimension in Metres Turning Radius
Overhang
Wheel base
Front
Rear
(Metres)
Overall Length
Overall
widrh
Height
Inner
Outer Passenger Car
P
Rigid Truck Semi-Trailer
Note:
SU
wB-15
3.4
0.90
I.5
5.8
2.1
1.3
4.2
|
l0
1.2
1.8
9.1
2.60
4.10
8.5
12.8
9. 10
0.9
0.60
16.1
2.60
4"10
5.8
13.7
6.
Marimum alIowable overall f"n Rigid vehicle - 12.2 m &0 ft\
(i)
(ii)
B.
Articutated vehicte _ 16.0 m (52.5 ft)
(iii) Semi-Trailer - I 2.5 m &tft\ (iv) Traiter - 9.0 m (29.5 fi) (v) Truck Trailer - 18.0 m (59 fr)
Maximum allowable overail width under currenf Maraysian Legisration is 2.5 m.
14
--)
FIGURE 3.1 : P DESIGN VEHICLE
,/4 1t. | |
/
/
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,
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it.,
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FIGURE 3.3 : WB-ls DESIGN VEHICLE
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MTTERS
--_
A Guide on Geometric Design of Roads
3.4
SPEED Speed is a primary factor in all modes of trahsportation, and is an important factor in the geometric design of roads. The speed of vehicles on a road depends, in addition to capabilities of the drivers and their vehicles, upon general conditions such as the physical characteristics of the highway, the weather, the presence of other vehicles and the legal speed limirations.
The speed is selected to meet the needs of the road to fulfil its function. Thus roads which are planned to provide long distance travel will be designed with a higher speed while those which provides shorter distance travel can be given a lower design speed.
3.4.1
Design Speed
Table 3-2A and 3-28 indicate the selection of design speeds rvith respect to rural and urban standards.
Design speed is defined as: "a speed selected to establish specific minimum geometric design elements for a particular section of highway". These design elements include vertical and horizontal alignment, and sight distance. Other features such as widths of pavement and shoulders, horizontal clearances, etc., are generally indirectly related to design speed.
The choice of design speed is influenced primarily by factors such as the terrain, economic, environmental factors, type and anticipated volume of traffic, functional classification of the highway, and whether the area is rural or urban. A highway in level or rolling terrain justifies a higher design speed than one in mountainous terrain. Where a difficult location is obvious to approaching drivers, they are more apt to accept a lower design speed than where there is no apparent reason for it. Where it is necessary to reduce design speed, many drivers may not perceive the lower speed condition ahead, and it is important that they be warned in advance. The changing conditions should be indicated by such traffic devices as speed zone signs and appropriate warning signs depicting the conditions ahead.
A highway carrying a large volume of traffic may justify a higher design speed than a less important faciliti' in a similar topography, pafticularly where the savings in vehicle operation and other cos.ts are sufficient to offset the increased cost of right of way and construction. On the contrary, a lower design speed should not be assumed for a secondary road where the topography is such that drivers are likely to travel at high speeds.
Taking into account the above considerations, as high a design speed as feasible should It is preferable that the design speed for any section of highway be a constant value. However, during the detailed design phase of a project, special situations may be used.
18
A Guide on Geometric Design of Roads
TABLE 3-2A: DESIGN SPEED FOR RURAL ROADS Design Speed (kph) Design Standard
Terrain
Catesorv of Road Flat
Rolling
Mountainous
R6
Expressway
110
r00
80
Highway Primary Roads
100
90 90
'7n
R5
R4
Primary Roads Secondary Roads
90 90
80
60 60
R3
Secondary Roads
80
60
50
60
50
40
50
50
30
R2
r00
80
70
Minor Roads
RI
TABLE
3-2B--
: DESIGN SPEED FOR URBAN ROADS Design Speed (kph)
Category of Road
Design
Area Type
Standard
il
m
U6
Expressway
100
90
80
U5
Arterials Collectors
90 80
70 10
60 60
U4
Arterials Collectors Local Streets
80 10 70
60 60 60
50 50 50
U3
Collectors Local Streets
60 60
50 50
40 40
50
4A
40
30
30 30
U2
UI Note
I
:
Local Streets
Type Type Type
I
-
II III -
relatively free in road location with very little problems
as regards land
acquisition, affected buildings or other socially sensitive areas. Intermediate between I and IIL Very restrictive in road location with probiems as regards land acquisition, affected buildinss and other sensitive areas.
19
A Guide on Geometric Design of Roads
arise in which engineering, economic, environmental, or other considerations make it impractical to provide the minimum elements established by the design speed. The most likely examples are partial or brief horizontal sight distance restrictions, such as those imposed by bridge rails, bridge columns, r'etaining walls, sound walls, cut slopes, and median barriers. The cost to correct such restrictions may not be justified. Technically, this will result in a reduction in the effective design speed at the location in question. Such technical reductions in design speed shall be discussed with and upprou"d by Road Authorities.
3.4.2
Operating Speed Operating speed is the highest overall speed at which a driver can travel on a given road under favourable weather conditions and prevailing traffic conditions without at any time exceeding the design speed on a section by section basis.
3,4.3
Design Sections In the design of relatively long road, it is desirable, although not always feasible to allow for a constant design speed. Changes in terrain and other physical controls may dictate a change in the design speed on certain sections. If a different design speed is introduced, the change should not be abrupt but should be effected over a ruffi.i"nt distance to permit drivers to change speed gradually before reaching the section of highway with lower design speed. There should be a transition section of at least I km per every t0 kph drop in design speed to permit drivers to change speed gradually before reaching the section of the road with a different design speed. The transition sections are sections with intermediate design speeds. Where the magnitude of the change in the design speed are large, more than one transition section will be required.
The intermediate design speed shall follow the sequence as established in Table 3-2A and 3-28, i.e. If the design speed of section A is I l0 kph and that of section B is 90kph, then the design speed of the transition section between them shall be 100knh.
3.5
CAPACITY The term highway capacity pertains to the ability of a roadway to accommodate rraffic. It is defined as the maximum number of vehicles that can pass over a given section of a lane or a roadway during a given time period under prevailing roadway and traffic conditions. Capacity considered here is applicable only to uninterrupted flow or open roadway conditions. Capacity for intemrpted flow as at intersections will be dealt with separately.
Capacity is often stated in terms of passenger car units (p.c.u). Table 3-3 gives the conversion factors to be used in converting the various classes of vehicles to passenger car units.
20
A Gui.de on Geometric Design of Roads
TABLE 3.3.CONVERSION FACTORS TO P.C.U. Equivalcnt Value in P.C.U Type of Vehicles
Rural Standard
Urban Standard
Roundabout Design
Traffic Signal Design
Passenger Car
1.00
r.00
1.00
1.00
Motor cycles
r.00
0.7.5
0.75
0.33
Light Vans
2.00
2.00
2.00
2.00
Medium Lorries
2.50
2.50
2.80
1.75
Heavy Lorries
3.00
3.00
2.80
2.25
Buses
3.00
3.00
2.80
2.25
Source
3.5.1
: 1. 2.
Road Research Laboratory "Research on Road Traffic", HMSO, London, 1965, pp201
Highway Planning Unit, Ministry of Works, Malaysia
Capacity under ideal condition Under ideal condition, the possible capacity for uninterrupted flow are as follows:
a. b.
For 2-lane two way (total) = 2,800 pcu/hr For multi lane (per lane) = 2,000 pcu/hr
Ideal conditions consist of the following for two -lane roads:
a. b. c. d. e. f. g. h.
Design speed greater than or equal to 100 kph Lane widths greater than or equal to 3.65m (i2') Clear shoulders wider than or equal to 1.83m (6') No "No passing zones" on the highway All passenger cars in the traffic streams A 50/50 directional split of traffic No impedient to through traffic due to traffic control or turning vehicles Level terrain
Where one or more of these conditions are not met, the actual capacity will be reduced. The effects of each are discussed in the Highway Capacity Manual which gives adjustment factors for the determination of the design capacity.
3.5.2
Design Volume Design volume is the volume of traffic estimated to use the road during the design year, which is taken as l5 years after the completion of the road. The derivation of the design hour volume (DHV) is as discussed in Section 3.2.2.
2l
A Cui.de on Geometric Design of Roads
Design of new highways or improvements of existing highways usually should not be based on current traffic volume alone. Considerations should be given to the future traffic expected to use the facilities. A highway should be designed to accommodate the traffic that might occur within the life of the facility under reasonable maintenance.
It is difficult to define the life of a highway because major segments may have different lengths of physical life. Furthermore, the design of highway based on life expectancy is greatly influenced by economics.
3.5.3
Service Volume The service volume is the maximum volume of traffic that a designed road would be able to serve without the degree of congestion falling below a preselected level as defined by the level of service which is the operating conditions (freedom to manoeuvre) at the time the traffic is at the design hour volume. Table 3-4 gives an indication of the levels of service used while Figure 3-4 gives a schematic concept of the relationship of level of service to operating speed and volume/capacity ratio.
TABLE 3-4 : LEVELS OF SERVICE Level of Service
Remarks
flow with low volumes, densities and high speeds. Drivers maintain their desired speeds with little or no delay.
A
Free
B
Stable flow. Operating speeds beginning to be restricted somewhat by traffic conditions. Some slight delay.
C
Stable flow. Speeds and manoeuvrability are more closely
can
controlled by higher volumes. Acceptable delay. D
Approaching unstable flow. Tolerable operating speeds which are considerably affected by operating conditions. Tolerable delay.
E
Unstable Flow. Yet lower operating speeds and perhaps stoppages of momentary duration. Volumes are at or near capacity congestion and intolerable delay.
F
Forced Flow. Speeds and volume can drop to zero. Stoppages can occur for long periods. Queues of vehicles backing up from a restriction downstream.
22
FIGURE 3.4 : RELATIONSHIP OF LEI'EL OF SERVICE TO OPERATING SPEED AIYD VOLUME
CAPACITY RATIO.
!t) !.)
ao. oll '.4
o
L
0,
o
o
Volume
/
Copocity Rotio
z5
i
A Guide on Geometric Design of Roads
3.5.4
Design Level of Service and Volume/Capacity Ratio The selection of the design level of serviqe is as given in Table 3-5A and 3-58. This level of service concept can be simplified into volume to capacity ration (V/C) for the purpose of design to determine the service volume as indicated in Section 3.5.3. Table 3-5A and Table 3-58 also gives the ranges of V/C ratio consistent with the levels of service for the purpose of design.
TABLE 3-5A : DESIGN LEVEL OF SERVICE AND VOLUME/ CAPACITY RATIO (RURAL) Road Category
Desisn Level of Service
Volume /Capacity Ratio
Expressway
C
0.70-0.80
Highway
C
0.70-0.80
Primary Road
D
0.80-0.90
Secondary Road
D
0.80-0.90
Minor Road
E
0.90- 1.00
TABLE 3-58 : DESIGN LEVEL OF SERVICE AND VOLUME/ CAFACITY RATIO (URBAN) Road Category
Desien Level of Service
Volume /Capacity Ratio
Expressway
D
0.80-0.90
Arterial
D
0.80-0.90
Collector
D
0.80-0.90
Local Street
E
0.90-1.00
.A
A Guide on Geometric Design of Roads
CHAPTER 4 - ELEMENTS OF DESIGN 4.1
SIGHT DISTANCE
4.1.1
General
Sight distance is the length of road ahead visible to drivers. Ability of a driver to see ahead is of the utmost importance to the safe and efficient operation of a road. The designer must provide sight distance of sufficient length in which drivers can control the speed of the vehicles so as to avoid striking an unexpected obstacle on the travelled way. Also, at frequent intervals and for a substantial portion of the length, Z-lane undivided roads should have sufficient sight distance to enable drivers to overtake vehicles without hazard. Decision sight distance, is the distance required for a driver to detect an unexpected hazard and to select an appropriate speed or path thus by completing a manoeuvre which avoid this hazard. Sight distance thus includes stopping sight distance, passing sight distance and decision sight distance.
4.1.2 Stopping Sight Distance The stopping sight distance is the length required to enable a vehicle travelling at or near the design speed to stop before reaching an object in its path. Minimum stopping sight distance is the sum of two distance.
(i) (ii)
the distance traversed by a vehicle from the instant the driver sights an object for which a stop is necessary, to the instant the brakes are applied; and
the distance required to stop the vehicle after the brake application begins.
(a)
Perception and Brake Reaction Time For safety, a reaction time that is sufficient for most operators, rather than for the average operator, is used in the determination of minimum sight distance. A perception time value of 1.5 seconds and a brake reaction time of a full second are assumed. This make the total perception and brake reaction time to be 2.5 seconds.
(b)
Braking Distance The approxirnate braking distance of a vehicle on a level roadway is determined by the use of standard formula:
d=
V2
254f
25
A Guide on Geometric Design of Roads
where
d = brake distance, (m) V= initial speed, (kph) f = coefficieirt of friction between tires and roadway
In this formula the f factor varies considerably due to many physical elements such as condition of tires, condition of pavement surface, the presence of moisture etc. The value of f for design are inclusive of all these factors.
(c)
Design Values
The sum of the distance traversed during perception and brake reaction time and the distance required to stop the vehicle is the minimum stopping sight distance. The values to be used for minimum stopping sight distances are as shown in Table 4-1.
TABLE 4.1 : MINIMUM STOPPING DISTANCE
Design Speed (kph)
Min Stopping Sight Dist. (m)
110
r00
2s0 205
90 80
r70 t40
t0
110
60 50
65
85 A<
40 30
(d)
AJ
30
Effect of Grades on Stopping Sight Distance When a road is on a grade the standard formula for braking distance is
D_
V2
zsatl,e)
in which g is the percentage of grade divided by 100. The effect of grade on stopping sight distance is as shown in Table 4-2.
1A
A Guide on Geometric Design of Roads
TABLE 4.2 : EFFECTS OF GRADES IN STOPPING SIGHT DISTANCE _ WET CONDITIONS
Design Speed (kph)
Stopping Sight Distance (m) for Downgrades
Assumed
for Condition Speed
Stopping Sight Distance (m) for Upgrades
37a
6Vo
97o
(koh)
30
30
3l
)L
40
46 66 89
48 69 94
50
50 60 70 80 90
'73
30 40 47
101
55
71
63 70
3Vo
6Vo
9Vo
29
29
)9,
+J
42
41
56
5Z 61
90
54 69 86
t07
102
83 98
1aA lLa
119
1ta I lJ
118
126
136
149
161
t76
181
t95
71
100
221
85
148
140
t34
110
267
241 293
714 261 327
91
r68
159
l5i
(e)
Every effort should be made to provide stopping distances greater than the minimum design values shown in Table 4-1 especially at locations where there are restrictions to sight in the horizontal plane.
(f)
Decision Sight Distance
Stopping sight distances are usually sufficient to allow reasonably competent and alert drivers to come to a hurried stop under ordinary circumstances. However, these distances are often inadequate when drivers must make complex or instantaneous decisions, when information is difficult to perceive, or when unexpected or unusual manoeuvres are required. Limiting sight distances to those provided for stopping may also prelude drivers from performing evasive manoeuvres, which are often less hazardous and otherwise preferable to stopping. Even with an appropriate complement of standard traffic control devices, stopping sight distances may not provide sufficient visibility distances for drivers to corroborate advance warning and to perform the necessary manoeuvres. It is evident that there are many locations where it would be prudent to provide longer sight distances. In these circumstances, decision sight distance provides the greater length that drivers need.
Decision sight distance is the distance required for a driver to detect an uriexpected or otherwise difficult-perceive information source or hazard in a roadway environment that may be visually cluttered, recognise the hazard or its potential threat, select an appropriate speed and path, and initiate and complete the required safety manoeuvre safely and efficiently. Because decision sight distance gives drivers additional margin for error and affords thern sufficient length to manoeuvre their vehicles at the same or reduced speed rather than to just stop. its values are substantially grcater than stupping sight distance. 21
A Guide on Geometric Design of Roads
Drivers need decision sight distance whenever there is a likelihood for error in either information reception, decision-making, or control actions. The following are examples of critical locations where these kinds of errors are likely to occur, and where it is desirable to provide decision sight distance: interchange and intersection locations where unusual or unexpected manoeuvres are required; and changes in cross section such as toll plaza and lane drops. The decision sight distances in Table 4-3 provide values to be used by designers for appropriate sight distances at critical locations and serve as criteria in evaluating the suitability of the sight lengths at these locations. Because of the additional safety and manoeuvrability these lengths yield, it is recommended that decision sight distances be provided at critical Iocations or that these points be relocated to locations where decision sight distances are available. If it is not possible to provide these distances because of horizontal or veftical curvature or if relocation is not possible, special attention should be given to the use of suitable traffic control devices for providing advance warning of the conditions that are likely to be encountered.
TABLE 4-3 : DECISION SIGHT DISTANCE Design Speed (kph)
Decision Sight Distance for Avoidance Manoeuvre (meters)
50 60 70 80 90
15 95 125
r00
ll0
B
C
160
145 115
20s 250
D 200
200 230 215
360
225
300 360 415
235 215 315
3r5
405
26s
,,t<<
335
42s
155
i85
Decision sight distance values that will be applicable to most situations have been developed from empirical data. The decision sight distances vary depending on whether the location is on a rural or urban road, and on the type of manoeuvre required to avoid hazards and negotiate the location properly. Table 4-3 shows decision sight distance values for various situations rounded for design. As can be seen in Table 4-3, generally shorter distances are required for rural roads and when a stop is the manoeuvre involved.
28
A Gui.de on Geometric Design of Roads
The following avoidance manoeuvres are covered in Table 4-3: . Avoidance Manoeuvre A : Stop on rural road. . Avoidance Manoeuvre B : Stop on urban road. . Avoidance Manoeuvre C : Speed/path/direction change on rural road. . Avoidance Manoeuvre D : Speed/path/direction change on urban road.
In computing and measuring decision sight distances, the 1070mm eye height and 150mm object height criteria used for stopping sight distance have been adopted. Although drivers may have to be able to see the entire roadway situation, including the road surface, the rationale for the 150mm object height is as applicable for decision as it is for stopping sight distance.
4.1.3
Passing Sight Distance
Most roads in rural areas are two-lane two way on which vehicles frequently overtake slower moving vehicles, the passing of which must be accomplished on a lane regularly used by the opposing traffic. Passing sight distance for use in design should be determined on the basis of the length needed to safely complete a normal passing manoeuvre.
The minimum passing sight distance for two-lane highways is determined as the sum of four distances:-
(i)
distance traversed during the perception and reaction time and during the initial acceleration to the point of encroachment on the passing lane.
(ii)
distance travelled while the passing vehicle occupies the passing
(iii)
distance between the passing vehicle at the end of its manoeuvre and the opposite vehicles.
(iv)
distance traversed by an opposing vehicle for two-thirds of the time the passing vehicle occupies the passing lane.
(a)
Design Values
lane.
,
The total passing sight distance is determined by the sum of the above four element" Table 4-4 gives the minimum values to be used for each design speed. In designing a road, these distances should be exceeded as much as practicable and such sections provided as often many as can be done with reasonable costs so as to present as many passing opportunities as possible exceeded.
29
A Guide on Geometric Design of Roads
TABLE 4-4 : MINIMUM PASSING SIGHT DISTANCES (2-LANE - 2 WAY) Design Speed (kph)
Minimum Passing Sight Dist. (m)
110
730 610 610 550 490 410
100
90 80 70
60 50 40 30
(b)
350
290 230
Effect of Grade on Passing Sight Distance Specific adjustment for design use is not available. The effect of grade is not considered in design as the effect is compensated in upgrade or downgrade. However, it should be realised that greater distances are needed for passing on grade as compared to level conditions. The designer should recognise the desirability of increasing the minimum as shown in Table 4.4.
(c)
Frequency and length of Passing Sections
Sight distance adequate for passing should be provided frequently on 2-lane roads. Designs with only minimum sight distance will not assure that safe passing can always be made. Even on low volume roads a driver desiring to pass may, upon reaching the section, find vehicles in the opposing lane and thus be unable to utilise the section or at least not be able to begin to pass at once. The percentage of length of road or section of road with sight distance Ereater than the passing minimum should be computed. The adequacy of sight distance is determined by analysis of capacity related to this percentage, and this would indicate or not alignment and profile adjustments are necessary to accommodate the design hour volumes. Generally this percentage should be about 60Vo for flat terrain, 40Vo for rolling terrain and 20Vo for mountainous terrain. The avaiiable passing sight distances should not be concentrated in one section but be evenly distributed throughout the entire road. It is inrportant to note that in order to cater for high volume of traffic at high level of service, it requires that frequent and nearly continuous passing sight distances be provided. I I I I I I
i
30
T
A Guide on Geometic Design of Roads
It is not necessary to consider passing
sight distance on roads that have two or more traffic lanes in each direction of travel. Passing manoeuvres on multilane roads are expected to occur within the limits of each carriageway allocated for that direction of travel. Thus passing manoeuvres that require crossing the centerline of four-lane undividual road are reckless and should be prohibited.
4.1.4
Criteria for Measuring Sight Distance
(a)
Height of Driver's Eye
The eyes of the average driver in a passenger vehicle are considered to be l070mm above the road surface.
(b)
Height of Object
A height of object of 150mm is assumed for measuring stopping sight distance and the height of object for passing sight distance is 1300mm both measured from
the road surface. Sight distance should be considered in the preliminary stages of design when both the horizontal and vertical alignment are still subject to adjustment. The sight distance should be determined graphically on the alignment plans and recorded at frequent intervals, for both direction of travel. This will enable the designer to appraise the overall layout and effect a more balanced design by minor adjustments in the plan or profile. Horizontal sight distance should be measured in the inside of a curve at the center of the inside lane. Vertical sight distance should be measured along the longitudinal profile of the central line, using the height of driver's eye (1070mm) and the object height (150mm). Figure 4-l and 4-2 provrde examples of measuring sight distance in both plan and profile respectively. For two lane twoway roads passing sight distance in addition to stopping sight distance and decision sight distance should be measured and recorded. Sight distances, in this case, are to be checked from the centerline of the road.
JI
v2
7 l-
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33
A Guide on Geometric Design of Roads
4.2
HORIZONTAL ALIGNMENT
4.2.1
General In the design of horizontal curves, it is necessary to establish the proper relation between the design speed and curvature and also their joint relations with superelevation and side friction. From research and experience, limiting values have been established for the superelevation (e), and the coefficient of friction (fl.
4.2.2 Superelevation Rates The maximum rates of superelevation usable are controlled by several factors such as climatic conditions, terrain conditions and frequency of very slow moving vehicles that would be subjected to uncertain operation. While it is acknowledged that a range of values should be used, for practical purposes in establishing the design criteria for horizontal alignment, a maximum superelevation rate of 0.10 is used for roads in rural areas and 0.06 for roads in urban areas.
4.2.3 Minimum Radius The minimum radius is a limiting value of curvature for a given speed and is determined from the maximum rate of superelevation and the maximum allowable side friction factor. The minimum safe radius (Rmin) can be calculated from the standard curve formula.
Rmin =
v2 127 (e+f)
WhereRmin = minimum radius of circular curve (m)
V=
Design Speed (kph)
e = maximum superelevation
rate
f = maximum allowable side friction
34
factor
Design of Roads 4 Guile on Geometric
Figure 4-3 illustrates the range of side friction factors (0 that has been derived using imperial methods for the various types of roads. FIGURE 4-3 : COMPARISON OF SIDE FRICTION FACTORS ASSUMED FOR DESIGN OF DIFFERENT TYPES OF ROADS
ion between t ,ation and si
blished for
r,
0.5
f
Maximum side friclion ./ lactor tor new tyres on / wet concrete pavement
actors SUCh r g vehicles th 'ange of valu,
horizont areas and 0.( for
A ; H
E
0.4
1l concrete
Assum ed for design
lyres on s el
lvement
p
I
of turning roadways
I I
u'r
r..
5 E !
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f
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f
O)
q)
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(a
--t ruraland high speed urban highways
0.1
is determine kiction facto U
formula.
30
40
60
50
70
90
80
100
Speed (kph)
Table 4-5 lists the minimum radius to be used for the designated speed and maximum superelevation rates.
TABLE 4.5: MINIMUM RADIUS Minimum Radius (m)
Design Speed (kph) e
= 0.06
e
= 0.10
s60 465 335 284
s00 315
195 150 100
n5
4A
60
50
30
35
30
110 100
90 80 70 60 50
35
305
230 125 85
I
l0
A Guide on Geometric Design
of
Roads
While the values in Table 4-5 lists the minimum radius that can be used, it should be noted, that the above minimum radii will not provide the required stopping sight distance and passing site distance within typical cross-sections. Hence, all elforti should be made to design the horizontal curves with radii larger than the minimum values shown for greater comfort and safety.
4.2.4 Transition (Spirat) Curves Vehicles follow a transition path as it enters or leave a circular horizontal curye. To design with builrin safety, the alignment should be such that a driver travelling at the design speed will not only find it possible to confine his vehicle to the occupied lane but will be encouraged to do so. Spiral transition curves are used for this purpose. Generally, the Eulers spiral, also known as the clothoid is used. The degree of curve varies from zero at the tangent end of the spiral to the degree of the circular arc at the circular curve end. a road
a)
Length of Spiral The length of spirals to be used are normally calculated from the Spiral formula or from the empirical superelevation runoff lengths. The following formula is used by some for calculating the minimum length of a spiral:
, L= where
L = minimum
0.0702v' RC .,'
length of spiral;
V-
speed (kph);
R=
curvo radius; and
C=
rate of increase of centripetal accelerating (m/sec3)
The factor C is an imperial value and a range of lto 3 have been used for highways. Highways design does not normally require the level of precision achieved using this formula. A more practical control for determining the length of a spiral is that *fri.tt equal the length required for superelevation runoff. Superelevation runoff is the general term denoting the length of highway needed to accomplish the change in cross slope from a section with adverse crown removed to a fully superelevated sectjon or vice versa. Current practice indicates that the appearance aspect of superelevation runoff largely governs the length. The length of superelevation runoff should not exceed a longitudinal slope as indicated in Table 4-6.
36
A Guide on Geometric Design of Roads
TABLE 4.6: RELATIONSHIP OF DESIGN SPEED TO MAXIMUM RELATIVE PROFILE GRADIENTS Maximum Relative Gradients (and Equivalent Maximum Relative Slopes) for Profiles between the Edge of Two-Lane Travelled Way and the Centerline (Vo)
Design Speed v (kph)
0.75 (l:133) 0.70 (l:143) 0.65 (l:150) 0.60 (1:167) 0.55 (l:182) 0.50 (1:200) 0.48 (l:210) 0.45 (I:222) 0.42 (l:238)
30 40 50 60 70 80 90 100 110
Table 4-74 and 4-78 gives for the various design speeds and degree of curvature, the minimum lengths of spiral, the superelevation rates and the limiting curvature for which superelevation is not required.
For pavements with more than 2 lanes and standard lane width of 3.5m, the superelevation runoff lengths should be as follows.
(i)
3 lane pavements
(ii)
4?aneundivided pavements
-
(iii) 6 lane undivided
1.2 times the length
pavements
37
,i
-
for 2-lane roads.
1.5 times the length for 2 lane roads.
*
2.0 times the length for 2 lane
roads.
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A Guide on Geometric Design of Roads
4.2.5 Methods of Attaining Superelevation (a) revolving the Three specific methods of profile design in attaining superelevation are edge pavement about the centreline profile (b) revolving the pavement about the inside profile, and (c) revolving the pavement about the outside edge profile. Figure 4-4 illustrates to the distance these three methods diagrammatically. The rate of cross slope is proportional from start of superelevation runoff.
(a)
Figure 4-4A illustrates the method where the pavement section is revolved about the centreline profile. This general method is the most widely used in design since the required .hung" in elevation of edge of pavement is rnade with less distortion then by the other methods. The centreline profile is the base line and one-half of the required elevation change is made at each edge'
(b)
the Figure 4-48 illustrates the method where the pavement section is revolved about the to inside edge profile. The inside edge profile is determined as the line parallel is made calculateJ centreline profile, One half of the required change in cross slope the by raising the centre line profile with respect to the inside pavement edge and other half by raising the respect to the centreline profile.
(c)
Figure 4-4C illustrates the method where the pavement section is revolved about outside edge profile involves similar geometrics as (b), except that the change is affected below the upper control profile.
Except when site condition specifically requires, method (a) shall be adopted for undivided roads.
4.2.6
Superelevation Runoff with Medians In the design of divided roads, the inclusion of a median in the cross section alters somewhat the superelevation runoff treatment, The three general cases for superelevation runoff design are as follows:
(a) ' (b)
The whole of the travelled way, including the median, is superelevated as a plane section. This case is limited to namow medians and moderate superelevation rates to avoid substantial differences in elevation of the extreme pavement edges because of the median tilt. Diagrammatic profile controls is sirnilar to Figure 4'4a^ except that the two rnedian edges will appear as profiles only slightly removed from centreline.
The medial is lield in a horizontal plane and the two pavements separately are rotated around the median edges. This case has most application with medians of intermediate width. Runoff design uses the median edge profile as the control' One pavement is rotated about its lower edge and the other about its higher edge. The diagrammatic profile control is similar to Figure 4-48 and 4'4C with the centreline grade control the same for the two pavements.
40
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(c)
4.2.7
The two pavements are separately treated for run-off with a resultant variable difference in elevation at the median edges. The differences in elevation of the extreme pavement edges are minimised by a compensating slope across tlie median' A fairly wide section is necessary to develop right shoulder areas and desired gentle slope between. Thus this case is more applicable to wide median of 9m or more' The pavement rotation can be made by any methods in Figure 4-4.
Pavement Widening on Curves Pavements on curves are widened to make operating conditions on curve comparable to those on tangents. Pavement widening on curves is the difference in pavement width required on a curve and that use in a tangent. Table 4-8 gives the widths of pavement widening that are required on open road curves.
Widening should be attained gradually on the approaches to the curve to ensure a reasonable or smooth alignment on the edge of pavement and to fit the paths of vehicles entering leaving the curve. Preferably, widening should be attained over the superelevation runoff length with most of all of the widening attained at the start of circular curve poin
4,2.8
Sight Distance on Horizontal Curves Another element of horizontal alignment is the sight distance across the inside of curves. Where there are sight obstructions (such as walls, cut slopes, buildings and guardrails), a design to provide adequate sight distance may require adjustment if the obstruction cannot be removed. Using the design speed and a selected sight distance as a control, the designer should check the actual condition and make the necessary adjustment in the manner most fitting to provide adequate sight distance.
4,2.9 General Controls for Horizontal Alignment In addition to the specific design elements for horizontal alignment, a number of general controls ur".".ognised and should be used. These controls are not subject to empirical or formula derivation but are important for the attainment of safe and smooth-flowing roads. These are:-
(a)
The horizontal alignment should be consistent with the topography and width preserving developed properties and community values. Winding alignment Lo*posed of short curves should be avoided. On the other hand very long straights should also be avoided. The maximum length of straight section of the highway should be limited, where possible, to 2 minutes travelling time.
ft)
The use of the minimum radius for the particular design speed should be avoided wherever possible. Generally flat curves should be used, retaining the maximum curvature for the most critical conditions.
(c)
Consistent alignment should always be sought. Sharp curves should not be introduced at thd end of long tangents. Where sharp curves must be introduced, it should be approached, where possible, by successively sharper curves from the generally flat curvature.
42
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(d)
For small deflection angles, curves shcluld be su{'f iciently long to avoid the appearance o1'a kink. Curves should be at least l-50m long for a central angle of 5o anci the length should be increased 30m foreach ludecrease in the centreline angle. The n-rinin-rurn length clf horizontal curve on main roads, should be about 3 times the
(e)
Any abrupt reversal in alignment should be avoided. The distance between reverse curves shoulcl be the sum o1-the superelevation runofl'lengths and the tangent runout lengths.
(f)
The 'broken back' an'angentent of curves should be avoided. Use of spiral transitions or a compound curve alignment is prelerable for such conditions if it is unavoidable.
4"3
(g)
other than tangent or flat curvature should be avoided <-in high, long embankment fills. In the absence of'cut slopes, shrubs, and tree above the roadway, it is difficult fbr drivers to perceive the extent of curvature and adjust their operation to the conditions.
(h)
Caution should be exercised in the use of compound circular curves. While the use of compound curves affords flexibility in fitting the highway to the terrain and other ground controls, the simplicity with which such curves can be used clften ternpts the
(i)
The designer should ensure thaL the required sight distance (i.e. decision sight ciistance) is providecl when approaching interchanges and intersections'
\/ERTICAL AI,IGNMF'NT
4.3.1 Maximum
Grades
The vertical profile of road affect tlie perfomrance of vehicles. The effect of grades on trucks which have weight power ratio of about 18Okg/kw, is considered. The maximuln grade controls in tenns of design speed is summarised in T'able 4-9A to 4'9F.
44
7 A Guide on Geometric
TABLE 4-9A: MAXIMUM GRADES FOR Rt. R2. Ul & U2 STANDARD ROADS Design speed (kph)
Type of Terrain/Area Flat & Type I Rolling & Type II Mountainous & Tvpe III
30
40
50
60
70
80
8
7
7
l
1
6
ll
ll
l6
l5
l0 l4
t0
9
8
t3
t2
l0
TABLE 4-9B : MAXIMUM GRADES FOR R3 AND R4 STANDARD R0ADS Design speed (kph)
Type of Terrain 50
60
Flat
1
7
7
Rolling
9
8
8
Mountainous
r0
IO
l0
9
70
90
r00
6
6
5
7
7
6
9
8
80
TABLE 4.9C : MAXIMUI\{ GRADES FoR u3 AND u4 STANDARD ROADS Design speed (kph)
Area Type 40
50
60
70
80
9
9
9
8
7
t2 t3
t1
10
9
8
I2
12
1l
10
Type I Type II Type III
TABLE 4.9D z MAXIMUM GRADES FOR R5 STANDARD ROADS Design speed (kph)
Type of Terrain 60
Flat Rolling Mountainous
-7n
80
r00
5
5
+
90 4
6
6
5
5
A
+
8
1
7
6
6
5
i
a
110 a
J
TABLE 4-98 : MAXIMUM GRADES FOR U5 STANDARD ROADS Design speed (kph)
Area Type of Terrain
Type I Type II Type III
s0
60
t0
80
90
r00
6
7
6
6
5
5
q
II
8
7
7
6
6
10
9
9
8
8
TABLE 4-9F : MAXIMUM GRADES FOR U6 AI{D R6 STAI'{DARD ROADS Design speed (k
Type of Terrain/Area Flat & Type I Rolling & Type II Mountainous & TVpe
III
the grade is The total upgrade for any section of road should not exceed 3000 m unless less than 4 Vo.
4.3.2 Minimum
Grades
percent may A desirable minimum grade or 0.5 percent should be used. A grade of 0.35 On straight be allowable where a high type pavement accurately crowned is used' of even flatter grades may stretches traversing across wide areas of low lying swamps use
water drainage outlet be allowabte witn lrior approval. However, the design of the stonn of the travelled should be considered carefully at these location to ensure that flooding
lanes is avoided. Where less than 0.5Volongrtudinal gradient is necessary, the superelevation runofflength along the used should be the minimum valul permitted. This will minimise flat area wearing carriageway where the cross falls are less than \Vo. The use of open textule hydropianing' course at this locations should be considered; this will heip in preventing
4.3.3 Critical
Grade Length
upgrade The term "critical grade length" indicates the maximum length of a designated in speed' To upon which a loaded truck can operate without an unreasonable reduction of trucks is establish the design values for ciitical grade lengths, for which gradeability the determining factor, the following assumptions are made:-
(i)
The weighr-power rario of a loaded truck is about 180 kglkw.
(ii)
the The average running speed as related to design speed is used to approximate speed of vehicles beginning an uphill climb'
(iii)
reduction in The common basis for determining the critical grade length is a that accident speed of trucks below the averageiunning speed. Studies show exceeds involvement rate increases siglificantly when truck speed reduction l5kph. For example accideniinvolvement rate is 2.4 times greater for 25kph reduction than for a l5kph reduction.
The length of any given grade that will cause the speed of a representative truck (lS0kglkw) enterin! the grade at 90k1r/h to reduce by various amounts below the speed reduction average running rp""O in shown graphically in Figure 4-5. The 25kph critical iength of curve should be used as the general design guide for determining the srade.
AA
aDlducnoN 50
300
500
400
"*liii
i
I
600
LEIIGTII OF GRADE (METERS)
GRADE FOR DESIGN FIGT]RE 4-5 : CRITICAL LENGTIIS OF ltOkslkW' OF TRUCK ffiAVY FOR TYFICAL
+t
A Cuide on Geometric Design of Roads
Where a particular section is made up of a combination of upgrade, the length of critical grade, measured from tangent point to tangent point, should take into consideration, the entire section of the combination.
Where the length of critical grade is exceededand especially where grade exceed5Vo, consideration should be given to providing an added uphill lane, that is, the climbing lane, for slow moving vehicles, particulariy where the volume is at or near capacity and the truck volume is hish.
4.3.4 Climbing
Lanes for Two Laned Roads
The following conditions and criteria, which reflect economic consideration, should be satisfied to justify a clirnbing lane:
(i)
Upgrade peak traffic flow rate in excess of 2AA vehicies per hour
(ii)
Upgrade truck peak flow rate in excess of 20 vehicles per hour
(iii)
One of the following conditions exist:
.
A 25kph or greater speed reduction is expected for typical heavy truck.
.
Level of service E or F exist on the grade
.
A reduction of two or more levels of services is experienced approaching the start of the grade.
Where climbing lanes are provided, the following requirement are to be followed:-
(a)
the climbing lanes should begin near the foot of the grade and should proceeded by a tapered section of at least 50m iong'
(b)
the width of the clirnbing lane should be the same as lhe main carriageway lane width and in any case should not be less than 3.25m.
(c)
the section of the road must be separated by a New Jersey Type Concrete Median with adequate signing and markings and the opposing lane widened also to 2 lanes to make it a four lane divided carriageway.
(d)
the climbing lane should end at least 60m beyond the crest and in particular should be at a point where the sight distance is sufficient to permit passing with safety. In addition, a coffesponding taper length as in (a) of l00m should be provided.
(c)
the shoulder on the outer edge of the climbing lane should be as wide as the shoulder on the normal two laned section. Where conditions dictate otherwise, a useable shoulder width of 1.25m is acceptable.
48
be
7 A Guide on Geontetic Design of Roads
4.3.5
Passing Lane Sections on Two-Lane Roads
Wliere a sufficient number and iength of safe passing sectiolt canuot be obtaincd in the design of horizontal and vertical alignment alone, an occasional section of four lanes may be introduced to provide more sections and length safe for passing. Such section are particularly advantageous in rolling terrain, especially where the alignment is winding or where the vertical profile includes critical lengths of grade. Four lane sections should be sufficiently long to permit its effective usage. The sections of four lanes introduced need not be divided. The use of a median, however is advantageous and should be considered on roads carrying 500 vehicles per hour or lnore. The transitions between the two lane and four lane pavements should be located where of change in width is in full view of the driver. Sections of four-lane road, particularly divided sections should not be longer than 3 km so as to ensure that the driver does not lose his awareness that the road is basicaliy a two lane facility. Where four lane sections are not practical, passing one direction lane sections shall be introduced at regular intervals. The passing lane must be at least 125m long and of full lane width and preceded by a taper of 50m at the beginning and 100m at the end.
4.3.6 Climbing
Lanes on Multilane Roads
Multilane roads more frequently have sufficient capacity to handle their traffic load, including the normal percentage or slow-moving vehicles without becoming congested. However where the volume is at or near capacity ( 1700 vehicle per hour per lane) and the truck volume is high (2107o of total flow) so as to interfere with the normal flow of traffic then addition of climbine lanes should be considered.
4.3.7 Vertical
Curves
Vertical curves are used to effect a gradual change between tangent grades. They should be simple in application and should result in a design that is safe, comfortable in operation, pleasing in appearance and adequate for drainage. For simplicity, the parabolic curve with an equivalent vertical axis centered on the vertical point of intersection is used. The rate of change of grade to successive points on the curve is a constant amount for equal increments of horizontal distance, and equals the algebraic difference between the intersecting tangent grades divided by the length of curve or A,/I- in percent per meter. The reciprocal L/A is the horizontal distance in metre required to effect a 1 percent change in gradient and is a measure of curvature. This quantity (L/A), termed k, is used in determining the horizontal distance from the beginning of the vertical curve to the apex or low point of the curve. The k value is also useful in determining the minimum lengths of vertical curves for the various design speeds. The lengths of vertical curves used should be as long as possible and above the minimum values for the design speeds where economically feasibie.
49
A,9uide on Geometic De
(al
Crest Vertical Curves
Minimum lengths of crest vertical curves are determined by the sight distance requirements. The stopping sight distance is the ma.ior controi for the safe operation at the design speed chosen. Passing sight disiances are not used as ir provides for an uneconomical design. An exception may be at decision areas such as sight distance to ramp exit gores where iong.r rengths are necessary. The basis formula for length of a parabolic vertical curve in terms of algebraic difference in grade and sight distince (using an eye height of l.Orn and object height of 0.15m) are as follows: Where S is less than L, L= AS2 404 Where S is greater than L, L= 25
_ --'404
Where L = length of vertical curve(m
)
S = sight distance (m)
A = algebraic difference in grades (percent) Table 4-t0A indicates the required K values that areto be used in design for the various design speeds.
TABLE 4-r0A: CREST VERTTCAL CURVE (K VALUES)
Minimum K value (b)
Sag Vertical Curves
At least four different criteria for establishing lengths of sag verticai curves are recognised. These are (r) headlight sight distan c'omforr, (3) drainage control and (4') a rule of thumb for general use "",\zlriaer and this criterion is used to establish the design vaiues for a range of lengths of sag vertical curves. It is again convenient to express the design control in terms of the K value. Table 4-108 indicates the minirnum K varues that are fo be used. Longer curves are desired wherever feasible and should be used, but where K values in excess of 55 are used, special attention to drainage must be exercised. shorter sag verticar curves may bliustified for economic ,Juronr, in cases where an existing element such as a structure which is not ready for replacement controls the vertical profile.
50
A Guide on Geometic Design of Raads
Drainage of curbed pavements are especially important on sag vertical curves where a gradeline of not less than 0.3 percent within 15m of the level point must be maintained.
TABLE 4-108 : SAG VERTICAL CURVE (K VALUES) Design Speed
(km/h) Minimum K value
110
100
90
80
10
60
50
40
30
62
51
40
a^ 3L
25
l8
T2
8
4
4.3.8 General Controls for Vertical Alignment In addition to the specific controls, there are several general controls that should
be
considered.
(a)
A smooth gradeline with gradual changes should be strived for in preference to a line with numerous breaks and short lengths of grade. While the maximum grade and the critical length are controls, the manner in which they are applied and fitted to the terrain on a continuous line determines the suitability and appearance of the finished product.
(b)
The 'roller coaster' or the 'hidden-dip' type of profile should be avoided. They are avoided by use of horizontal curves or by more gradual grades.
(c)
A broken back gradeline should be avoided, particularly in sags where the full view of both vertical curves is not pleasing. This effect is very noticeable on divided roadways with open median sections.
(d)
On long grades, it is preferable to place the steepest grades at the bottom and lighten the grades near the top of the ascent or to break the sustained grade by short intervals of higher grade instead of a uniformed sustained grade that might be only slightly below the allowable minimum-
(e)
Where intersections at grade occur on sections with moderate to steep grades, it is desirable to reduce the gradient through the intersection.
5l
4"4
Horizontal and vertical alignment should not be designed independent.ly. They complement each other. Excellence in their design and in the desrgn of their combination increase ufility and safety, encourage uniforrn speecl, and improve
appearance, armost arways without acr
Proper combination of horizontal alignment. and profire is obtained by engineenng study and consideration of the following general
controls.
(a)
Curvature and grades should be in proper balance. Tangent alignment or flat curvature at the expense ofsteep or long grades, and excessive curvature with flat grades, are both poor design. A logicaio-esign is a compromise between the two, which offers the most in safety, capacity, ea-se and uniiormity of operation, and pleasing appearance within the pracrical limits of terrain and area traversed.
(b)
vertical curvature superimposed upon ho.izontal curvature, or vice versa, generally results in a more pleasing iacility but it should be analysed for effect upon traffic' Successive changes in profile not in combination with horizontal curvature may result in a series of humps visible to the driver for some distance, ahazardous condition.
(c)
Sharp horizontal curvature should not be introduced at or near the top of a pronounced crest vertical curve. Their condition is hazardous in that the driver cannot perceive the horizontal change in alignment, especially at night when the headlight beams go straight ahead into ,pu.!. Thehaiardor tnis ariangement is avoided if the horizontal curve is made longer than the vertical curve. Also, suitable design can be by using design valuesiboue the minimum for the design
speed.
(d)
(e) ' (0
Somewhat allied to the above, sharp hori zontal curvature should not be introduced at or near the low point of u p.onoun.ed sag verlical curve. Because the road ahead is foreshortened any but flat horizontal curvature assumes an undesirable distorted appearance, fufther, vehicular speeds, particularly of trucks, often are high at the bottom of grades and erratic operation may result, especially at night.
on 2-lane
roads the need for safe passing sections at frequent intervals and for an appreciable percentage ofthe length olthe road often supersedes the general desirability for combination of horizontal and vertical alignment. In these cases it is necessary to work toward long tangent sections to secure sufficient passi'g sight distance in design.
Horizontal curvature and profile should be made as flat as feasible at intersections where sight distance along both roads is important and vehicles may have to slow down or stop.
52
(e)
On divided roads, variation in the width of median and the use of separate profiles and horizontal alignments should be considered to derive design and operational advantages of one-way roadways. Where traffic justifies provision of 4 lanes, a superior design without additional cost generally results from the concept and logical design basis of one-way roadways.
(h)
In residential areas the alignment should be designed to minimise nuisance factors to the neighbourhood. Generally, a depressed road makes it less visible and less noisy to adjacent residents. Minor horizontal alignment adjustments can sometime be made to increase the buffer zone between the road and the
residential areas.
(i)
Examples of poor and good practices are illustrated in Figure 4-6 z A to 4-6 : N below:
TANGENT
AIIGNMENT
PREFERRED--\
(A) PRONLE WITII TA}IGENT ALIGNMENT NorE
:-
AVoID DESIGNING LITTLE LocAt DIps IN AN orrIERwIsE LoNc. UNIFORMGRADE,
PREFERRED
(B) PROFILE WITH CUR.VED ALIGNMENT NOTE :- sHoRT HUMps IN TIIE GRADE SHOULD BE AVOIDED.
F'IGURE4-6:A&B
LrNE oF
srcHrnslTsgEN
BoTToMLAND
PREFERRED
PROFILE
(C) DISTANT VIEW SHOWING BUMPS IN PROFILE GRADE LINE NOTE :- A DISTANT SIDE VIEW OF A LONG GRADE TANGENT WILL
REVEAIEVERYBUMP ONIT.
ALIGNMENT
(D) SHORTTAI\IGENT ON A CREST BETWEEN TWO HORIZONTAL CUR\rES NoTE :- THIS COI.ItsINATION IS DEFICIENT FOR TWO REASONS' THE TANGENT BEfi[ryEN T}iE CUR\ES IS TOO SHORT, AND TIIE REVERSE OCCI.'RS ON A CREST.
FIGURE4-6zC&D
55
AlIGMvTENT
PROFILE
(E) SEARP ANGLE APPEARANCE NOTE:- TT{IS COMBINAfiON PRESENTS A poORA?PEARANCE. T}tE HORZONTAL CLJRVE LOOKS LIKE A SHAR-P ANGLE,
(F") DTSJOTNTED NorE
EFFECT
:- A DISTOINTED EFFECT occ{tRs WHEN Tr{E BEGINNI}JG oF A HoRIzoNTAL CTIRVE IS HIDDEN FROM TI{E DRTVER BY A INTERVENING CREST WHILE
TIIE CONTINUATION OF TI{E CTIRVE IS VISIBLE IN THE DISTANCE BEYOND THE INTERVENING CREST..
FIGURE4-6:E&F
56
V
SLrt\{MIT
PROFILE
(G) COINCIDTNG CURVES IN HORTZONTAL AND VERTICAL DIMENSIONS NOTE :- WHEN HORIZONTAL AND VERTICAL CttRVEs SATISFACTORY APPEARANCE RESULTS.
coINcIDE A VERY
ATIGNMENT
Gr) oPPosrNG cuRvES rN HORTZONTAL AND VERTICAL DIMENSIONS NOTE :- WHEN HORIZONTAI AND VERTICAT CURVES OPPOSE' A VERY SATISFACTORY APPEARANCE RESTILTS.
CTIRVES FOR TFM
DESIGNSPEED / -MIMMUM
ALIGNMENT
\ \/l-\
TI) .,
./L
DESIRABLE CURVE FOR APPeeRaNce
-]
FLAT C{IRVES APPROPRIATE FOR HORIZONT,AL WITTI SMALL CENTRAL A}IGLES REGARDLESS OF PROFILE NOTE:- VERY LONG FLAT CLJRVES, EVEN WHERENOT REQUIRED BY TI{E DESICN SPEED, AIJO HAVE A PLEASING APPEARANCE WHEN TTIE CE}]TRAL ANGLE IS VERY SMALL.
FIGURE4-6:G,H&I
57
-]_-*{-
HORTZONTAL AIIGNMENT
t
t
\
L -
-
t\_
--\
t-
I
I
l-
-
PROFILE
(o COINCIDING
CURVES VERTTCES rN HORTZONTAL AND VERTICAL DIMENSIONS
NOTE :- THE CLASSIC CASE OF COORDINATION BETWEEN HORIZONTAL AND VERTICAI ALIGNMENT IN WHICH TT{E VERTICES OF HORIZONTAL AND VERTICAL CURVES COINCIDE. CREATING A zuCH EFFECT OF THREE-DIMENSIONAL S.CLIRVES, COMPOSED OF CO}.IVEX AND CONCAVE IIELD(ES,
lI I
I
HORIZONTAI AI,IGNMENT _-I
j -
-
1I
'\ \-_-
\
-/PROFILE
(K) COINCIDING VERTICES WITH SINGLE.PHASE SKIP NOTE :- A LENGITIMATE CASE OF COORDINATION : ONE pHASE IS SKIPPED IN TI{E HORTZONTAL PLANE, BUT VERTICES STILL COINCIDE. TI{E LONG TANGENT INPI.ANE IS SOF-|ENED BY VERTICAI CT,IRVATURE,
/
_----l { I
I
-..-*--/
--i_
I
HORLZONTATAUGN},IE{I 1 tl ll__r_ .-----l--"\ \-!--'
I
FROFILE
(L) WEAK COORDINATTON OF EORTZONTAL AND VERTICAL ALIGNMENTS NOTE :- A CASE WTIH WEAK COORDINATION WHERE TI{E VERTTCAT ATTGNMENT IS SHIFTED HALF A PTIASE WTTH RESPECT TO HORIZONTAI ALIGNMENT SO T}IE VERTICES COINCIDE WTru POINTS OF IhIFLECTION. TIIE SUPERELEVATION IN T}IIS CASE OCCURS ON GRADE, WHILE CRESTS AND SAGS TTAVE NORMAL CROSS SECTIONS IN TIIE FIRST CASE, SIIPERSLEVATION OCCI]RS ON CRESTS SAGS, WHILE GRADES I{A!E NORh,T,AL CROSS SECTIONS.
FIGURE 4-6 z J,K&L
58
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H 3Fi
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BsE fitr.1
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3 iHa
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i'a"l"
Z{rltH Z ixFl E FT;H
D F TEFE HAfi5 l+l l{ Atll
rvH*
E6 vz
59
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-
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60
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-{
A Cuide on Geometric Design
o.f Roads
CHAPTER 5 - CROSS SECTION ELEMENTS
5.1 5.f
.1
PAVEMENT Surface Type The selection of the pavement type is determined by the volume and composition of traffic, soil characteristics, weather, availability of materials, the initial cost and the overall annual maintenance and service life cost. Pavements may be considered as three general types according to the volume of traffic they carry - high, intermediate and low. High traffic volume pavements have smooth
riding qualities and good non-skid properties in all weather conditions. Intermediate traffic volume pavements vary from high traffic volume pavements only slightly, are less costly and with less strength than high traffic volurne pavements. Low traffic volume pavements range from surface treated earth roads and stabilized matertals to loose surface such as earth and gravel.
The important characteristics of surface type in relation to geometric design are the ability of a surface to retain the shape and dimensions, the ability to drain, and the effect on driver's behavior. Table 5-l gives the general selection of the pavement surface types for the various road standards. For minor roads of grades exceeding 87o, the surface and shoulder should be sealed to prevent erosion. The structural design of flexible pavement should be in accordance to Arahan Teknik (Jalan) - 5/85 "Manual On Pavement Design".
TABLE 5-1 : TYPICAL PAVEMENT SURFACE TYPE Design
Description
Standard
R6ru6 R5ru5
R4N4 R3ru3
RzN2
Rlrul 5.1.2 Normal
Asphaltic Concrete/Concrete Asphaltic Concrete/Concrete Dense Bituminous Macadam/Asphaltic Concrete/Concrete B ituminous Macadam/Concrete Surface Treatment/Semi srout Gravel/Semisrout
Cross Slope
Cross slopes are an important element in the cross-section design and a reasonably steep lateral slope is desirable to minimise water ponding on flat sections of uncurbed pavements due to pavement imperfections or unequal settlements and to control the flow of water adjacent to the curb on curbed pavements. The range of cross slopes for various
pavement types varies from 2.5Vo to 6.0Vo.
6l
A Guide on Geometic Design
The capacity of a llqnwav primarily depends on rhe number of traffic lanes and rheir widths' The lane width is determin"d uy the size of vehicle, averagedaily traffic volume of cornmercial vehicies and the requi.emenfs for overtaking and passing.
Marginal
strip is a nalrow pavement strip attached to both edges of a cariageway. It is paved to the same standard as the traffic lanes. For divided roads, the marginal strips are provided on both sides of the carriageways. The marginal strip is included as part of the
shoulder width' Edge line ma.kingire applied to demarcate the through lane and the marginal strip and is to be located within the marginal strip.
Table 5'2 indicates the lane and marginal strip width that are to be used for the various road standards.
TABLE 5.2 : LANE & MARGINAL STRIP wIDTHs Design Standard
Lane Width (m)
Marginal Strip Width (m)
Interchange Rampl Single Lane Multi Lanes Single Lane
Note
5.3
4.50 3.50 4.50
Lt Lt Lr
1.50 Rt 0.50 0.50 Rt 0.50 1.50 Rt 0.50
: x denotes the total two_way width
SHOULDERS
5"3.1 Definition A shoulder is defined as the portion of the roadway continuous with the traveled way for accommodation of stopped vehicles, for emergen.f ur. and for laterar support to the pavement. A typical cross section showing shoulder and verge is as shown in Figure 5.r.
az
re*ia':r%
a)
c)
l!)
o u0 L q)
o
-
B
A
I
cq
h
I
-
c)
a I
I
--+
I {Jl
-t4 a tl
.61 l-r
lt)
'H
T-
q.(
I
a rh
I
I
r..
c)
I
-
-l[)I; >l t
.FI
+J
Iq)
0
0 rn
r\L \J -c!
.-I
g
t-
F bo 'f F
t-l
(J
tn q)
L FI
bo .r-. H
63
A Guide on Geometric Design of Roads
5.3.2 Functions The main functions of shoulders are
(a) (b)
:_
space is provided for emergency stoppin space is provided
g freeo{ the traffic iane.
for the occasirnal motorist who desires to stop for various
reasons.
(c) (d)
space is provided to escape pofential accidents or reduce their severity. the sense of openness created by shoulders
of adequate width contributes to
driving ease and comfort.
(e) (0 (g) (h) 5.3.3 Width
sight distance is improved in cut sections, thereby improving safety. highway capacity is improved and uniform speed is encouraged. lateral clearance is provided for signs and guardrairs.
structural support is given to the pavement. of Shoulders
The normal usable shoulder width thatshould be provided along high type facilities is 3m' However' in difficult terrain and on low volume roads, usable shoulders of this width may not be feasible.
Table 5-3A and 5-38 gives the widths of shoulders for the various road standards in rural and urban areas. The shoulder widths on structures are defined in Section 5.10. The width of shoulders should also take into consideration car parks.
TABLE 5-3A : USABLE SHOULDER WIDTH (RURAL) Usable Shoulder Widrh (m)
: ::
R6
3.00 3.00 3.00
Rollin
Mountainous
2.s0 2.00
R3
)\n
R2
2.40
3.00 3.00 3.00 2.50 2.0a
R1
1.50
1.50
R5
R4
64
2.5A
2.AA
r.50 r.50
A Guide on Geometric Design of Roads
TABLE 5-38 : PAVED SHOULDER WIDTH ruRBAN) Paved Shoulder Width (m)
Area Type *
U6 U5 U4 U3 U2 U1
* x*
I
II**.
III**
3.00 3.00 3.00 2.50 2.00
3.00 3.00 2.50 2.00 1.50 1.50
2.50 2.50 2.04
1.50
1.50 1.50
1.50
For Area Type definition, see Table 3-28 For Area Type II & III, U1 to U4, shoulder may be replaced by sidewalk
5.3.4 Type of Shoulders There are basically two types of shoulders namely paved and unpaved. Paved shoulders can be either bituminous or concrete surfaced while unpaved shoulders can be either crushed rock. earth or turf shoulders.
The paved shoulder widths are as shown in Table 5.4. The figures given are in addition to the width required for parking.
TABLE 5.4 : PAVED SHOULDER WIDTH (RURAL) Design Standard R6 R5 R4 R3
Paved Shoulder Width (m)
Min 2.5 Min 2.0 1.5 1.5
The structure and the cross slope of these shoulders are different and they are described
below.
.
5.3.5 Shoulder Cross Slope Ali shoulders should be sloped sufficiently to rapidly drain surface water but not to the extend that vehicular use would be hazardous. Because the type of shoulder construction has a bearing on the cross-slope, the two should be determined jointly.
Bituminous and concrete surface shoulders should be sloped from 2.5 to 6 percent, gravel or crushed rock shoulders from 4 to 6 percent and turf shoulders 6 percent. Where kerbs are used on the outside of the shoulders, the minimum cross-slope should not be less than 4 percent to prevent ponding on the roadway.
65
4 9!id" on Geometric De
At superelevated areas, the shoulder cross-slopes should be adjusted to ensure that the maximum ,.roll over,'does not exceed 6 percent.
5.3.6 ShoulderStructure For shoulders to function effectively, they must be sufficiently stable to support occasional vehicle loads, in all kinds of weather without rutting. paved or stabilised shoulders offer manY advantages in this aspect. The structu.e oi the paved shoulders should be similar to that of the carriageway for ati design standards. H"*;;;, ;;;; gradients exceed 8vo the shoulders snouti also be close turfed or sealed to prevent erosion for R2 and Rl roads.
5.4
KERBS
5.4.1
General Consideration The type and location of kerbs appreciably affect driver's behaviour and in turn the safety and usage of a road. Kerbs are uied for drainage control, pavement edge delineation, aesthetics, delineation of pedestrian walkways and to assist in the orderly development of the roadside. Kerbs are needed mostly on roads in urban areas. In rurar areas, the use of kerbs should be avoided as far as possible except in localised areas which has predominant aspects of an urban condition.
5.4.2
Types of Kerbs The two general classes of kerbs are barrier kerbs and mountable kerbs and each has numerous types and detail design may be designed as a separate unit or integrally with the pavement. They may also lach be designed wltn I gutter to form a combination kerb and gutter sectjon. Barrier kerbs are relatively high and steep faced and are designed to inhibit or discourage vehicles from leaving the roadway. They should not be used on expressways. They should also not be used where the design speeds 70 kph or in combination with traffic barriers' They are recommended foi """""a use in builrup areas adjacent to footpaths with considerable pedestrian traffic, where pedestrian traffic is light, a semi-barrier type may be used.
Mountable kerbs are used to define pavement edges of through carriageways. For channelisation islands, medians, outer separators or any other required delineation within the roadway, a semi-mountable type may be used.
Figures 5'2A and 5'28 show the various standard types of kerbs that are to be used.
66
t7
FIGURE 5.2A : ROAD KERB DETAILS USE
TYPE OF KERB
USF
TYPE OF K-ERB
3.)MOWSUtrS
I,)BA.RRHffiS
Moutrblc Lcrbs t12e Ml ud
Euiatsts w ucd to inhr'bit vcbicla ftom leving thc rudrry. Tbcycmmadcd f6uc ia boilt-rp rru Edi&@t to fmPatis or o partr with
|sOF@
oridablo pctsiu
od
lcs
wt
ovd-dllmtold volcl6.
traffic
thc daigl spccd shotdd bc rh^n 70 lpL
Typc outcr
B1
rc dcigncd to discowc trEfrc fioE moutBg a t'affic irlud Roudebou'l s @adis! qc?t for lo.ng q M2
Bl kab ir garally
wc
uscd at
of thc supclcvarod
scdiou-
p Wot@l t
gl
i t-
I
Typc 82 tctb shoulda bc ucC
fc aorosl ms stim
sGcb ud scd.ioB al
l
"r
thc aupaclov8ted
=L
i.Mmclo
@lleisuf&cmE
1.)
t" l"
TypB M2 kdb6 sc d6igred colldt BEf8cc mo{f ud b allow vchicld to @s ovd
ud
to psrk
cltr
to
of tbc
cmirgmy duing magmcY.
GAISIEL I€RJS
Cbrucl kabouldbcwd
wbc irfad reoffis colidmbtY
alonr thc pavod sboulda tbc
Nomalfy EOP 'N
@o?ls
@
wd fo
&l.inariou
m dl int rs6tioil. lt i6 sitsblc
ud
drsiDrgc
luee. It is nomrllY wcd oo d-bshot wbdc dniD hs !o bc phccd dmg tbo pavod sboulds i--cdiatly i! Asnt of b$iq
(GudreilN*
fq
this chancl nfcly.
NOTES:-
I.
JaneY
Briq)
It ws dairncd to minimiz thc avce cffi(' of thc cbrucl on tbc stairg md vcbiclc sboulds bc sblo to drivc ova
all rcads including expnxs*ays. It ir alrc fc uc o bridga whicb b offsst &on hridgc nililg.
CONCRETE FOR CAST INSMU OR PRECAST CONCRETE KSRB SHALL BE GRADE 30/20.
2. CONCRTTE BEDDING SHALL BE GRADE 15,20. 3. THE FIMSHED KERB SHALL BE TRUE TO LINE, GRADE AND LEVEL TO WITHIN 15M SIIALL PRESENT A SMOOTH A?PEAXANCE FREE FROM KINKS AND DISTORTION.
61
AND
FTGURE 5.28: ROAD KERB DETAILS 20
i00
50
100
(mi!.)
--mnm$
SECTION A-A
WITH RECTA IEULAR OPENING
()2 ya
1300r FRONTVIEW OF KERB
l00m CONCRETE
FOI'NDATION
TE PIPE
68
.7 A Guide on Geomelric Design
d
Roads
The width of kerbs are considered as cross section elements entirely outside the traffic lane width. However, for drainage kerbs, the gutter section may be considered as pan of the marginal strip. Where roadways do not have any marginal strip, kerbs should be offsetted by a minimum of 0.25 m.
5.4.3 Channel Kerbs Channel kerbs should be considered for roads built on hieh embankment and for roads with wide roadway.
SIDEWALKS Sidewalks are accepted as integral parts of urban roads and should be provided except on Urban Expressways or Major Arterials where the presence of pedestrians are minimal. However, the need for sidewalks in many rural areas is great because of the high speed and general lack of adequate lighting and due consideration must be given for it especially at points of community development such as schools, local business and industrial plants that results in high pedestrian concentrations. While there are no numerical warrants, the justification for a sidewalk depends on the vehicle-pedestrian hazard which is governed by the volumes of pedestrian and vehicular traffic, their relative timing and the speed of the vehicular traffic.
In urban areas, sidewalks can be placed adjacent to the kerbs and raised above the pavement. In the absence of kerbs, a strip of a minimum width of l.0m must be provided between the sidewaik and the travelled way.
In rural areas, sidewalks must be placed well away from the travelled way and separated from the shoulder by at least 1.0m.
A desirable width of 2.0m is to be provided for all sidewalks. Where there are restrictions on right of way, a minimum of clear width of 1.5m can be considered. When provided, sidewalks must have all weather surfaces. There must be no obstructions on sidewalks. 5.6
TRAFFIC BARRIERS Traffic barriers are used to minimise the severity of potential accident involving vehicles leaving the traveled way. Because bariers are ahazard in themselves, emphasis should be on minimising the number of such installations. Arahan Teknik (Jalan) l/85 "Manual On Design Guidelines of Longitudinal Traffic Barrier" (May, 1984) should be used for the design of longitudinal traffic bariers.
5./
MEDIANS
5.7.1 General A median is a highly desirable element on all roads canying four or more lanes and should be provided wherever possible. The principal functions of a median are to provide the desired freedom from the interference of opposing traffic, to provide a recovery area for out-of-control vehicles, to provide for speed change and storage of right-tunring vehicles and to orovide for future lanes.
69
A Guide on Geometric
For maximum efficiency, a median should be highly visible both night and day and in definite contrast to the through traffic lanes.
5.7.2 Median Width Medians may be depressed, raised or flush with the pavement surface. They should be as wide as feasible but of a dimension in balanceo wltn other components of the crosssection. The general range of median width varies from a minimum of l.5m in a Area Type III urban situation to a desirable width of l8 m on a rural expressway. on wide medians, it is essential to have a depressed centre or swale to provide for drainage.
Table 5-5A and 5-58 gives the minimum and desirable widrhs and types of medians rhar are to be applied to the various road standards. The median widths u, are the dimensions between the through lane edges and ""prrrr.d any. includes the right shouiders
if
TABLE 5-5A : MEDIAN WIDTH (RURAL)
Minimum
Median Width (m Terrain Rollin Minimum Desirable
Mountainous
Minimum
TABLE 5.58 : MEDIAN WIDTH ruRBAN) Median Width (m)
Minimum
5.8.1
Minimum
General Service roads are generally found in urban areas and they can have numerous functions, depending on the type ofroad they serve and the character of the sunounding area. They may be used to control access or function as a street
facility serving adjourning property.
They segregate local traffic from the higher speed through traffic and intercept driveways of residences and commercial establishments along the road. Service roads also not only provide more favourable access for commercial ind residential development than the faster moving afterials but also helps to preserve the safety and capacity of the latter.
70
'17 A Guide on Geometic Design of Roads
5.8.2
Design Requirements From an operational and safety standpoint, one way service roads are much preferred to two-way and should be considered. One-way operation inconveniences local traffic to some degree, but the advantages in reduction in Vehicular and pedestrian conflicts at intersecting streets often fully compensate for this inconvenience. Two-way service roads may be considered for partially developed urban areas where the adjoining road system is so irregular and disconnected that one-way operation would introduce considerable added travel distance and cause undue inconvenience. Two-way service roads may also be necessary for suburban or rural areas where points of access to the through facility are infrequent; where the service roads are widely spaced or where there is no parallel street within reasonable distance of the service roads in urban areas that are developed or likely to be developed. The design of a service road is affected by the type of service it is intended to provide. When provided, they should always cater for at least one side on-street parking. They should be at least I .25 m wide for one-way operation and9.25 m for two-way operation. Figure 5.3 gives the recommended layout for a two-way operation service road fronting an urban arterial where the distance between junctions is greater than 0.5 km. The minimum reserve width for a service road is 12 m.
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5.9
U.TURN
5.9.1
General U-turns are categorised into two (2) types:
i)
Dedicated U-turn
- These U-turns are grade separated from the through
traffic.
This form of U-tum should be considered for high speed and high volume highway as the diverging and merging movements are on the slow lanes.
ii)
Median Opening U-turn.
5.9.2 Location of U-Turns U-turns may be required in the following circumstances:
a)
Beyond intersections to accommodate minor turning movements not otherwise provided in the intersection or interchange area.
b)
Just ahead of an intersection to accommodate U-turn movements that would interfere with through and other turning movements at the intersection. Where a fairly wide median on the approach roadway has few openings, U-turning is necessary to reach roadside areas. Advance separate openings for U-turn outside the intersection proper will reduce interference.
c)
In conjunction with minor crossroads where traffic is not permitted to cross the major road but instead is required to turn left, enter the through traffic stream, weave to the right, U-turn, then retum. On high-speed or high-volume roads, the difficulty and long lengths required for weaving with safety usually make this design pattern undesirable unless the volumes intercepted are light and the median is of adequate width. This condition may occur where a crossroad with high volume traffic, a shopping area, or other traffic generator that requires a median opening nearby.
d)
Locations occurring where regularly spaced openings facilitate maintenance operations, policing, repair service of stalled vehicles, or other highway-related activities. Openings for this purpose may be needed on controlled-access roads and on divided roads through undeveloped areas.
e)
Locations occuming on roads without control of access where median openings at optimum spacing are provided to serve existing frontage developments and at the sanre time minimize pressure for future ntedian openings.
The locations of U-turns are very important and traffic safety should be given due consideration. As a general guide, Table 5.6 can be used to detennine the desirable distances between U-turns.
13
A Guide on Geometric Design
TABLE 5.6 : DESIRABLE DISTANCE BETWEEN U.TURNS Distance Between U-Turns R6 R5 R4
No U-turns allowed
3km 2km
Design j Standard I U6 U5 U4 U3
Distance Between U-Turns No U-turns allowed
2km
lkm lkm
5.9.3 DesignConsideration u-turning vehicles interfere with through traffic by encroaching on part or all of the through traffic lanes. They are n11de uilo* speeds and the required speed change is normally mad^e the through traffic Ianes with weaving to and riom ttre outer lanes. As 91 such, U-turn facilities are potentialhazard areas. Careful consideration should be made to design and locate the u-turn that are safe for ail the road users. For direct u-turns, the width of the highway, including the median should be sufficient to permit the turn to be made without encroachment beyond the outer edges of the pavements' Figure 5'4 gives the minimum width of the median required for the various types of maneuvers for direct U_turns. The U-turn should be designed such that the through traffic approaching the U-turn are travelling at much lower than the highway desigipeed. Traffic signage and calming devices, etc. should be considered.
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5.10
5.10.1 Width of Shoulders The width of the usable shoulders should follow that as indicated in Table 5-3A and 5'38. The width of the shoulder of bridges and structures should be the same as that of the carriageway. Figures 5.5 show the typical bridge cross section.
5.10.2 Required Clearance (a)
(b)
The vertical height clearance of all structures above the roadway shall be a minimum of 5.3 meters over the entire width of traffic lanes, auxiliary Ianes and shoulders. It is recommended that an additional 0.1 meter be allowed for future pavement resurfacing. Whenever resurfacing will reduce the clearance to less than 5.3 meters, milling will be required to maintain the minimum clearance. For ciearance requirements over railways and waterways, reference should be
made to the relevant authorities. (cJ
5.11
Figure 5.6 indicates the clearance requirements for undemasses of various cross sections.
BUS LAYBYES Bus laybyes serve to remove the bus from the through traffic ianes. Its location and design should therefore provide ready access in the safest and most efficrent manner possible. The basic requirement is that the deceleration, standing and acceleration of the buses be effected on pavement areas clear of and separated fromihe through traffic lanes. The locations of bus laybyes are impoftant so as not to impede the normal flow of traffic. In this respect, bus laybyes must not be located on any interchange ramps or srmctures, slip roads or within 60 m of any junction or intersection. The distance between laybyes too should not be less than 150 m. Liaison will need to be made also with the relevant bus companies and the respective Local Authority.
For school areas, provision of bus laybyes and parkings areas should be specifically studied so as to ensure the unirnpederj flow of triffic. Figure 5.7 shows the typical layout and dimensions of bus laybyes that are to be used in both rural and urban areas. If possible, a dividing area between the outer edge of the roadway shoulder and the edge of the bus laybye lane should be provided. This dividing area should be as wide as possible but not less than 0.6 m. The pavement areas of the laybyes should be of concrete or other material fbr contrast in coiour and texture with the through traffic Ianes to discourage through traffic from encroaching on or entering the bus stop.
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FIGURE 5.6 : CLEARANCES AT UNDERPASS LEFT CLEARANCES
RIGHT CLEARANCES
3.0 m (Desirable) 1.5 m
(Minimum)
2.0 m (Desirable) 1.25
n
(Minimum)
TRAfFIC LANES
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;ff,lHtrff]
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(Minimnm)
5.3 m
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CHAPTER 6 . OTHER ELEMENTS AFFECTING GEOMETRIC DESIGN 6.1
ROAD SAFETY Road safety has been a much neglected area in the design of roads, With an increasing number of accidents each year, attention to road safety should be emphasised, While the road element is oniy one of the three groups of influences causing accidents, it is nonetheless the responsibility of the designer to provide as safe a road environment as possible,
In this aspect, the road design should be one with uniformly high standards applied consistently over a section. It should avoid discontinuities such as abrupt major changes in design speeds, transitions in roadway cross-section, the introduction of a short-radius curve in a series of longer radius curves, a change from full to partial control of access, constrictions in roadway width by narrow bridges or other structures, intersections without adequate sight distances, or other failures to maintain consistency in the roadway design. The highway should offer no surprises to the driver, in terms of either geometrics or traffic controls. The tendency to base- road designs upon minimum standards rather than to adopt an optimum design is unfortunate and must be avoided. A more liberal and optimum design must always be considered, and although it would cost a little more in terms of capiLl cost of the project, it would increase the road's level of safety substantially.
All too often' minimum
design standards attempt to rely, on the use of warning signs or other added roadside appurtenances for the safe operat;on that could have been built in by the use of higher design standards. Often tle safety deficiencies generated by minimum design are impossible to correct by any known tiaffic device or ippurtenance. A warning sign is a poor substitute for adequate geometric design. the light to improve road safety, Road Safety Auditing (RSA) has been introduced in ]n Malaysia since 1994-_It is a formal process to identify deficiencies in road design at different stages as welr as to identify hazardous features on existing roads for erimrnation so as to make the road safer for all road users.
RSA is carried out in five stages. The feasibility and planning stage (Stage r), preliminary design stage (stage2), detailed design itage (stage 3), pre-opening stage (Stage 4) and operationaVaudit of existing roads lStage S;.
j
Rgference on RSA should be made to the "Guidelines For The Safety Audit Of Roads In Malaysia" published by the Roads Branch, pubiic works Department Malaysia. 6.2
D&AINAGE Road drainage facilities provide for carrying water across the right of way and for removal of storm water from the road itself. These facilities include bridges, culverts, channels, gutters and various types of drains. Drainage design considerations are an integral part of geometric design and flood plain encriachments fiequently affect the highway alignmenr and profile.
80
A Guide on Geometric Design of Roads
The cost of drainage is neither incidental nor minor on most roads. Careful attention to requirements for adequate drainage and protection of the highway from floods in all phase of location and design will prove to be effective in reducing costs in both construction ant maintenance. Reference should be made to the relevant JKR manual on this subiect. 6.3
LIGHTING Lighting may improve the safety of a road and the ease and comfort of on operation thereon. Lighting of rural highways may be desirable but the need is much less than on roads in urban areas. They are seldom justified except on critical portions such as interchanges, intersections, railroad grade crossings, narrow or long bridges, tunnels and areas where roadside interference is a factor.
Where lighting is being considered for future installation, considerable savings can be effected through design and installation of necessary conduits under the pavements and kerbs as part of the initial construction and should be considered. Reference should be made to the relevant JKR manual on this subiect. 6.4
UTILITIES All
road improvements, whether upgraded within the existing right of way or entirely on
new right of way, generally involves the shifting of utility facilities. Although utilities generally have little effect on the geometric design of the road, full consideration should be given to measures necessary to preserve and protect the integrity and visual quality of the road, its maintenance efficiency and the safety of traffic.
Utilities relocation shouid form part of design and the designer should liaise closely with the relevant service authorities in determining the existing utilities and their proposed relocation. 6.s
SIGNING AND MARKINGS Signing and marking are directly related to the design of the road and are features of traffic control and operation that the engineer must consider in the geometric layout of suchfacility. The signing and marking should be designed concurrently with the geometrics as an integral part, and this will reduce significantly the possibility of future operational problems. The signing and marking should follow the standards that have been established. Reference should be made to Arahan Teknik (Jalan) 2A,2F.,2C and 2D on the design, usage and application of signs and markings.
6.6
TRAFFIC SIGNALS
Traffic control signals are devices that control vehicular and pedestrian traffic by assigning the right of way to various movements for certain pretimed or traffic actuated intervals of time. They are one of the key elements in the function of many urban roads and should be integrated with the geometric design so as to achieve optimum operational efficiency.
81
6.7
ENVIRONMENTAL IMPACTS Erosion prevention is one of the major factors in design, construction ancl matntenance of highway. It should be considered early in the location and design stages. Some degree of erosion control can be incorporated into geometric design, particularly in the crosssection elements. Landscape development should be in keeping with the character of the road and its environment. The general are of improvement include the followins: (a) preservation of existing vegetation
(b) transplanting of existing vegetation where feasible (c) planting of new vegetation (d) selective clearing and thinning (e) regeneration of natural plant slecies and material. Landscaping of urban roads assume an additional importance in mitigating the many nuisances associated with urban traffic. Reference stroulO be made to the relevant JKR
manual on this subject
The highway can and should be located and designed to complement its environment and serve as a catalyst to environmental improvement. The area surrounding a proposed highway is an interrelated system of natural, manmade and sociologic variables. Changes in one variable within this system cannot be made without some effect on other variable. Some of these consequences may be negligible, but others may have strong and lasting impact on the environment, including the sustenance and quality of human lif.. B".uur" highway location and design decisions have an effect on udju""n t areadevelopment, it
is important that environmental variables be given full consideration. Environmental impacts include social, economic and physical impacts. In geometric design, only physical impact- are aasessedl while the social and economic impacts would have been taken care of during the planning stage.
82
1W!1EF@@jfwF1,I@. ile
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A Guide on Geometric Design of Roads
REFERENCES
l.
ARAHAN TEKNIK JALAN 8/86: A Guide on Geometric Design of Roads,
2.
ISMAIL, MOHD. A: Thesis on Review of Highway Geometry Design
1986
Standards in
Malaysia, 1996
3.
AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALS (AASHTO): A Policy on Design of Urban Highways And Arterial Streets, r913
4.
THE INSTITUTION OF HIGHWAYS AND
TRANSPORTATION WITH
DEPARTMENT OF TRANSPORT: Roads and Traffic in Urban Areas, 1987
5.
TRANSPORT ROAD RESEARCH LABORATORY AND OVERSEAS DEVELOPMENT ASSISTANCE. UNITED KINGDOM : TowaTds SafeT ROAdS iN Developing Country, 199 I
6.
JABATAN PERANGKAAN MALAYSIA: Banci Penduduk Malaysia,
7.
JABATAN PERANCANG BANDAR DAN DESA: Manual Piawaian Perancangan, JPBD (IP) S.273 FN/HHffeb. 1998
8.
AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALS (AASHTO): A Policy on Design of Highways And Streets, 1994
9.
TRANSPORTATION AND RESEARCH BOARD: Highway Capacity Manual, Special Report 209,1985
10.
HIGFIWAY PLANNING UNIT, MINISTRY OF WORKS MALAYSIA: Trip Generation Manual 1997.
1991
83
*n5+f.!ry,'qw-:w
A Guide on Geometric Design of Roads
ACKNOWLEDGEMENTS PUBLIC WORKS DEPARTMENT, MALAYSIA
Dato'Ir. Hj. Zainibin Omar STANDING COMMITTEE ON TECHNOLOGY AND ROAD MANAGEMENT Dato' Ir. Chew Swee Hock (Chairman)
TECHNICAL COMMITTEE ON GEOMETRICS ffC3) Ir. Han Joke Kwang Ir. Koid Teng Hye Ir. Aznam Abdul Rahim Ir. Tai Meu Choi Ir. Sam El-Ashwawi
(Chairman) (Secrerary) (WG Leader) (WG Leader) (WG Leader)
84
