CE 323-Highway Engineering

FACULTY OF ENGINEERING Department of Civil and Building Engineering

YEAR III, SEMESTER II

LECTURE NOTES

Researched and Compiled by

Okello Francis Eugene

February 2010

Table of Contents

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Course Structure Third Year; Semester II

Core Course:

Yes

Lecture Hours [L]:

45

Contact Hours [CH]:

60

Practical Hours [P]:

30

Credit Units [CU]:

4

Class Hours

Day

Monday:

Evening

1400 – 1600 hrs

1700 – 1900 hrs

Course Assessment

Course Work:

40%

Final Examination:

60%

[Assignments 15%, Tests 25%]

Normal Progress

Grade Point [GP]

2.0 [50%]

Course Outline • •

Introduction: History and Development of

•

Urban Roads;

roads;

•

Single and Double Carriageways;

Planning and Layout of Roads;

•

Junctions;

• Route Surveys;

• Intersections;

• Selection of Routes;

•

• Site Investigation;

• Road Furniture;

• Soil Survey;

•

•

• Maintenance of Roads.

Types of Roads: Low Cost Roads, Granite

Roundabouts;

A Case Study of Uganda;

Sets, Flexible & Rigid roads;

•

Soils Technology for Roads;

Field Exercise:

• Soil Stabilisation;

Planning of one Layout of Length of a New

•

Construction Techniques;

Road Using Available Contoured Maps

•

Drainage;

• Street Lighting; •

Highways;

•

Rural roads;

Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514 806514

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Table of Contents

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Table of Contents Table of Contents ....................................................................... ..................................................................................................... ......................................................... ............................. ii List of Tables ........................................................ ...................................................................................... ............................................................ ................................................ .................. vi List of Figures ............................................................................... ............................................................................................................. ..................................................... .......................vii Symbols and Abbreviations ..................................................................................... .............................................................................................................. .........................viii 1.0 History and Development of Roads .................................. ............................................................... ....................................................... .......................... 1 1.0 1.1

Introduction ............................................................ .......................................................................................... ......................................................... .................................... ......... 1 Definitions of some common terms .................................... .................................................................. ..................................................... ....................... 1

1.2 1.3 1.4

Early Age Road Development ....................................................................... ................................................................................................. .......................... 1 Middle Age Road Development ............................................................................. .............................................................................................. ................. 2 th 19 Century Roads ...................................... .................................................................... .............................................................. ............................................... ............... 4

1.5 1.5.1 2.0

Roads in the World Today ...................................................... .................................................................................... ................................................. ................... 4 References ...................................................... ..................................................................................... ........................................................... ........................................... ............... 6 Planning and Layout of Roads .......................................... ....................................................................... ....................................................... .......................... 7

2.1 2.2 2.3 2.3.1

Introduction ............................................................ .......................................................................................... ......................................................... .................................... ......... 7 Goals and Objectives .......................................................... ....................................................................................... ..................................................... ........................ 7 The Project Cycle .......................................... ......................................................................... ........................................................... ............................................ ................ 8 Components of the Project Cycle....................................................... Cycle.................................................................................... ...................................... ......... 8

2.3.2 2.3.3

Problem Identification.................... Identification.................................................. ............................................................ ............................................................ ..............................8 Pre-feasibility ........................................................... ......................................................................................... ........................................................... .................................. ..... 9

2.3.4 2.3.5

Feasibility ....................................................... ...................................................................................... ............................................................ ........................................... .............. 9 Design ......................................................... ...................................................................................... ............................................................. ................................................ ................ 9

2.3.6 2.3.7 2.3.8

Commitment and negotiation .......................................... ........................................................................ ......................................................... ........................... 9 Implementation .............................................................. ............................................................................................. ........................................................ .........................10 Operation.......................................................... ......................................................................................... ........................................................... ....................................... ........... 10

2.3.9 2.4

Monitoring and Evaluation .............................................................................. .................................................................................................... ...................... 10 Overview of Road Appraisal in Developing D eveloping Countries ......................................................... ......................................................... 10

2.4.1 2.4.2

Define Objectives.......................................................... ........................................................................................ ......................................................... ...........................11 Determining alternative ways of meeting Objectives ...................................................... ............................................................ ...... 11

2.4.3 2.4.4 2.4.5 2.4.6

Preliminary considerations............................................................ ......................................................................................... ......................................... ............12 Assess Traffic Demand .............................................................. ............................................................................................. ............................................ ............. 12 Design and Cost different Options ........................................................... ......................................................................................... .............................. 12 Determine Benefits of each Alternative ......................................................... ................................................................................. ........................ 13

2.4.7 2.4.8

Economic Analysis and comparison of alternatives .............................................................. .............................................................. 13 Recommendations ...................................................... ................................................................................... ......................................................... ............................... ... 13

2.5 2.6 2.6.1 2.6.2 2.6.3 2.6.4

A Typical Road Project Appraisal Process in Uganda .......................................................... .......................................................... 13 Economic Evaluation of Highway Projects P rojects ..................................................... ........................................................................... ...................... 16 Role of Economic Evaluation ................................................................. ............................................................................................. ............................... ... 16 Some Basic Principles .................................. ................................................................... ............................................................... ......................................... ........... 16 Time Value for Money....................................................... ..................................................................................... .................................................... ...................... 17 Costs and Benefits .................................. ................................................................ ............................................................ .................................................. .................... 17

2.6.5

Evaluation Techniques ........................................................................ .................................................................................................... ................................... ....... 20


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Table of Contents 2.6.6

Comparison of the Various Methods of Economic Evaluation ............................................. 22

2.6.7 2.7 2.7.1

Selection of the Discount Rate ............................................................................................... 22 Selection of Routes ................................................................................................................ 25 Introduction ............................................................................................................................ 25

2.7.2 2.7.3

Overview of the Location Process .........................................................................................26 Location Surveys in Non-Built-Up Areas .............................................................................. 26

2.7.4 2.7.5

Road Location in Built up Areas............................................................................................ 28 References .............................................................................................................................. 29

3.0 3.1 3.2 3.2.1 3.2.2

The Road User and the Vehicle ............................................................................................. 30 Introduction ............................................................................................................................ 30 Human Factors Governing Road User Behaviour ................................................................. 30 Human Body as a complex System........................................................................................ 30 Vision ..................................................................................................................................... 30

3.2.3 3.2.4

Hearing ................................................................................................................................... 30 Perception, Intellection, Emotion and Volition ..................................................................... 31

3.3 3.3.1

Pedestrian Characteristics ...................................................................................................... 31 Speed ...................................................................................................................................... 31

3.3.2 3.4 3.5

Space Occupied by Pedestrians..............................................................................................31 Vehicle Characteristics .......................................................................................................... 31 References .............................................................................................................................. 32

4.0 4.1

Geometric Design of Highways .............................................................................................33 Introduction ............................................................................................................................ 33

4.2 4.3

Highway Design Standards in Uganda .................................................................................. 34 Division of Roads into Functional Class................................................................................ 34

4.4 4.4.1 4.4.2 4.4.3

Design Controls and Criteria ................................................................................................. 34 General ................................................................................................................................... 34 Topography ............................................................................................................................ 35 Traffic..................................................................................................................................... 35

4.4.4 a)

Design Vehicle Dimensions ...................................................................................................37 Design Vehicles ..................................................................................................................... 37

b) c)

Dimensions of Design Vehicles ............................................................................................. 37 Selection of the Design Vehicle ............................................................................................. 38

4.4.5 4.4.6

Design Speed..........................................................................................................................38 Control of Access ................................................................................................................... 38

4.5 4.5.1

Sight Distance ........................................................................................................................ 39 General ................................................................................................................................... 39

4.5.2 4.5.3 4.5.4 4.5.5 4.6 4.6.1 4.6.2

Stopping Sight distance, SSD ................................................................................................ 40 Full Overtaking Sight Distance, FOSD.................................................................................. 42 Sight Distance for Multi-Lane Roads .................................................................................... 43 Set-back Distance at Obstructions of Horizontal Curves....................................................... 43 Horizontal Alignment ............................................................................................................ 46 Basic Formula for Movement of Vehicles on Curves............................................................ 46 Value of the Coefficient of Lateral Friction, µ ...................................................................... 48

Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Table of Contents 4.6.3

Maximum super-elevation Value, e max ..................................................................................48

4.6.4 4.6.5 4.6.6

Super-elevation Rates ............................................................................................................ 48 Radii of curves for which no super-elevation is required ...................................................... 49 Method of Attainment of Super-elevation .............................................................................49

4.6.7 4.6.8

Transition Curves ................................................................................................................... 52 Curve Widening ..................................................................................................................... 54

4.6.9 4.7

General Controls for Horizontal Alignment .......................................................................... 57 Vertical Alignment.................................................................................................................58

4.7.1 4.7.2 4.7.3 4.7.4 4.7.5

Major Requirements of Vertical Curves ................................................................................ 58 Gradients ................................................................................................................................ 58 Climbing Lanes ...................................................................................................................... 59 Cross falls............................................................................................................................... 59 Vertical Curves ...................................................................................................................... 60

4.7.6 4.7.7

Vertical Crest Curve Design and Sight Distance Requirements ............................................ 63 Vertical Sag Curve Design and Sight Distance Requirements .............................................. 64

4.7.8 4.8

General Controls for Vertical Curve Alignment .................................................................... 66 Cross-Sectional Elements ...................................................................................................... 70

4.8.1 4.8.2 4.8.3

General ................................................................................................................................... 70 Road Reserve ......................................................................................................................... 71 Carriageway Width ................................................................................................................ 71

4.8.4 4.8.5

Central Reservation (Median) Strip ....................................................................................... 71 Shoulders................................................................................................................................ 72

4.8.6 4.8.7

Laybys and bus bays .............................................................................................................. 72 Kerbs ...................................................................................................................................... 72

4.8.8 4.8.9 4.9 4.9.1

Camber ................................................................................................................................... 73 Side slope ............................................................................................................................... 73 Intersection Design and Capacity .......................................................................................... 73 General ................................................................................................................................... 73

4.9.2 4.9.3

At-grade and Grade Separated Junctions ............................................................................... 74 Basic Forms of At-grade Intersections................................................................................... 74

4.9.4 4.9.5

Overview of the Design Process ............................................................................................ 75 At-grade Intersection Types (from a design perspective) ...................................................... 75

4.9.6 4.9.7

Capacity of a T-Junction ........................................................................................................ 83 Design Reference Flow (DRF) .............................................................................................. 85

4.9.8 4.9.9

Delay ...................................................................................................................................... 85 Rotary Intersections (Roundabouts)....................................................................................... 87

4.10 5.0 5.1 5.2 5.2.1 5.2.2 5.3

References .............................................................................................................................. 93 Design of Flexible Pavements................................................................................................ 94 Introduction ............................................................................................................................ 94 Types of Pavements ............................................................................................................... 94 Flexible Pavements ................................................................................................................ 94 Rigid Pavements .................................................................................................................... 95 Elements of a Flexible Pavement and their significance ....................................................... 95


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Table of Contents

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5.3.1

Surfacing ................................................................................................................................ 95

5.3.2 5.3.3 5.3.4

Roadbase ................................................................................................................................ 95 Subbase .................................................................................................................................. 96 Capping Layer (Selected or Improved Subgrade).................................................................. 96

5.3.5 5.4

Subgrade................................................................................................................................. 96 The Pavement Design Process ............................................................................................... 96

5.4.1 5.4.2

Traffic Assessment.................................................................................................................97 Subgrade Assessment.............................................................................................................97

5.4.3 5.5 5.6 5.6.1 5.6.2

Material Selection .................................................................................................................. 97 Approaches to Design ............................................................................................................ 97 Highway Design Standards .................................................................................................... 98 Uganda Road Design Manual ................................................................................................99 Kenya Road Design Manual .................................................................................................. 99

5.6.3 5.7

TRL Road Note 31 ................................................................................................................. 99 The AASHTO Approach to Pavement Design ...................................................................... 99

5.7.1 5.7.2

The AASHTO Design Equation ............................................................................................ 99 Regional Adjustment............................................................................................................100

5.7.3 5.7.4 5.8

Design Tables.......................................................................................................................100 Steps involved in the AASHTO method of Design ............................................................. 102 References ............................................................................................................................ 107

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List of Tables

vi

List of Tables Table 1.1: International Comparison of Road Statistics ...................................................................... 5 Table 1.2: Car Ownership Rates (Cars per 1000 persons) ................................................................... 6 Table 2.1: Computation of NPV, B/C Ratio and IRR ........................................................................ 23 Table 4.1: Terrain Classification ........................................................................................................ 35 Table 4.2: Conversion Factor of Vehicle into Passenger Car ............................................................ 36 Table 4.3: Dimensions of Design Vehicles ........................................................................................ 37 Table 4.4: Level of Access Control ................................................................................................... 39 Table 4.5: Stopping Sight Distance on Level Ground for Wet Pavement Condition ........................41 Table 4.6: Coefficient of Lateral Friction as Recommended by AASHTO ....................................... 48 Table 4.7: Maximum Grades as recommended by MoWH&C ......................................................... 58 Table 4.8: Minimum Radii for Crest Curves as Recommended by MoWH&C ................................ 63 Table 4.9: Minimum Radii for Sag Curves as recommended by MoWH&C .................................... 63 Table 4.10: Types of At-grade Intersections as recommended by MoWH&C .................................. 76 Table 4.11: The Limits of the Parameters used in Roundabout Capacity Equation .......................... 89 Table 5.1: Subgrade Classes ............................................................................................................ 100 Table 5.2: Traffic Groups................................................................................................................. 101 Table 5.3: Average Vehicle Equivalence Factors, C i ...................................................................... 101 Table 5.4: Traffic Classes ................................................................................................................ 101 Table 5.5: Determination of DSN for different Subgrade and Traffic Classes................................ 101 Table 5.6: Layer Coefficients........................................................................................................... 102 Table 5.7: Compacted Thickness Ranges ........................................................................................ 102

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List of Figures

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List of Figures Figure 1.1: Cross Section of Early Roads ............................................................................................ 4 Figure 2.1: Typical Road Project Appraisal in Uganda ..................................................................... 14 Figure 4.1: Stopping and Passing Sight Distances on a crest curve .................................................. 43 Figure 4.2: Sight Distance Requirements on a horizontal curve with S ≤ L ...................................... 44 Figure 4.3: Sight Distance Requirements on a horizontal curve with S > L ...................................... 45 Figure 4.4: Forces acting on a vehicle on a horizontal curve ............................................................ 46 Figure 4.5: Stages involved in attainment of super-elevation ............................................................ 50 Figure 4.6: Attaining Super-elevation by revolving about the centre line ......................................... 51 Figure 4.7: Main Elements of a Circular Curve Provided with Transitions ...................................... 52 Figure 4.8: Widening on Curves ........................................................................................................ 54 Figure 4.9: Climbing Lane outside the ordinary lane ........................................................................ 59 Figure 4.10: Highway Cross falls ...................................................................................................... 59 Figure 4.11: Typical Vertical Curves................................................................................................. 60 Figure 4.12: A Simple Symmetrical Parabolic curve ........................................................................ 60 Figure 4.13: Sight distance over crest curves when a) S ≤ L and b) when S > L .............................. 62 Figure 4.14: Single Carriageway Cross-section Elements ................................................................. 70 Figure 4.15: Dual Carriageway Cross-section Elements ...................................................................71 Figure 4.16: Basic Intersection Forms ............................................................................................... 75 Figure 4.17: Typical Access Layout showing Visibility Requirements ............................................ 76 Figure 4.18: Typical T-Intersections.................................................................................................. 77 Figure 4.19: Typical Designs for Control Intersections ..................................................................... 78 Figure 4.20: Selection of Intersection Category based on Safety ...................................................... 80 Figure 4.21: Selection of Intersection Category based on Capacity .................................................. 81 Figure 4.22: Selection of Priority Intersection type based on Safety................................................. 82 Figure 4.23: Selection of Control Intersection Type.......................................................................... 83 Figure 4.24: Selection of Control Intersection Type.......................................................................... 84 Figure 5.1: Definition of Pavement layers .........................................................................................95 Figure 5.2: Summary of the Pavement Design Process ..................................................................... 98

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Symbols and Abbreviations

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Symbols and Abbreviations AADT

Annual Average Daily Traffic

AASHTO

American Association of State Highways and Transportation Officials

ADT

Number of average daily traffic

ALD

Average Least Dimension

CBR

California Bearing Ratio

E.S.A

Equivalent Standard Axle

GB3

Granular Base-material type 3

GIS

Graphical Information Systems

HW

Allowable Headwater depth

KUTIP

Kampala Urban Transportation plan

LL

Liquid Limit

LS

Linear Shrinkage

M.S.A

Millions of equivalent standard axle

MC

Moisture Content

MDD

Maximum Dry Density

OMC

Optimum Moisture Content

ORN

Overseas Road Note

PI

Plasticity Index

PL

Plastic Limit

TRRL

Transport Road Research Laboratory

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History and Development of Roads

1.0

History and Development of Roads

1.0

Introduction

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Everybody travels, whether it is to work, play, shop, do business, or simply visit people. All foodstuffs and raw materials must be carried from their place of origin to their place consumption [O’Flaherty, 2002]. Historically, people have travelled and goods have been moved by: a) Road i.e. using cars, wagons, cycles and motor vehicles; b) Water i.e. using ships and boats; c) Rail i.e. initially using animals and then the steam oil or electric powered locomotives to pull passenger carriages and goods wagons; d) Air i.e. using airships and aeroplanes (20 th Century) 1.1

Definitions of some common terms

Some terms like ‘highways’, ‘roads’ and ‘streets’ have precise meanings, though they are often used loosely in practice. A ‘highway’ is an arterial road facility designed for high speed and high volume traffic in non-urban areas. For example, the national road network of a country is called the National Highway Network. A ‘road’ is a lower order facility, designed for relatively lower speed and lower volume traffic in the non-urban areas. For example, they can be district roads or village roads. A ‘street’ is an urban road facility. An ‘Expressway’ or ‘Express Highway’ is a superior type of highway facility with full or partial control of access. It is generally consists of divided carriageway that caters for very high speeds.

1.2

Early Age Road Development

The origin of roads dates back to the period before the advent of recorded history. While the birth of the road is lost in the mist of antiquity, there is no doubt but that the trails deliberately chosen by early man and his pack animals to facilitate his movements were the forerunners of today’s road. As civilization developed and people’s desire for communication increased, the early trails became pathways and the pathways evolved into recognized travelways. Many of these early travel ways-termed ‘ridge ways’- were located high on hillsides where the underbrush was less dense and walking was easier; they were also above soft ground in wet valleys and avoided unsafe wooded areas. As civilization advanced, the growth of agriculture took place and human settlements began to be formed. The invention of the wheel in 5000BC and the domestication of animals saw the advent of chariots and carts. These carts enabled heavy loads to be carried more easily Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Middle Age Road Development

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and gave rise to wider travelways with firmer surfacings capable of carrying concentrated loads, but with less steep connecting routes down to/up from valleys and fordable streams. Thus trackways evolved along the contours of lower slopes e.g. they were sufficiently above the bottoms of valleys to ensure good drainage but low enough to obviate unnecessary climbing. The trackways eventually become well established trade routes along which settlements developed and these gave rise to hamlets and villages - Some of which, eventually, became towns and cities. Early manufactured roads were stone-paved streets of Ur in the Middle East (4000BC), the corduroy – log paths near Glastonbury, England (3300BC), and brick pavings in India (3000BC): The oldest existing wooden pathway in Europe, the 2km long Sweet Track, was built across (and parts subsequently preserved in) marshy ground near Glastonbury. The oldest existing stone road in Europe was built in Crete in 2000BC.

1.3


Notwithstanding the many examples of early man-made roads that are found in various parts of the world, it is the Romans who must be given credit for being the first ‘professional’ road-makers. At its peak the Roman road system, which was based on 29 major roads radiating from Rome to the outermost fringed of the empire, totalled 52,964 Roman miles (approx. 78,000km) in length. Started in 312BC, the roads were built with conscripted or forced labour; their purpose was to hold together the 113 provinces of the empire by aiding imperial administration, extension of the territorial limits of the empire and quelling rebellions after a region was conquered. The roads were commonly constructed at least 4.25m wide to enable two chariots to pass with ease and legions (large group of soldiers) to march abreast. It was common practice to reduce gradients by cutting tunnels, and one such tunnel on the Via Appia was 0.75km long. Most of the Roman roads well built on embankments 1m to 2m high so as to give the troops a commanding view of the country side and make them less vulnerable to surprise attacks; this had the engineering by-product of helping to keep the carriage way dry. The roads mainly comprised of straight sections as they provided the most direct routes to the administrative areas; however deviations from the straight line were tolerated in hilly regions or if suitable established track ways were available. The withdrawal of the legions from Britain in AD 407; foreshadowed the breakdown of the only road system in Europe until the advent of the 17 th century. While the Roman roads in Britain continued to be the main highways of internal communications for a very long time; they inevitably began to decay and disintegrate under the actions of weather, traffic and human resourcefulness. Eventually, their condition became so appalling that when sections became impassable, they were simply abandoned and new tracks created about them. The onset of the 18 th century also saw foreign trade become more important to Great Britain’s steadily developing manufacturing industries and soon long trains of carts and Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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wagons were common sights as they laboriously dragged coal from mines to iron works, glassworks and potteries and manufactured goods to harbours and ports, along very inadequate ways. Confronted by the above pressures and the terrible state of the roads, parliament passed in 1706, the first of many statutes that eventually created over 1,100 Turnpike Trusts. These trusts which administered some 36,800km of road were each empowered to construct and maintain a specified road length and levy tolls upon certain types of traffic. The development of the toll road system, especially in the century following 1750, was important for many reasons, not least of which were: a) It promoted the development of road making techniques in Britain and allowed the emergence of skilled road makers e.g. Thomas Telford, John Loudon Mc Adam and Pierre Tresaguet. b) It established that road users should pay some road costs. c) It determined the framework of the 20 th century pre-motorway trunk road network The steam-powered railway service in 1825 marked the beginning of the end for the Turnpike Trusts as the transfer of long distance passengers from road to rail was almost instantaneous and towns were accessed by railway. Pierre Tresaguet, the inspector general of roads in France was the first to recognize the importance of drainage of roads and its methodical maintenance. He appreciated the role of moisture in soils and pavements and how moisture affects the performance of road beds. Camber began to be introduced in roads during his time. Thus, he can be rightly called the father of modern highway engineering. The name of John Metcalf is associated in Britain with the art of building good and stable roads in the latter part of the 18 th century. He used boulders to achieve strong foundations for roads and spread gravel as a surface layer. He pioneered the construction of good roads on soft ground, using a sub base of bundles of heather (Low s preading bush with small pink purple flowers). Thomas Telford (1757-1834) is yet another illustrious name in highway engineering, immortalized by naming the hand-packed boulder foundation of roads as Telford base. The construction technique held the sway for nearly 150years since Telford introduced it in the early part of the 19th century. A run of names of eminent highway engineers is incomplete without John McAdams (17561836). He was a Scottish road builder who has influenced road construction so profoundly that the term ‘Macadam’ is frequently used in pavement specifications even to this day. His two important principles of good road construction were; a) It is the native soil that supports the traffic load ultimately and when the soil is maintained in a dry state, it can carry heavy loads without settlement. b) Stones which are broken to small angular pieces and compacted can interlock each other and form a hard surface. Thus Mc Adam’s specifications were at variance with Telford’s in that smaller pieces of stones with angular faces were favoured than larger hand packed boulders. He is reported to have given a practical hint to engineers in selecting the size of stones; the size is good if the Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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19th Century Roads

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stone can be put into the mouth. How valid his advice is even to this day! Other than the innovative specifications he introduced, Mc Adam is also remembered for his foresight in urging the creation of a central highway authority to advise and monitor all matters relating to roads in Britain. His recommendation is valid even now in Uganda [Kadiyali, 2006].

1.4

th

19 Century Roads

A significant development which revolutionized road construction during the 19 th century was the steam road roller introduced by Eveling and Barford. The development of Portland cement in the first decades of the 19 th century by Aspin and Johnson facilitated modern bridge construction and use of concrete as a pavement material. Tars and asphalts began to be used in road construction in the 1830’s, though it was the pneumatic tyre vehicle which gave a real push to extensive use of bituminous specifications. The automobile had its slow development in the 19 th century, but the First World War, 1914-18, gave momentum to its growth. Thus the road was given a new lease of life [O’Flaherty, 2002].

(a) Roman Roads

(b) British Roads

(c) French Roads Figure 1.1: Cross Section of Early Roads

Source: Mathew & Rao (2007)

1.5

Roads in the World Today

Roads are the principal arteries of traffic in the present-day world. The right indicator of a country’s prosperity is its road length and vehicle ownership. Table 1.1 gives an international comparison of road length in some selected countries. The following inferences can be drawn: a) America has the largest network of roads (6.3million km) b) India, with its 3.3million km of network comes second. c) The density of roads (km/sq-km) is very high in countries like Germany and Japan which are small in area. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Roads in the World Today

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d) In countries which are large in area, the density is low. India has a density of 1km/sq km, USA 0.67km/sq km and China 0.12km/sq km. e) The percentage of roads paved is very high in countries like UK (100 per cent), Germany (99 per cent) and USA (91 per cent). Nearly the entire length is paved. f) In India, the percentage of paved roads is 50. In USA, the percentage is 91. Table 1.1: International Comparison of Road Statistics s/n 1 2 3 4 5 6 7 8 9 10 11

Counrty Roadlength (km) Roaddensity (km/sq-km) Per cent paved USA 6,300,000 0.67 91 INDIA 2,009,600 0.63 50 BRAZIL 1,939,000 0.23 9 CHINA 1,157,000 0.12 90 JAPAN 1,136,347 3.77 73 GERMANY 650,700 5.97 99 INDONESIA 372,414 0.19 47 U.K. 366,999 1.5 100 MALAYSIA 93,975 0.29 75 THAILAND 62,000 0.12 97 NIGERIA 32,810 0.04 83

Source: Kadiyali, 2006

NB: All values are for 1998 India’s road length now is 3.3million km and the road is 1km/sq km.

In modern times, Europe saw the beginnings of the Expressway system of World War II. Italy, under Mussolini, started the ‘Autostrade’. The famous German ‘Autobahns’ were planned in the late 1920s and Hitler accelerated their completion. The Autobahns became a key part of the war-time infrastructure for the movement of tanks and other military vehicles UK started its Motorway construction rather late, in the 1950s. These form the arterial road grid of the country linking London to major cities like Manchester, Liverpool, Hull, Bristol, Edinburgh and Newcastle. Perhaps the largest arterial system, the US interstate, was started after World War II as a national defence system. The construction of the 41, 000 miles system was approved in 1956. It was funded by the Federal Government to an extent of 90 per cent, the balance being state’s matching share. It linked all the major cities of the nation. It is toll-free. USA also pioneered the modern super highway - a limited access, high-speed facility. The Bronx River Parkway constructed in 1925 was the forerunner to many such to come later. The inter-state system of USA The world’s best road system is perhaps in the USA. The interstate system was taken up after the Second World War as a defence system. It is now fully functional. The USA now has a length of 88,400km of express ways, of which 5,000km (6 per cent) was tolled. The remaining length is toll-free. Autobahns of Germany Germany began constructing its express ways, which were known as Autobahns in the late 1920s. Before the start of the Second World War, Germany had about 4,000 km of express ways. The country has now 11,238km of express ways most of which are non-toll.


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References

6

Express ways in France. France started the construction of its express ways in the 1950s. The work was carried out through semi-government public companies. Private companies were involved in the work in the 1970s. The network was developed as a toll system. By 1996, the country had a network of 8,768km of express ways, 72 per cent of which are tolled. Vehicle Ownership Since road transport gives mobility to persons, the vehicle ownership rate has been increasing at a fast rate round the world. Table 1.2 gives a comparison of the car-ownership rate (cars per 1000 persons) in some selected countries. The rate is very high in USA. (One car per two persons), and is currently low in India (one car per 250 persons). This rapid growth calls for modernization of the road system. Table 1.2: Car Ownership Rates (Cars per 1000 persons) China India Pakistan Indonesia Egypt Thailand Brazil Malaysia South Korea Japan U.K. Australia Germany USA

3 4 6 10 19 22 76 113 114 342 248 459 459 504


1.5.1

References

1. 2. 3.

Kadiyali, L.R., 2006. Principles and Practices of Highway Engineering (including th Expressways and Airport Engineering), 4 Edition. Khanna Publishers, New Delhi. Mathew, T.V. and Rao K.V.K., 2007. Introduction to Transportation Transport Engineering. O’Flaherty C.A., 2002. Highways: The Location, Design, Construction and th Maintenance of Pavements. 4 Edition, Oxford, Butterworth Heinemann.

6


6

Planning and Layout of Roads

2.0

Planning and Layout of Roads

2.1

Introduction

7

Transport is an important infrastructure for development. It occupies a pivotal position in the growth of developing countries. Planning for economic development is now an accepted tool widely followed in most of the countries. So far there is very little evidence of a scientific approach in planning at the national level in the transport sector. However, the outlays and targets are normally adopted after a careful study of the existing facilities, their deficiencies and immediate needs. Very often it has been experienced that investment decisions are taken after a bottleneck situation develops. The transport plan should be integrated in the countries overall economic plan since transport in its own sake has no meaning. It assumes importance only in as far as its serves the ultimate goal of development i.e. transport plans must translate overall development objectives and potentials into transport requirements [Kadiyali, 2006].

2.2

Goals and Objectives

The goals and objectives of the transport plan should be clearly identified and expressed. This alone will facilitate the formulation of a realistic plan. The following points give general guidance in this regard: a) The transport plan should not conflict with the broad goals and objectives of the national plan for development. It should help in translating the goals and objectives of the national development plan. b) The transport plan should aim at coordinated development of all modes of transport without prompting unhealthy competition. c) The transport plan should aim at conserving scarce resources such as oil fuels, coal and electricity. d) The transport plan should generate employment potential and should favour labourintensive technologies to the extent feasible and desirable. e) The transport plan should aim at a balanced development of the country, keeping in view the special needs of inaccessible areas and backward classes of society. f) The transport plan should aim at a balanced development of rural and urban settlements. While urbanisation is an inevitable result of and a pre-requisite for economic development, growth of cities beyond manageable limits leads to undesirable effects. Transport should be used as a tool for dispersal of activities to result in overall health of the economy. g) Transport plans should recognise the need to exploit the natural resources of the country and provide for quick exports to earn valuable foreign exchange to developing countries h) Transport plans should facilitate the growth of new industries, agricultural production and processing of raw materials. Functional linkages between industry and hinterland should be established. i) Environmental impact of transport plans should be established.


7

7

The Project Cycle 2.3

The Project Cycle

2.3.1

Components of the Project Cycle

8

Projects are planned and carried out following a sequence of activities, often known as the ‘project cycle’. There are many ways of defining the s teps in this sequence but the following terminology will be used here: 1) Problem identification 2) Pre-feasibility 3) Feasibility 4) Design 5) Procurement and negotiation 6) Implementation 7) Operation 8) Monitoring and evaluation. The first three steps (1-3) make up the planning phases of the project cycle, though evaluation (step 8) may also be considered integral to the planning process by providing feedback on the wisdom and processes of past decisions. Figure 2.1 provides an outline of the stages of the project cycle. The planning phases of the cycle involve a gradual process of screening and refining alternative options (for resolving an earlier identified problem). In this process there are clear decision points (at the end of each stage) when potential projects are either rejected or taken forward for further and more detailed analysis. Dubious projects should be rejected at an early planning stage (and before feasibility) as they gain a ‘momentum of their own’, and hence become increasingly difficult to stop at the later stages in the cycle when minor changes of detail are often all that are possible. Within each of the planning phases (project identification, pre-feasibility and feasibility), the same basic process of analysis is adopted. Differences occur largely in the level of detail applied. Sometimes phases are merged, with pre-feasibility becoming an extension of the project identification, or a first step in the feasibility stage [TRL, 2005].

2.3.2

Problem Identification

The first stage of the cycle is to find potential projects. General planning identifies key transport constraints and sketches solutions at a global or macro level, and should prioritize these as to the need and urgency for resolution. The planning process takes into account government policies and programmes (in all relevant sectors) which impact on transport development. The need for general road development is therefore examined in a very wide socio-economic and policy-orientated context. The framework for general planning could be cross-sectoral in nature or it could also be focused specifically on transport issues. In all cases, however, the scope is ‘macro’ in nature, taking in a complete region or city. Examples of such spatial (or structure) plans and transportation studies include: a) A national or regional development study (e.g. regional spatial plans) b) An urban development study (or master plan) c) A national or regional transport study (sometimes known as a multi-modal or intermodal transport study) d) An urban land-use/transportation study e) An integrated rural accessibility plan f) A road safety strategic plan


8

8

Pre-feasibility 2.3.3

9

Pre-feasibility

At the start of the pre-feasibility stage there is a clearly defined transport problem (identified in general planning), but no strong evidence that this problem could be solved by road improvement, or any other transport solution (e.g. improvements to transport services) in an environmentally or economically acceptable manner. By the end of the pre-feasibility stage, there will be clear evidence whether or not a road improvement project is worthwhile. If it is, the pre-feasibility will normally identify what type of project would be suitable, checks that the project is not premature and provides the information needed to commission a feasibility study. Typically, this phase might identify ‘corridors’ that require a new road. An affirmative pre-feasibility study will also trigger the inclusion of a ‘line-item’ in the long-term road preparation budget (of the ministry or its highway agency). It gives advance warning that monies will need to be budgeted for the future implementation of this particular project. The pre-feasibility study may indicate that the proposed road improvement project would not be effective in solving the problem, or should be reconsidered later, perhaps when there is more traffic). In that case the process should be terminated or shelved without incurring the high cost of a feasibility study.

2.3.4

Feasibility

The feasibility study finds the most suitable road improvement project for solving or helping to solve an identified transport problem. At the start of the study there is a clearly defined problem with an expectation that the problem can be solved by some form of road improvement, in a manner that is environmentally, socially and economically acceptable. This expectation is backed up by the evidence needed to justify the considerable cost of carrying out a feasibility study (identified in a pre-feasibility study). The level of detail of this study will depend on the complexity of the project and how much is already known about the proposal. By the end of the study there should be a clear recommendation for a specific road improvement project. The study will provide evidence that this particular project should be carried out and that this project provides the most suitable solution to the problem, taking into account its operational benefits and its environmental and economic implications. It will also provide a detailed description and a preliminary engineering design (PED) and associated drawings of the proposed project to enable costs to be determined at a level of detail to enable funding decisions to be made. The feasibility study will also provide an input to the road preparation budget process, giving greater detail (than earlier phases) of costs that will be incurred and project timings.

2.3.5

Design

The final engineering design (FED) is often very costly (up to 15 per cent of project costs) and usually follows provisional commitment to the project. Numerous decisions which will affect economic performance are taken throughout design; and economic appraisal often results in redesign. In this stage, working drawings and bills of quantities are normally prepared.

2.3.6

9

Commitment and negotiation

Commitment of funds often takes place in a series of stages. This is followed by invitations to tender and negotiations with contractors, potential financiers and suppliers. At this stage, there are still considerable uncertainties. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

9

Implementation 2.3.7

10

Implementation

Several aspects of the earlier stages in the project cycle will affect the success of the implementation. The better and more realistic the plan, the more likely it is that the plan can actually be carried out and the full benefits be realised. A flexible implementation plan should also be sought. It is almost inevitable that some circumstances will change during the implementation. Technical changes may be required as more detailed soils information becomes available or as the relative prices of construction materials change. Project managers may need to change and re-plan parts of the project to take account of such variations. The more innovative and original the project is the greater is the likelihood that changes will have to be made during implementation.

2.3.8

Operation

This refers to the actual use of the road by traffic; it is during this phase that benefits are realised and maintenance is undertaken.

2.3.9

Monitoring and Evaluation

The final phase of the project cycle is evaluation. This consists of looking back systematically at the successful and unsuccessful elements of the project experience to learn how planning can be improved in the future. For evaluation to be successful, it is important that data about the project is collected and recorded in a systematic way throughout all stages of the project cycle. Without this, it is usually impossible to determine details of events and information that were available during periods leading up to the taking of important decisions. Evaluation may be carried out by many different people. The sponsoring organisation or external agency may undertake evaluation. In large and innovative projects, a separate unit may be needed to monitor each stage of the project by collecting data for identifying problems that need to be brought to the attention of the project's management. In some cases, outside staff will be used to provide an independent audit and specialist university staff may well be suited to undertake such a task. The evaluation should result in specific recommendations about improving aspects of the project design which can be used to improve ongoing and future planning.

2.4

Overview of Road Appraisal in Developing Countries

Feasibility studies of road schemes in developing countries are undertaken along the following steps: 1) Define objectives 2) Determine alternative ways of meeting objectives 3) Make preliminary considerations 4) Asses traffic demand 5) Design and cost different options 6) Determine benefits of each alternative 7) Economic analysis and comparison of alternatives 8) Recommendations The steps are not necessarily sequential and involve iteration. The above steps will now be discussed:


10

10

Define Objectives 2.4.1

11

Define Objectives

A road project is wherever possible set against the background of a national or regional transport plan or at least a road plan. Definition of project objectives provides the basic framework for carrying out feasibility studies. The objective of providing a new road could be any of the following: a) b) c) d)

To support some other developmental activity; To provide fundamental links in the national or a district road network; To meet a strategic need; To increase the structural capacity or traffickability of an existing road to cope with higher traffic flows; e) To provide an alternative to an existing transport link or service; f) To address a major safety hazard, environmental or social problem; g) To rectify damage or failure that has caused sudden deterioration of the existing road.

Depending on the objectives of the investment, the project is appraised against different sets of criteria. Development Banks like World Bank and the Asian Development Bank are increasingly getting involved in strategic planning of road networks in developing countries. This calls for alignment of a country’s Transport Plan with a Development Bank’s country strategy. 2.4.2

Determining alternative ways of meeting Objectives

This may involve making a modal choice say between rail, road, air and water transport to solve a transport problem or deciding between different technical solutions to highway problems. These technical solutions include: a) Upgrading and new construction – Upgrading projects aim at providing addition capacity for a road towards the end of its design life or because of a change in route function. Examples are paving of gravel roads and providing overlays on paved roads; b) Reconstruction and rehabilitation - Major repair on an existing road; c) Stage construction – Planned improvements are made to the pavement standards of a road at fixed stages through the project life. Although stage construction may be appropriate in achieving an optimal economic balance, practice has shown that budgetary constraints have often prevented later upgrading phases of stage construction projects leading to lower rates of return. d) Maintenance projects – These consist of either building up the institutional capability of the maintenance organisation to improve its efficiency or overcoming a short term problem through project specific interventions like surface dressing, supply of maintenance equipment and technical assistance. The later type of project could be a component of the former. Community involvement in the early stages of development of projects in developing countries is now recognised as fundamental for project success because of the local wealth of knowledge possessed by the community concerning the solution to a problem in the context of an area’s physical and socio economic constraints.


11

11

Preliminary considerations 2.4.3

12

Preliminary considerations

The underlying issues are taken into account during the feasibility study: a) Analysis period and design life – Most road projects are analysed on a 15 year time horizon. The analysis period may be partly dictated by the nature of the investigation. For example, long periods are useful when comparing mutually exclusive projects, whereas short periods may be appropriate for small projects (such as regravelling of rural access roads), where the life of the investment is expected to be limited to a few years. b) Uncertainty and risk – Projects in developing countries are always set against a background of economic, social and political uncertainty to some degree. The steps taken to reduce uncertainty include risk analysis using probabilistic techniques for well defined projects and scenario analysis in explanatory projects. c) Choice of technology – According to the Transport and Road Research Laboratory (TRRL, 1998), engineers have to decide between mechanised and labour based techniques in preparing designs and specifications of works. d) Institutional issues – The major institutional issues to be considered include: • The institutional framework in which the roads are set including the aspects of organising, staffing, training, procedures, planning, maintenance, funding and controls. • Strengthening the institutions responsible for implementing the project; and • The funding and maintenance capability of road maintenance organisations. • Legislative requirements of the study (if applicable) e) Socio-economic considerations – The major issues that are assessed in terms of the impact of the project on the target community are social changes, construction consequences, road accidents, severance, minorities like gender issues and availability of local expertise and resources. f) Environmental Conditions – The impact of the road project on the surrounding environment is taken into consideration. The impact is more significant for new projects penetrating an undisturbed country tan for upgrading projects because the latter usually follow an existing alignment.

2.4.4

Assess Traffic Demand

For the purpose of geometric design and evaluation of economic benefits, the volume and composition of current and future traffic needs to be known. For structural design purposes of paved roads, the axle loading of only heavy goods vehicles is relevant thus for this purpose traffic appraisal considers volumes of Heavy Goods Vehicles (HGVs). The Road Maintenance Initiative (RMI) (World Bank, 1998) observes that far too few countries in Africa have permanent road data banks, locally managed and regularly updated, based on objective technical data. 12 2.4.5

Design and Cost different Options

Cost estimates should encompass analytical techniques and rigorous procedures of risk management to produce realistic estimates. The major activities undertaken in this step include: Route location, pavement design, geometric design and design of drainage Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

12

Determine Benefits of each Alternative

13

structures. In this stage an optimal balance between cost of provision and user cost is important.

2.4.6

Determine Benefits of each Alternative

Estimates are made of both the costs associated with the project and the benefits expected to occur. The benefits normally considered are: a) b) c) d) e)

2.4.7

Direct savings in the cost of operating vehicles Economies in road maintenance Time savings by travellers and freight Reduction in road accidents Wider effects on the economic development of the region

Economic Analysis and comparison of alternatives

The best option representing the option with the minimum level of maintenance is carefully chosen and used as a basis against which other options are compared. A cost benefit analysis procedure is then used to assess the net contribution the road investment makes to the country as a whole. The cost benefit analysis uses either Net Present Value (NPV) or Internal Rate of Return (IRR) rules. A positive NPV means a project is justified at the given discount rate. Results of financial, social and environmental appraisals are also considered in deciding the best project. The IRR acts as a guide to the profitability of the investment but gives no indication of the costs or benefits of the project. A difficult approach is normally required for rural access projects so that the cost of the appraisal is justified in terms of project costs. All investment decisions have political, social and environmental consequences besides economic effects. According to TRRL (1998), in planning main road investment, economic/engineering implications are usually paramount in the decisions to upgrade existing road surfaces. Foster (2000) observes that the financial aspects of the project appraisal receive more systematic treatment than non-financial aspects.

2.4.8

Recommendations

The feasibility study report marks the end of the appraisal process and recommends whether the project should go ahead and the standards to which it should be built. The depth and detail to which the report covers certain aspects depends on who the report is being made for. An analysis carried out for a development bank covers financial aspects very thoroughly. Projects prepared for aid agencies normally dwell heavily on the socioeconomic factors.

2.5

A Typical Road Project Appraisal Process in Uganda

This section will be based upon the process that was followed for the feasibility study of the Kampala-Fort Portal Road. 13


13


1. Objective

14

2. Problem Identification

3. Determine Alternatives

4. Project Strategy

5. Engineering, Economic and Environmental analyses

6. Draft Recommendation for preferred solution

7. Review by Ministry of Works, Housing and Communications

8. Finalisation of Recommendation

9. Submission for Funding

10. Detailed Design, Tender and Construction

11. Post Implementation Review

Figure 2.1: Typical Road Project Appraisal in Uganda

Source: MoWH&C, 1998

The process shown in the figure above will now be described: Step 1: Objectives

The study objectives were derived from two major sources namely: a) The 10 year road sector development programme (RSDP); b) The strategy related to the Trans-African Highway.


14

14


15

Step 2: Problem identification

Past feasibility studies from 1972 to 1995 were used as a basis for establishing the existing problems on the Kampala to Fortportal road. Step 3: Determine alternatives

The pre-appraisal study by GIBB consultants on behalf of Danida brought together information from steps 1 and 2 above and challenged the assumptions made in previous studies. Arising out of this study were five options for further evaluation. Step 4: Project strategy

Arising out of the results the pre-appraisal study in step 3 above, a draft project strategy was prepared consisting of a two stage construction strategy. Step 5: Engineering, Economic and Environmental analysis

Danida as the financier commissioned COWI-DRD to carry out further engineering, economic and environmental analysis on the project strategy and compare different upgrading options under the strategy with the existing route under optimal and prevailing maintenance respectively over 16 study sections. Traffic studies were part of the economic evaluation. Step 6: Draft recommendations on preferred solution

Resulting from the analyses in step 5, recommendations were made on the feasibility of options along an environmentally preferred route alignment in terms of Economic Internal Rate of Return (EIRR). Step 7: Review by Ministry of Works, Housing and Communications

In Uganda, step 1 to 6 usually lead to the production of a draft detailed engineering report three (3) months from the start of the study. The report is reviewed by the Ministry of Works, Housing and Communications on behalf of Government as the client leading to comments that are taken into account in preparing the final detailed engineering report (Ministry of Works, Housing and Communications, Gauff Ingenieure, 1993). Step 8: Finalising recommendations

Adjustments are made to the draft report in accordance with the recommendations of the client. The consultant then concludes the final report 30days from the receipt of information from the (MoWH&C and Gauff Ingenieure, 1993). Step 9: Submission for funding

On conclusion of recommendations, the Ministry of Works, Housing and Communications would submit the feasibility study report to the financier as was the case in the 1993 study by Scott Wilson Kirkpatrick. The consultants would then submit the reports to the financier as their employer. 15 Step 10: Detailed design, Tender and Construction

If the financier approved the study, funds would be released for detailed design, tender and construction of the road. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

15

Economic Evaluation of Highway Projects

16

Step 11: Post implementation review

External financiers like the World Bank usually evaluate a project when it is handed over to the client to assess success and compliance with objectives. Such reviews provide valuable lessons as inputs into subsequent projects to improve on project success. Interestingly, the study period for the Kampala-Fort portal road took 26years (1972-1998). Yet the process would ordinarily take three years.

2.6

Economic Evaluation of Highway Projects

2.6.1

Role of Economic Evaluation

A developing country like Uganda has serious shortages of resources needed for economic development. The outlay for various sectors of economic activity is decided by planning at the national level, keeping in view the national goals and policies. Within the allocation earmarked for the highway sector, a number of schemes can be taken up, each enjoying its own urgency and attractiveness. It thus becomes necessary to screen and evaluate the various alternatives so that a wise decision can be reached on the most appropriate choice. This is achieved by modern techniques of economic evaluation of projects [Kadiyali, 2006]. Economic evaluation is a rational approach at quantifying the future benefits and costs of proposed highway improvements with a view to determine the extent to which the projects will contribute to the goal of raising the living standard of the people and their general welfare. It provides for a systematic and unbiased procedure for selection of schemes for implementation under the Ten Year Road Sector Development Plans. It ensures that the most worthwhile projects are given the highest priority. Economic evaluation of highway projects can also be carried out to weigh other alternative transport projects, such as railway projects, pipe-lines or inland water transport projects, in order to select the most beneficial scheme. The following are some of the specific objectives in carrying out an economic evaluation: 1. To decide whether the scheme under consideration is worth investment at all; 2. To rank schemes competing for scarce resources in order of priority; 3. To compare various alternative schemes and select the one most economical; 4. To assist in phasing the programme (stage construction) depending upon the availability of resources. 2.6.2

Some Basic Principles

Economic evaluation involves a number of basic principles discussed below: a) Economic evaluation makes it possible to choose the best of the various alternatives. The question before the analyst is to suggest the most attractive of them. Often the choice is between ‘do-nothing’, and other improvement schemes. b) In economic evaluation, all past actions are irrelevant. What is of prime importance is the future flow of costs and benefits. c) In highway projects, the appraisal is carried out from the view-point of the nation as a whole, and is not restricted to any sub-set like the highway agency, truckers, private motorists and bus operators. d) Economic analysis should not be misunderstood with financial analysis. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

16

16

Time Value for Money

17

e) Economic evaluation should take place within a set of established criteria such as minimum attractive rate of return, interest rate etc. f) Opportunity cost of capital and resources should be considered wherever they are important. g) The period of analysis need not be too long in view of the uncertainties associated with the future traffic and benefits. In any case, the discounted cash flows of a distant future period are insignificant. For highway projects, it is enough if the analysis covers a period 15-25 years after opening to traffic.

2.6.3

Time Value for Money

The fundamental premise on which all methods of economic evaluation rests is that money earns income over a period of time. For example, US$ 100 today will be worth US$ 672.75 at the end of 20 years if invested at 10 per cent compound rate of interest. So also, a sum of US$ 672.75 which might become due to an individual after 20years from today is worth only US$ 100 at the present, assuming the same rate of interest. These facts point to the need for devaluing the future benefits and costs to the present time to determine their present worth. The process of calculating the present worth of a future payment is known as ‘discounting’ and the interest rate used is called the ‘discount’ rate. The following formulae are very useful in dealing with the problems in economic evaluation: a) The amount A to which US$ 1 will increase in n years with a compound interest rate of r will be given by;

  1

….2.1

b) The present value P of US$ 1, n years therefore when discounted at a rate r will be given by;

1    1   1

2.6.4

….2.2

Costs and Benefits

In economic evaluation, the main objective is to compare the costs and benefits of various alternative schemes and select the one, most advantageous. The first step is, therefore to determine the costs and benefits. There is a great deal of confusion in the designation of what constitutes ‘costs’ and what constitutes ‘benefits’. The simplest description is that the negative effects of a scheme constitute the costs. They indicate the cash out-flows. On the other hand, the positive effects are called benefits and they represent cash in-flows. As long as sufficient care is taken to see that the signs are assigned properly, it is immaterial whether the economic consequence is labelled as ‘costs’ or ‘benefits’. Costs and benefits can be traced to the provider of the facility (highway department), the highway users and non-users. In economic analysis, since all consequences are to be considered, the costs and benefits to all parties are to be reckoned. Some consequences can be quantified into monetary terms whereas some cannot. The aim of the analyst should be to quantify as many elements as can be monetarily quantified. Those which cannot be ultimately quantified into monetary terms are kept separately apart and a judgement value can be accorded to them before a final decision is taken.


17

17

Costs and Benefits

18

The economic evaluation of highway projects is generally done by computing the total transport cost which consists of the following components: a) Cost of construction of the facility b) Cost of maintenance of the facility c) Road user cost d) Cost to the society The Government, which is often the agency providing the facility, incurs expenditure on constructing a road. This includes land acquisition, earthwork, road pavement and structures. The government also invests money on maintenance and up keep annually. The road user cost, which is borne by the actual user of the highway facility (passenger, crew of vehicles, operator, consignor of goods, pedestrian, cyclist etc.) is composed of: a) Vehicle operating costs i) Fuel; ii) Lubricants; iii) Tyre; iv) Spare parts; v) Maintenance labour; vi) Depreciation; vii) Crew costs; and viii) Fixed costs such as: • Interest on capital • Insurance • Taxes • Registration fee • Grading charges • Fines, tolls, etc • Permit charges • Loading and unloading charges • Commission on booking • Overhead charges such as rent, salary, electricity, postal, telephone, stationery b) Travel Time Cost i) Time value of vehicle occupants ii) Time value of goods in transit iii) Time value of vehicles in transit c) Accident Costs i) Cost of fatality ii) Cost of injuries iii) Cost of damages to property d) Cost to Society i) Impact on the environment (noise pollution, air pollution, vibration). ii) Loss of aesthetics iii) Changes in land values iv) Land severance v) Discomfort and inconvenience.


18

18

Costs and Benefits

19

Benefits from highway projects in effect represent the difference in costs with the new facility and the old facility. Benefits can be grouped under the following: a) Benefits to the existing traffic, by way of reduced road user costs. b) Benefits to the generated traffic c) Benefits to traffic diverted from other routes and modes d) Benefits to traffic operating on other routes and modes where reduction in traffic has been caused by the opening of the facility. Vehicle operating costs are affected by a number of factors such as: a)

i) ii) iii) iv) v) vi) vii) viii)

Vehicle Factors Age Make Horse-power, engine capacity Load carried Condition of Vehicle Level of maintenance input Type of fuel used Type of tyres (rayon, nylon, radial ply, cross ply etc.)

i) ii) iii) iv) v) vi) vii) viii)

Roadway Factors Roughness of the surface Type of the surface Horizontal curvature Vertical profile Pavement width Type and condition of shoulder Urban and rural location Number of junctions per km

b)

c)

i) ii)

Traffic Factors Speed of travel Traffic volume and composition.

i) ii) iii)

Environmental Factors Altitude Rainfall Temperature

d)

Research has shown that the vehicle operating cost components are closely governed by (i) roadway factors such as roughness, pavement width, rise and fall and horizontal curvature, (ii) vehicle factors such as age and load carried and (iii) traffic factors such as speed and volume of traffic. It follows therefore, that good roads result in lower vehicle operating costs. Highway improvements result in speedier travel. Savings in travel time are enjoyed by occupants of vehicles, goods in transit and the vehicles in transit Road accident rates are governed to a certain extent by the condition of the road. Highway improvements can thus bring about a reduction in road accidents. The cost of road accidents, which have been eliminated by highway improvements, represents a benefit.


19

19

Evaluation Techniques

20

When carrying out economic analysis, costs and benefits are considered exclusive of taxes. Taxes do not represent an economic cost and represent only a transfer within the community. Insurance premiums are also excluded from economic analysis since the savings in accidents already account for this element. In a developing country, there are certain resources which are scarcer than the others. The prevailing market prices, therefore, do not reflect the true economic value of the resources. In order to correct such distortions and imperfections, ‘shadow pricing’ is done. A case in example is the cost of imported fuel in Uganda. Since foreign exchange reserves are very precious, such imported items are shadow priced at a higher value than the market price when carrying out the economic evaluation. Similarly, unskilled labour is surplus in Uganda and the prevailing wage rate (which is statutorily fixed) may not truly reflect this situation. A shadow-pricing of such labour at a slightly lower level would be appropriate. Inflation is disregarded in economic analysis, as it is generally assumed that all prices increase in the same proportion, but relative prices remain constant. But if differential inflation is expected to occur among commodities, necessary adjustments need to be made.

2.6.5


The methods commonly adopted for economic evaluation are: a) Net present value (NPV); b) Benefit/cost Ratio (B/C Ratio); c) Internal Rate of Return (IRR); d) First Year Rate of Return (FYRR). a) Net present value (NPV) Method The NPV method is based on the discounted cash flow (DCF) technique. In this method, the stream of costs and benefits associated with the project over its time horizon is calculated and is discounted at a selected discount rate to give the present value. Benefits are treated as positive and costs are treated as negative. Any project with a positive NPV is treated as acceptable. In comparing more than one project, a project with the highest NPV is selected.

The NPV is algebraically expressed as:



     1  

Where; NPV0 Bi Ci r n

= = = = =

….2.3

Net Present Value in the year 0; Value of benefits which occur in the year i; Value of costs which occur in the year i; Discount rate per annum; Number of years considered for analysis.

b) Benefit-cost (B/C) Ratio Method There are a number of variations of this method, but a simple procedure is to discount all costs and benefits to their present worth and calculate the ratio of the benefits to costs. Negative flows are considered costs, and positive flows as benefits. Thus the savings in the transport costs are considered as benefits. If the B/C ratio is more than one, the project is worth undertaking. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

20

20


   1          1 

21

….2.4

Where C is the total cost of the project In the AASHTO practice of road –user analysis the B/C ratio expresses the ratio of the net annual benefits to the net annual costs. The benefits are determined for a simple reference year, which for convenience can be the first year of operation after construction or the median year of the analysis period [Kadiyali, 2006].

c) Internal Rate of Return (IRR) Method The internal rate of return is the discount rate which makes the discounted future benefits equal to the initial outlay. In other words, it is the discount rate at which the present values of costs and benefits are equal i.e. NPV = 0. Calculation of the IRR is not as straight forward as for NPV and is found by solving the following equation for r;



    0  1 

….2.5



Solutions are normally found graphically or by iteration. However, with a computer program, the work is rendered simple. The IRR gives no indication of the sizes of the costs or the benefits of a project, but acts as a guide to the profitably of the investment [Thagesen, 1996]. If the internal rate of return calculated from the above formula is greater than the rate of interest obtained by investing the capital in the open market, the scheme is considered acceptable. d) First Year Rate of Return (FYRR) Method The FYRR is simply the present value of the total costs expressed as a percentage of the sum of benefits in the first year of trafficking after project completion. Thus FYRR is given by;

      100 ….2.5 ,%     1      FYRR = 100

B j

j −1

∑ C (1 + r )

.... (2.6) j −1

i

j = 0

Where j is the first year of benefits, with j = 0 in the base year, and other notation is as before. If the FYRR is greater than the planning discount rate, then the project is timely and should go ahead. If it is less than the discount rate, but the NPV is positive, the start of the project should be deferred and further rates of return should be calculated to define the optimum starting date.


21

21

Comparison of the Various Methods of Economic Evaluation

22

It is should be noted that the results of the cost-benefit analysis are no better than the assumptions and input data on which it is based. The data and parameters used in the analysis of a road project can be prone to substantial errors and it is important to recognise that these exist and to take steps to minimise them [Thagesen, 1996].

2.6.6

Comparison of the Various Methods of Economic Evaluation

The three methods of economic evaluation described above have their own advantages and short comings. The B/C ratio method is very widely used by the highway engineers. It, however, suffers from the following drawbacks: a) It requires an assumption of a discount rate, which should bear relation to the opportunity cost of capital. It is however, rather difficult to know the opportunity cost of capital accurately. b) The significance of the B/C ratio is ambiguous, and its relative value is difficult to understand and interpret. For instance, if there are two proposals, one with a B/C ratio of 1.05 and the other with a ratio of 1.10, the difference is very difficult to appreciate. c) It is somewhat confusing and difficult to decide which items should be termed as costs and placed in the denominator and which as benefits and placed in the numerator. The IRR method is popular with international lending agencies like the World Bank. It l ends itself admiringly well for use in a computer-aided design model. It avoids the need for selecting a discount rate initially. The rate derived from computations can be easily compared with the market rate of interest, with which economists, financial experts and bankers are familiar. Its disadvantage is that the computations are tedious and a solution can only be obtained only by trial and error. The NPV method suffers from the same disadvantage as in case of B/C ratio method in that a rate of discount has to be assumed.

2.6.7

Selection of the Discount Rate

As seen from the discussions above, the selection of an appropriate discount rate (or interest rate) is crucial in the B/C ratio and NPV methods. The choice of the discount rate is governed by a number of complex factors, and is dependent on the future availability of finance and the various opportunities for its use. The attitude of the society towards present consumption as against savings for future is an important factor. Will the present generation prefer to consume the resources now or conserve it for future use by the current or future generation? The answer to this question will give the ‘social time preference rate of interest’. Another approach is to find out the social yield that the resources employed by a marginal public project would have otherwise generated. This determines the ‘social opportunity cost rate of interest’. In a truly competitive economy, the two rates of interest would be equal and investments and consumption would then be ideally allocated. But such a situation is difficult to find, and more so in a developing country where capital is very scarce. In such situations, some general guidelines can be given for selecting an appropriate discount rate. Such a rate should not be less than the rate of borrowing or lending by the government or the market rate of interest. A rate of 12 per cent is generally being adopted.


22

22

Selection of the Discount Rate

23

Example An existing single lane road, 30 km long, is to be widened to two lanes. The cost of widening is US$ 10,000 per km. The vehicle operating costs, accident costs and maintenance costs, with and without widening, for a 10 year period are tabulated in Table 2.1. The discount rate is 12 per cent. Is the project worthwhile? Compare the results of NPV, B/C ratio and IRR methods. Solution Cost of project

=

US$ 10,000 x 30 = US$ 300,000

Table 2.1: Computation of NPV, B/C Ratio and IRR Computation of NPV, B/C Ratio andIRR (All Values in Thousands of US$) DiscountedRate for NPV & B/C Ratio, r:

12%

Total Cost of the Project, Co (in US$1000) Year

RoadUser Costs With Impr. Without Impr.

300.000 Accident Costs Maintenance Costs With Impr. Without Impr. With Impr. Without Impr.

(By Trial andError) approx, zero

0.0000087571284

DiscountedRate for IRR

17.843439000%

Benefits

DiscountedBenefits DiscountedBenefits (NPV, B/C) (IRR)

1

101.5

160.7

2.5

3.6

10

7.5

57.8

51.607

49.048

2

105.6

168.2

2.6

3.7

10

7.5

61.2

48.788

44.070

3

110.2

176.3

2.7

3.8

10

7.5

64.7

46.052

39.536

4

116.2

185.2

2.8

3.9

10

7.5

67.6

42.961

35.053

5

122.3

190

2.9

4

10

7.5

66.3

37.620

29.173

6

128.4

199

2.9

4

10

7.5

69.2

35.059

25.839

7

135.6

210

3

4.1

10

7.5

73

33.021

23.130

8

143.2

219.5

3.1

4.2

10

7.5

74.9

30.251

20.139

9

149.1

228.2

3.2

4.3

10

7.5

77.7

28.019

17.729

10

154.6

240.1

3.2

4.3

10

7.5

84.1

27.078

16.283

380.458

300.000

Total Project is Economically Justified

NPVo

Project is Economically Justified Project is Economically Justified

B/C Ratio IRR

80.458 1.268 17.84%

Example The Ministry of Works and Transport (MoW&T) has proposed an upgrade of the Kampala-Jinja road to a dual carriageway and to improve some of its junctions. The time for construction of the scheme has been set at two years, with the benefits of the s cheme accruing to the road users at the start of the third year. The three main benefits considered are time savings, accident cost savings and vehicle operating cost reduction. Construction costs are incurred mainly during the two years of construction, but ongoing annual maintenance costs must be allowed for throughout the economic life of the project which is expected to be 10years after the road has been commissioned. The following basic data has already been ascertained by experts in highway economics for this analysis:

Accident Rates:

0.85 per million vehicle-kilometres (Existing road) 0.25 per million vehicle-kilometres (Upgraded road)

Average Accident Cost:

US $10,000

Average vehicle time savings:

US $2.00 per hour

Average Vehicle Speeds:

40km/hr (Existing Road) 85km/hr (Upgraded Road)

Average Vehicle Operating Cost:

0.012

23

  0.00005 in US $ per km

V is the average vehicle speed


23

Selection of the Discount Rate Discount rate:

6%

Predicted Flow in Year 3, F:

250 mil.veh-km/yr

24

The traffic flows and the construction/maintenance costs for the highway proposal are shown in the table below Traffic flows and costs throughout the economic life of the highway proposal Year

1 2 3 4 5 6 7 8 9 10 11 12

Predicted Flow (106 veh-km/yr) 250 260 270 280 290 300 310 320 330 340

Construction Costs (in US $) 150,000,000 10,000,000 -

Operating Cost (in US $) 500,000 500,000 500,000 500,000 500,000 500,000 500,000 500,000 500,000 500,000

As a consultant engineer to the Ministry of Works & Transport (MoW&T), you have been assigned the task of ascertaining whether the project is economically justified, or not, using both the NPV and B/C ratio techniques of economic evaluation. Briefly comment on your results.

Solution

1.0 Data Summary

a) Accident rates: b) c) d) e) f)

R e R u Average accident cost: Ca Average vehicle savings: St Average vehicle speeds: Ve Vu Average vehicle operating cost: C o Discount rate, r

= = = = = = = =

0.85/mil.veh-km (Existing road) 0.25/mil.veh-km (Upgraded road) US$10,000 US$2.00/hr 40km/hr (Existing road) 85km/hr (Upgraded road) 0.01[2+35/V+0.00005V 2] 6%

2.0 Re quired Computations

Total Benefit, B = Ba + Bt + Bo Where the above terms are defined as below for the 3 rd year, F being the predicted flow; Accident Savings;

   ..  0.850.2510000250  $ 1,500,000/   0.0135 1 10.00005 .  0.0135140  1850.000054085  25010  $ 454,963/

Operating Cost Savings;

Time Savings; Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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24

Selection of Routes

25

  1 1..  140  1852.0025010  $ 6,617,647/ Therefore the total benefit is given by;

  15000004549636617647  $ 8,572,610/    8572610  $ 7,197,729   1  10.06 3.0 Computation of discounted benefits and costs

Year

1 2 3 4 5 6 7 8 9 10 11 12

Flow A ccident Cos t F Savings mil.veh-km/yr US$/yr

250 260 270 280 290 300 310 320 330 340

1,500,000 1,560,000 1,620,000 1,680,000 1,740,000 1,800,000 1,860,000 1,920,000 1,980,000 2,040,000

Operating Cos t Savings US$/yr

454963 473162 491360 509559 527757 545956 564154 582353 600551 618750

BENEFITS Travel Time Savings US$/yr

6,617,647 6,882,353 7,147,059 7,411,765 7,676,471 7,941,176 8,205,882 8,470,588 8,735,294 9,000,000

Total Us er Benefits, B US$/yr

8,572,610 8,915,515 9,258,419 9,601,324 9,944,228 10,287,132 10,630,036 10,972,941 11,315,845 11,658,750 ∑PVB

COSTS Dis counted Cons truction & Dis counted Benefits (PVB) Maintenace Costs Cost, (PVC) US$/yr US$/yr US$/yr 15,000,000 14,150,943 10,000,000 8,899,964 7,197,729 500,000 419,810 7,061,923 500,000 396,047 6,918,429 500,000 373,629 6,768,555 500,000 352,480 6,613,480 500,000 332,529 6,454,274 500,000 313,706 6,291,902 500,000 295,949 6,127,233 500,000 279,197 5,961,046 500,000 263,394 5,794,042 500,000 248,485 65,188,613 PVB 26,326,133 ∑

4.0 Computation of NPV & B/C Ratio

     65,188,61326,326,133  $38,862,480 /   626,5,1388,26,611333 2.476 5.0 Concluding Remarks

All the above indicators point to the economic strength of the project under examination. Its NPV at just over US$ 38million is strongly positive, and its B/C ratio at just below 2.5 is well in excess of unity. A further computation of the IRR reveals a value of over 28% which is over four times the agreed discount rate of 6%. Together these indicators give strong justification for the upgrading of Kampala-Jinja road to a dual carriageway and improvement of its junctions.

2.7

Selection of Routes

2.7.1

Introduction

25

The location of a new or major road requires consideration of many complex and interrelated factors and brings together different professionals namely economists, geologists, planners, surveyors and road engineers. The process of defining the physical location of a new road must be preceded by the analysis of data on traffic volumes, planning Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Overview of the Location Process

26

intentions in the area to be traversed and preliminary estimates of the anticipated design of the new road. Route location consists of selecting the best compromise between demand factors and terrain factors. Demand factors determine the areas to be served by the new road standard and terrain factors influence the engineering cost. Terrain factors include: Ground conditions, materials for construction, earthworks, drainage both surface and subsurface and the need for structures. The choice of route is normally associated with the problem identification and feasibility stages of the project life cycle. Road locations are easier to determine through low cost relatively undeveloped lands than through well developed rural and urban areas.

2.7.2

Overview of the Location Process

Once the need for a new road has been justified by the transport planning process, the approach to the selection of an appropriate route location becomes a structured decision process. The first step requires the fixing of end termini and then defining a region which will determine all feasible routes between these two points. In a non-urban setting this region can be one third as wide as it is long. The region is then searched using reconnaissance techniques to obtain a limited number of broad bands within which further searches can be concentrated. Such a band can be up to 16km wide for a rural motorway. Within these bands, further reconnaissance searches result in the selection of say three narrower corridors each 3-8km wide that can be labelled A, B and C. A comparison of these may then suggest that C will provide the best route and then Route E is generated through it. In rural setting route E may be 1-1.5km wide. The next step is preliminary location where route E is searched and one or more feasible alignments is located within it each perhaps 30m wide containing minor design differences. These alignments are then compared during the final location phase of the analysis and the most suitable one is selected for further development in terms of design and construction. The above process is iterative in nature. Tangible considerations that might influence the selection process include topographic, soil and geological survey data, land usage and population distributions, travel demands and road user costs, construction and maintenance costs and safety factors. Intangible considerations of a political, social and environmental nature requiring extensive public consultation may need to be considered as well.

2.7.3

Location Surveys in Non-Built-Up Areas

The approach relies on three types of survey namely: reconnaissance, preliminary location and final location/design. a) Reconnaissance Survey The reconnaissance stage of the survey process takes place during the identification stage of the project where alternative possible routes are determined in terms of the corridors in which they lie. The first step in the reconnaissance survey is to carry out a major desk study of the bands/corridors being evaluated within the region. The types of information typically gathered for a desk study include:

a) General land survey – locating the site on maps. Dated air photographs, site boundaries, outlines of structures, meteorological information e.t.c.


26

26

Location Surveys in Non-Built-Up Areas

27

b) Permitted use and restrictions – Planning restrictions of an area according to planning legislation, local authority regulations and byelaws, ancient monuments, burial grounds, environmental restrictions.etc. c) Approaches and access – Checking road ownership, closed railway lines etc. d) Ground conditions – Geological maps, seismicity etc e) Sources of material for construction f) Drainage and sewerage – Authorities in charge, location of sewer lines, location of storm drains etc. g) Water supply – Authorities concerned location of existing lines etc. h) Electricity supply – Information on concerned authorities and existing lines i) Telecommunications – Information on concerned authorities and existing lines. Next, armed with questions from the desk study, the reconnaissance engineer visits the field to fill in omissions in information gathered from the desk s tudy and further limit the corridor under study into a more suitable terrain and provide further data useful for design. The reconnaissance study should be low key so as not to attract attention of local residents who may pre-empt the development of the project. On completion of the reconnaissance survey the engineer should have sufficient information which when combined with economic, environmental, planning, social and traffic inputs enable the selection of the feasible corridor routes. The renaissance report describes the preferred corridor routes; a state of criteria satisfied by the project, presents tentative project cost estimates, provides provisional geotechnical maps and shows characteristics of important engineering features. It also states special issues that may lead to design and construction problems.

b) Preliminary Location Survey This is the feasibility stage of the project where corridors are appraised to select the best route. It is a large scale study of one or more feasible routes within the corridor whose purpose is to collect all the physical information that may affect the location of the proposed road way. It results in a paper location that defines the line for the subsequent final location survey. Site investigations are carried out of alternative routes guided by terrain evaluation.

In the course of carrying out the preliminary survey, a ground survey, which is one of the approaches, the other being an aerial survey is taken by means of traversing and levelling to produce a strip map of the proposed corridor for the route showing the physical features along the route, locations of soft ground, locations of water bodies, power lines pipelines, houses monuments etc. These are converted into a topographic map that shows both horizontal and vertical data usually with the aid of contour lines that enable the road alignment to be defined in both horizontal and vertical planes. The survey area should be greater than the roadway width of the proposed route. The next step is the determination of the centreline of the proposed road. It should fit the topography while meeting the intended traffic service requirements. It is a trial and error process were trial centrelines are drawn on the strip map and are adjusted according to the skill and judgement of the engineer. Sketching can be by the method of arcs or the method of tangents. The process of sketching on paper should go hand in hand with field observations. Many considerations influence the choice of centreline finally selected. These include; a) Locating the road along property edges rather than through them in rural areas; b) Avoiding alignments that cause the motorist to drive into the rising or setting sun for long periods; Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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27

Road Location in Built up Areas

28

c) locating the road such that the users are able to view a prominent scenic feature; d) Minimising the destruction of manmade culture, cemeteries etc; e) Avoiding highly developed expensive land areas and seeking alignments that cause minimum environmental damage; f) Avoiding the locating bridges on or near curves; g) Minimising the use of alignments that require excavation of rock; h) Balancing excavation quantities with embankment quantities. i) If a vertical curve is superimposed on a horizontal curve, ensure the horizontal curve is longer or make the tangent/straight points coincident to those of the longer curve ; j) Avoiding the introduction of a sharp horizontal curve at the top of a pronounced crest curve or the low point of a pronounced sag vertical curve. For safety reasons, make horizontal and vertical curves as flat as possible at junctions with other roads; Finally cost comparisons are made of alternative alignments to assist in recommendation of the best route. c) Final Location Survey The final location survey involves fixing the final, permanent centreline of the road, while gathering additional physical data needed to prepare construction plans. The centreline that is pegged during the final location survey should closely follow the paper location of the preliminary survey map. Levels should be taken at regular intervals along the centreline. This should be extended say 175m beyond the start and end of the proposed scheme. Cross section levels should be taken at right angles on both sides of the centreline ensuring the width is greater than the proposed roadway width. The levelling data obtained in the final location survey are fundamental to the vertical alignment, earthworks and drainage designs. Main ground investigations for design are carried out during the final location survey. The subsurface investigations should provide borrow pit information. Benchmarks are also established during the final location survey.

2.7.4

Road Location in Built up Areas

It takes a longer time to establish a major road in a built up area than in an undeveloped area. The search for the line of a new road involves a combination of a reconnaissance preliminary survey (dominated by transport planning activities) and a final location survey. The reconnaissance-preliminary survey involves a transport planning investigation carried out in conjunction with a desk based physical site survey. The steps can be summarized as: i) ii) iii) iv) v)

Determine approximate traffic volume along a general corridor; Select road type, number of lanes to carry traffic load, level of service; Preliminary designs; Assign traffic to selected routes to determine design traffic volumes; Compare alternative locations using feasibility study criteria.

The final location survey is similar to the one described above for rural areas except it is more complex to carry out. Setting out may need more complicated offsetting and reference methods.


28

28

References 2.7.5

29

References

1. 2.

3.

4. 5. 6. 7.

Kadiyali, L.R., 2006. Principles and Practices of Highway Engineering (including th Expressways and Airport Engineering), 4 Edition. Khanna Publishers, New Delhi. Ministry of Foreign Affairs, Danida, 1998. Kampala to Fort-Portal Road Upgrading Project, Uganda: Design, Tendering and Supervision of Mityana-Mubende-Kyenjojo Section. Interim Engineering Report, COWI in Joint Venture with Road Directorate, Denmark Ministry of Transport. Ministry of Works, Housing and Communications and Gauff Ingenieur, 1993. Transport Rehabilitation Project – Upgrading, Regravelling and Rehabilitation of Roads. IDA Credit No. P593 – UG, Part 1, Volume I, Engineering Report, Detailed Engineering Study. Ministry of Works, Housing and Communications, 2004. Draft Road Design Manual. O’Flaherty C.A., 2002. Highways: The Location, Design, Construction and th Maintenance of Pavements. 4 Edition, Oxford, Butterworth Heinemann. Thagesen, B., 1996. Highway and Traffic Engineering in Developing Countries. 1st Edition. E & FN Spon Publishers, London, Uk. Transport and Road Research Laboratory, 2005. A Guide to Road Project Appraisal. Overseas Road Note 5. Crowthorne, England.

29


29

The Road User and the Vehicle

3.0

The Road User and the Vehicle

3.1

Introduction

30

A highway engineer is required to design road facilities which will be used by pedestrians, cyclists animal-drawn vehicles and a variety of motor-powered vehicles. The great variation in road user behaviour and vehicle characteristics necessitates an understanding of these variations as a precondition to highway design. Human factors that govern the behaviour of the driver, cyclist and the pedestrians have a considerable effect on the design elements. Knowledge of how this behaviour is influenced by various external conditions is useful in designing the road facility. The characteristics of the different types of vehicles will influence the geometric design elements of the highway and will determine the safety of traffic using the road [Kadiyali, 2006].

3.2

Human Factors Governing Road User Behaviour

3.2.1

Human Body as a complex System

The human body has a complex mechanism exhibiting varied reactions to external stimuli. When dealing with highway engineering design, human behaviour can be studied under the following groups: a) Physiological i) Vision; ii) Hearing; b) Psychological i) Perception; ii) Intellection; iii) Emotion; iv) Volition 3.2.2

Vision

Pedestrians, cyclists and drivers are able to use the road safely because of the help received by the eyes in seeing the road and traffic thereon and in evaluating the size, shape, colour, distance and speed of approach of various objects on the road. Safety of traffic depends upon the ability of the road users to see traffic lights, traffic signs, vehicles on the road, safe gap and safe crossing places. The drivers are able to cross, overtake, stop, accelerate and decelerate their vehicles on seeing the road conditions, the traffic conditions and the environmental conditions affecting safe traffic movement.

3.2.3

30

Hearing

For safe driving, cycling and walking, sound is an invaluable aid. Horns can alert the road user. Similarly the sound of a nearing vehicle or that of skidding may alert the road user and Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Perception, Intellection, Emotion and Volition

31

avert an accident. Efforts are being made to take measures for the control of noise pollution on roads.

3.2.4

Perception, Intellection, Emotion and Volition

The psychological traits of a road user are extremely important to highway engineers. The characteristics which are important are perception, intellection, emotion and volition abbreviated as PIEV. The time taken for these processes is known as PIEV time. Perception is the process of using the senses (e.g. seeing, hearing, feeling, smelling and the thinking) to acquire information about the surrounding environment or situation. The next stage is intellection, which means the identification of the stimuli by the development of new thoughts and ideas. When a person receives certain stimuli, new thoughts and ideas may form leading to better understanding of the stimuli. Emotion is a strong feeling about somebody or something. It is an individual trait of a person governing his decision making process. Volition is the ability to make conscious choices or decisions. It is a person’s will to react to any given situation. In highway design practice, the time that elapses between the perception of danger by a road user and the decision to take action (Perception and brake-reaction time) is an important consideration especially in the design of sight distances. The perception time is that time required for a driver to come to a realisation that brakes must be applied. The brake – reaction time is that time between the perception of danger and the effective application of brakes. The AASHO practice is to use a combined perception and brake-reaction time of 2.5 seconds.

3.3

Pedestrian Characteristics

3.3.1

Speed

Speed of walk of pedestrians is needed for design of traffic signals and other pedestrian facilities. The average walking speeds range from 0.75m/s to 1.8m/s. The rate assumed in the Manual on Uniform Traffic Control Devices (MUTCD) for timing pedestrian signals is 1.2m/s. The designers may keep in mind that many pedestrians consider themselves as not being governed by any laws. In addition, any regulations pertaining to the movement of pedestrians are not being enforced. 3.3.2

Space Occupied by Pedestrians

For design of a subway, foot bridges and other facilities, the space required by a pedestrian is generally taken as an ellipse with a major axis of 0.6m and a minor axis of 0.45m Area approx. (0.2121m2). The spacing between pedestrians while walking is generally taken as 2.5m which roughly corresponds to a time headway of 2 seconds (i.e. 2.5m/1.2m/s) [Kadiyali, 2006]. 3.4

Vehicle Characteristics

The major vehicle characteristics considered in design include: Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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31

References a) b) c) d) e) f)

3.5

32

Size; Power performance of vehicles; Rolling resistance; Air resistance; Grade resistance; and Inertial forces during acceleration.

References

1. 2.

Kadiyali, L.R., 2006. Principles and Practices of Highway Engineering (including Expressways and Airport Engineering), 4th Edition. Khanna Publishers, New Delhi. MoWH&C, 2005, Road Design Manual Vol.1 – Geometric Design Manual, Kampala, Uganda.

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32

Geometric Design of Highways

4.0

Geometric Design of Highways

4.1

Introduction

33

Geometric design is an aspect of the highway design dealing with the visible dimensions of a roadway. It is dictated, within economic limitations, by the requirements of traffic and includes the design elements of horizontal and vertical alignment, sight distance, crosssection components, lateral and vertical clearances, intersection treatment, control of access etc [Kadiyali, 2006]. The purpose of geometric design is to reduce the number and severity of road accidents while ensuring high traffic flow with minimum delay to vehicles [Thagesen, 1996]. The safe, efficient and economic operation of a highway is governed to a large extent by the care with which the geometric design has been worked out. Safety or the lack of it is an immediate corollary of the various design features of the highway. Efficient and comfortable operation of traffic is possible only if the design elements have been meticulously considered. A well designed highway has to be consistent with economy. Too liberal standards may not fit in with the available resources, whereas if the standards are too low, the cost of operation may mount up [Kadiyali, 2006]. The basic inputs are the Design speed and the Design hourly volume. The design speed governs the design of vertical and horizontal curvatures while design hour volume governs capacity required. The design engineer has to consider the following points when selecting the design standards for a highway. a) Adequate geometric design in planning a highway facility ensures that the facility will not become obsolete in the foreseeable future. Hence the volume and composition of traffic in the design year should be the basis of design. b) Faulty geometrics are costly, and in some cases impossible to rectify at a later date and so, due to consideration should be given to geometric design at the initial stage itself. c) The design should be consistent with and the standards proposed for different elements should be compatible with one another. Abrupt changes in design should be avoided. d) The design should embrace all aspects of geometrics of the road, including signs, markings, proper lighting, intersections, etc. e) The highway should be considered as an element of the total environment and its location and design should enhance rather than degrade the environment. The highway should be aesthetically satisfying. The design elements should strive to control pollution. f) The design should be so selected that not only the initial cost of construction of the facility, but also the total transportation cost, including maintenance cost and road user cost should be minimised. g) Safety should be inbuilt into the design elements. h) The design should enable all the road users (motor vehicles, cyclists, pedestrians and animal drawn vehicles) to use the facility. The performance of the vehicles using the facility should be given due consideration. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

33

33

Highway Design Standards in Uganda 4.2

34

Highway Design Standards in Uganda

Some geometric standards in Uganda have been formulated by the Ministry of Works, Housing and Communications e.g. The Uganda Road Design Manual Vol.1- Geometric Design Manual 2005. The AASHTO Standards represent the American practice, whereas the Department of Environment (UK) standards give the current British practice. It is important for engineers to exercise judgement in the use of a given design standard to ensure that they come up with an economical solution for a geometric design. Sometimes, more than one design standard is used for the purposes of comparing one pavement design with another so that the comparison guides the engineer in selecting the most economical option. 4.3

Division of Roads into Functional Class

The rural roads in Uganda are divided into the following 5 classes according to their major function in the road networks. Class A: International Trunk Roads Roads that link International Important Centres; Connection between the national road system and those of neighbouring countries; Major function is to provide mobility; Class B: National Trunk Roads Roads that link provincial capitals, main centres of population and nationally important centres. Major function is to provide mobility; Class C: Primary Roads Roads linking provincially important centres to each other or to a higher class roads (urban/rural centres). Linkage between districts local centres of population and development areas with higher class road. Major function is to provide both mobility and access; Class D: Secondary Roads Roads linking locally important centres to each other, to a more important centre, or to a higher class road (rural/market centres) and linkage between locally important traffic generators and their rural hinterland. Major function is to provide both mobility and access; Class E: Minor Roads Any road link to minor centre (market/local centre) and all other motorable roads; Major function is to provide access to land adjacent to the secondary road system;

Roads of the highest classes, A and B, have as their major function to provide mobility and have longer trip lengths. They are required to provide a high level of service with a high design speed. The roads of Classes C and D serve a dual function in accommodating shorter trips and feeding the higher classes or road. For these roads an intermediate design speed and level of service is required. Road Class E has short trip length and their primary function is to provide access. Design speeds and level of service for these roads may be low [MoWH&C, 2005].

4.4

Design Controls and Criteria

4.4.1

General

34

There are certain basic design controls and criteria which govern the geometric features of a highway. These are: topography, traffic (its volume, directional distribution, and Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

34

Topography

35

composition, including the future estimates), speed, capacity design vehicle and control of access.

4.4.2

Topography

Topography and physical features play an important role in the location and design of a highway. The various design elements should be related to topographical features if an economical and sound judgement is to emerge. The classification of terrain is normally done by means of the cross (transverse) slope of the country, i.e. the slope approximately perpendicular to the centre-line of the highway location. Table 4.1: Terrain Classification Type of terrain Flat

Description Level or gently rolling country which offers few obstacles to the construction of a road having continuously unrestricted horizontal and vertical alignment (transverse terrain slope around 5%)

Rolling Rolling, hilly or foothill country where the slopes generally rise and fall moderately gently and where occasional steep slopes may be encountered. It will offer some restrictions in horizontal and vertical alignment. (20% ≥ transverse terrain slope > 5%) Mountainous Rugged, hilly and mountainous country and river gorges. This class of terrain imposes definite restrictions on the standard of alignment obtainable and often involves long steep grades and limited sight distances (70% ≥ transverse terrain slope > 20%) Escarpment

In addition to the terrain class given above, a fourth class is added to cater for those situations whereby the standards associated with each of the above terrain types cannot b e met. Escarpment situations are where it is required to switchback road alignments or side hill traverse sections where earthwork quantities are huge (transverse terrain slope >70%) Source: Uganda Road Design Manual, 2005

4.4.3

Traffic

a) Importance of traffic data in Geometric Design Of crucial importance in highway design is the traffic data – both current and future estimates. Traffic volume indicates the level of service for which the highway is being planned and directly affects the geometric features such as width, alignment, grades etc. Without traffic data, it is futile to design any highway. b) Design Hour Volume (DHV) The general unit for measuring traffic on a highway is the annual average daily traffic volume, abbreviated as AADT. It is equal to the total annual volume of traffic divided by the number of days in the year. Know ledge of traffic in terms of AAD T is not of much use in geometric design, since it does not represent the variations in traffic during various months of the year, days of the week and hours of the day. A commonly used unit for geometric design is the 30 th highest hourly volume abbreviated as 30 HV. It is defined as the 30 th highest hourly volume during the year. Hence the design hourly volume (DHV) should be the 30 HV of the design (future) year chosen for design. Exceptions may be made on roads with high seasonal fluctuation, where a different volume may need to be used [MoWH&C, 2005].

35 DHV is then expressed as DHV = AADT x K or ADT x K where K is estimated from the ratio of the 30th HV to the AADT from a similar site. The 30th HV is the 30th highest hourly volume during the year. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

35

Traffic

36

The 30th HV is expressed as a fraction of ADT can vary as indicated in the following table. Traffic Conditions Rural Arterial (Average Value) Rural Arterial (Maximum Value) Heavily Trafficked road under congested urban conditions Normal Urban Conditions Road Catering for recreational or other traffic of seasonal nature Source: Uganda Road Design Manual, 2005

th

30 HV as a fraction of ADT 0.15 0.25 0.08 - 0.12 0.10 - 0.15 0.20 - 0.30

c) Directional Distribution of Traffic For 2-lane highways, the design hour volume is the total traffic in both directions of travel. For highways with more than 2-lanes, it is desirable to know the directional distribution of traffic. Though this distribution has to be found from traffic surveys, a rough approximation can be to assume 67% of total traffic to travel in one direction under the design conditions. The design has to take into account both the morning and evening situations [Kadiyali, 2006]. d) Traffic Composition Traffic composition has a vital effect on capacity and other design considerations. In Uganda, the traffic is heterogeneous in character, consisting of fast driven cars, trucks, and buses. It is customary in this country to express the traffic volume in terms of passenger car units (PCUs). The values in indicated in the table below. Table 4.2: Conversion Factor of Vehicle into Passenger Car

Vehicle Type

Level

Terrain Rolling PCU 1.0 1.5 5.0 8.0 4.0 1.0 0.5

Mountainous

Passenger cars 1.0 1.5 Light goods vehicle 1.0 3.0 Medium goods vehicle* 2.5 10.0 Heavy goods vehicle 3.5 20.0 Buses 2.0 6.0 Motor cycles, Scooters 1.0 1.5 Pedal cycles 0.5 NA * Also representative for combined group of medium and heavy goods vehicles and buses . Source: Uganda Road Design Manual, 2005

The following definitions apply to the different vehicle types mentioned in the above table. Passenger cars:

Medium goods vehicle: Heavy goods vehicle: Buses:

Passenger vehicles with less than nine seats. Light goods vehicle: Land rovers, minibuses and goods vehicles of less than 1,500kg un-laden weight with payload capacities less than 760 kg. Maximum gross vehicle weight 8,500 kg. Gross vehicle weight greater than 8,500 kg. All passenger vehicles larger than minibus.


36

36

Design Vehicle Dimensions

37

e) Future Traffic Estimates The design of the geometric elements has to be prepared for the traffic likely to use the road in the design year. The design period used for a flexible pavement generally varies from 15 to 25 years. A period of 20 years is widely used as a basis for design. The future traffic estimates should be computed to include normal, diverted and generated traffic.

4.4.4 a)

Design Vehicle Dimensions Design Vehicles

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. The dimensions and operating characteristics of a vehicle profoundly influence geometric design aspects such as radii, width of pavements, parking geometrics, etc. The weight of the axles and the weight of the vehicles affect the structural design of the pavement and structures, as also the operating characteristics of vehicles on grades. Because of its crucial importance the standardisation of the dimensions and the weights of design vehicles is the first step in formulating geometric design standards. This has been done in many countries. In Uganda, the Ministry of Works, Housing and Communications’ Uganda Road Design Manual Vol.1 - Geometric Design Manual 2005 , is being followed [MoWH&C, 2005].

b) Dimensions of Design Vehicles

The present vehicle fleet in Uganda includes a high number of four-wheel drive passenger/utility vehicles, buses and overloaded trucks. Accordingly the five design vehicles indicated in Table 5.1 will be used in the control of geometric design until a major change in the vehicle fleet is observed and detailed information on the different vehicle types using the roads in Uganda becomes available. Table 4.3: Dimensions of Design Vehicles (Extracted from AASHTO Geometric Design Manual of Highway and Streets)

Overhang (m)

Overall (m) Design Vehicle type

4x4 passenger car Single unit truck Single unit bus Semitrailer combination large Interstate Semitrailer

Symbol

t h g i e H

h t d i w

h t g n e L

t n o r F

r a e R

) m ( e s a b l e e h W

Minimum design turning radius (m)

Minimum inside radius (m)

DV-1

1.3

2.1

5.8

0.9

1.5

3.4

7.3

4.2

DV-2

4.1

2.6

9.1

1.2

1.8

6.1

12.8

8.5

DV-3

4.1

2.6

12.1

2.1

2.4

7.6

12.8

7.4

DV-4

4.1

2.6

16.7

0.9

0.6

6.1 & 9.1

13.7

5.8

DV-5

4.1

2.6

21.0

1.2

0.9

6.1 & 12.8

13.7

2.9

37

Source: Uganda Road Design manual Vol.1, 2005


37

Selection of the Design Vehicle c)

38

Selection of the Design Vehicle

The selection of the design vehicle for the design of a highway is governed by the type and volume of traffic that is expected to use the highway. For instance the design of a superior facility such as a motorway or an expressway should be based on the largest design vehicle. The design of streets and junctions primarily in residential areas can be done by using the passenger car design vehicle.

4.4.5

Design Speed

a) Speed as a Design Factor The value of a highway is largely indicated by the speed, safety and convenience afforded by the facility for travel. Speed is important for economic operation and has a great bearing on safety of the highway. It plays a vital role in determining the geometric design of any given highway. b) Design Speed Design speed is the speed determined for design and correlation of the physical features of a highway that influence vehicle operation. It is the maximum safe speed that can be maintained over a specified section of a highway when conditions are so favourable that the design features of the highway govern. The design speed obviously has to be correlated with terrain conditions and the classification of the highway. There is considerable variation in the speed adopted by different drivers and by different types of vehicles. This raises the question of what value of speed should be adopted for design. The value selected should accommodate nearly all demands with reasonable adequacy, yet the design should not fail completely under severe or extreme load. The speed adopted should s atisfy nearly all drivers with exception of those few who drive at extremely high speed [Kadiyali, 2006].

The standard design speeds are 50km/h, 60km/h, 70km/h, 85km/h, 100km/h and 120km/h. These speed bands are based on the premise that for a given highway, it is considered acceptable if 85% of the drivers travel at or below the designated design speed, generally inducing a situation where approximately 99% of the drivers travel at or below one design speed category above the design speed. Thus if a chosen design speed is by definition the 85th percentile speed for the highway, then the next speed band up will constitute the 99 th percentile speed. Speed bands are related to each other as follows:

99  8 5   √ 2 85  50 

….4.1

The 85th percentile speed is selected as the design speed on the basis that it constitutes the most appropriate choice. Use of the 99 th percentile speed would be safer but extremely expensive while use of the 50 th percentile speed would be unduly unsafe for faster travelling vehicles [Rogers, 2003]. The curve depicting the cumulative distribution of speeds has a typical “S” shape. It is important to note that designers use typical data previously obtained on similar roads. 38 4.4.6

Control of Access

Uncontrolled access to road side development along whose major function is to provide mobility will result in an increased accident hazard, reduced capacity and early obsolescence of the roads. In order to preserve major roads as high standard traffic facilities it is necessary Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

38

Sight Distance

39

to exercise access control, whereby the right of owners or occupants of land to access is controlled by the Road Authority. Although control of access is one of the most important means for preserving the efficiency and road safety of major roads, roads without access control are equally essential as land service facilities. The following three levels of access control are applicable: (1) Full access control: - means that the authority to control access is exercised to give preference to through traffic by providing access connections with selected public roads only and by prohibiting direct access connections. (2) Partial access control:- means that the authority to control access is exercised to give preference to through traffic to a degree in that, in addition to access connections with selected public roads, there may be (some) private access connections. (3) Unrestricted access: - means that preference is given to local traffic, with the road serving the adjoining areas through direct access connection. However, the detailed location and layout of the accesses should be subject to approval by the Road Authority in order to ensure adequate standards of visibility, surfacing, drainage, etc. Road function determines the level of access control needed. Roads of higher classes have their major function to provide mobility, while the function of lower classes is to provide access. Motorways should always have full control of access. For all purpose roads the following general guidelines are given for the level of access control in relation to the functional road classification:

Table 4.4: Level of Access Control Level of Access Control Desirable Reduced A Full Partial B Full or Partial Partial C Partial or Unrestricted Partial D Partial Unrestricted E Partial or Unrestricted Unrestricted Source: Uganda Road Design Manual, 2005 Functional Class

The reduced levels of access control may have to be applied for some road projects because of practical and financial constraints. Control of access is accomplished either by the careful location of accesses, by grouping accesses to reduce the number of separate connections to the through traffic lanes or by constructing service roads which intercept the individual accesses and join the through lanes at a limited number of properly located and designed junctions. In every case the location and layout of all accesses, service roads and junctions should be carefully considered at the design stage and include in the final design for the project [MoWH&C, 2005]. 4.5

Sight Distance

39 4.5.1

General

Sight distance is defined as the length of carriageway that a driver can see in both the horizontal and vertical planes. There are two types of sight distance: stopping sight distance and overtaking sight distance [Rogers, 2003]. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

39

Stopping Sight distance, SSD

40

The design of a highway with adequate sight ahead of a travelling vehicle results in safe operation. Knowledge of the sight distance requirements is needed in designing vertical curves. It also governs the set-back of buildings, slopes, fence, and other obstructions adjacent to a carriageway on a horizontal curve [Kadiyali, 2006]. 4.5.2


This is defined as the minimum sight distance required by the driver in order to be able to stop the car before it hits an object on the highway. It is primary importance to the safe working of a highway. Because of its importance to safety, all highways must be designed for the minimum stopping sight distance. It is made up of two components: a) The distance travelled during perception and brake-reaction time; and b) The distance travelled during the time the brakes are under application till the vehicle comes to a stop. When sensations received through the eyes, ears or body are strong enough to be recognised and interpreted, they become perceptions. In the cases of a motorist, it is the time which elapses between the instant the driver perceives the object on the carriageway and the instant that he realises that braking is needed. The time lag or the brief interval between the perception of danger and the effective application of the brakes is called the brake-reaction time. The perception time and the brake reaction time depend upon a variety of factors, e.g. age, sex, alertness and visual acuity of the driver, visibility, vehicle design, the size and type of the object etc. According to Ugandan practice, a perception reaction time of 2.5s, eye height of 1.07m above the road surface and an object height of 0.15m are used in computing stopping sight distance. The distance travelled during this interval, d 1 is given by:

   0.278

Where; d1 v V t

= = = =

….4.2

distance travelled in metres; speed in m/s; speed in km/h; perception and brake reaction time in seconds (2.5 seconds)

The braking distance is the distance within which a moving vehicle comes to a stop after the application of the brakes. On a level road, the distance is given by;

   254

Where; d2 V f

= = =

….4.3

braking distance travelled in metres; speed in km/h; coefficient of longitudinal friction between the tyre and the pavement.

The coefficient of friction (for a wet pavement condition) is assumed to vary from 0.40 at 30 km/h to 0.28 at 120 km/h. The above considerations yield the values in Table 4.5 below as recommended by MoWH&C.


40

40


41

Table 4.5: Stopping Sight Distance on Level Ground for Wet Pavement Condition Brake Reaction Design Speed [km/hr]

Assumed Speed for Conditio n [km/hr]

30

Coefficient of friction for wet pavement conditin (f)

Breaking distance on level [m]

Stopping sight distance for design [m]

Time [sec]

Distance [m]

30-30

2.5

20.8-20.8

0.40

8.9-8.9

29.7-29.7

40

40-40

2.5

27.8-27.8

0.38

16.6-16.6

44.4-44.4

50

47-50

2.5

32.6-34.7

0.35

24.8-28.1

57.5-62.8

60

55-60

2.5

38.2-41.7

0.33

36.1-42.9

74.3-84.6

70

63-70

2.5

43.8-48.6

0.31

50.4-62.2

94.2-110.8

80

70-80

2.5

48.6-55.6

0.30

64.3-84.0

112.9-139.5

90

77-90

2.5

53.5-62.5

0.30

77.8-106.3

131.3-168.8

100

85-100

2.5

59.0-69.4

0.29

98.1-135.8

157.0-205.2

110

91-110

2.5

63.2-76.4

0.28

116.4170.1

179.6-246.5

120

98-120

2.5

68.1-83.3

0.28

135.0202.5

203.1-285.8

Source: Uganda Road Design Manual Vol.1, 2005

Some slight adjustments are needed in the values of the braking sight distance to take into account the effect of grades. The following amended formula may be used to calculate d 1.

   254 

Where; G

=

….4.4

Longitudinal grade in percent (%).

The positive sign is used when the gradient is upgrade and the negative sign may be used if the gradient is downgrade. Correction for grade should not be applied on undivided roads with two-way traffic but must invariably be considered for divided highways which have independently designed profiles. The safe stopping sight distance, SSD is given by d 1 + d2.

    0.278  254 

….4.5

Example: Calculate the safe stopping distance of a vehicle travelling at a speed of 80kph on an upward gradient of 2%. Make suitable assumptions. Solution 1.0

Data Summary a) Vehicle running speed, V b) Longitudinal gradient, G c) Perception reaction time, t d) Coefficient of friction, f

41 = = = =

80kph 2% (upgrade = +0.02) 2.5s (Assumed) 0.3 (Assumed between 0.40 and 0.28)


41

Full Overtaking Sight Distance, FOSD

2.0

42

Safe Stopping Sight Distance, SSD From equation 4.5, SSD is given by

  80   0.278802.5 2540.300.02  55.6078.74  134.34

4.5.3

Full Overtaking Sight Distance, FOSD

Overtaking sight distance is that distance which should be available to enable the driver to overtake another vehicle safely and comfortably without interfering with the speed of an oncoming vehicle travelling at the design speed should it come into view after the overtaking manoeuvre is started [Kadiyali, 2006]. Overtaking sight distance is of central importance to the efficient working of a given section of highway. Overtaking sight distance only applies to single carriageways. There is no full overtaking sight distance (FOSD) for a highway with a design speed of 120km/h since this speed is not suitable for a single carriageway road. Full overtaking sight distances are much larger in value than stopping sight distances. Therefore, economic realities dictate that they can only be complied with in relatively flat terrain where alignments, both vertical and horizontal, allow the design of a relatively straight and level highway [Rogers, 2003]. Full overtaking sight distance is measured from vehicle to vehicle (the hazard or object in this case another car) between points 1.05m and 2.00m above the centre of the carriageway. FOSD is made up of three components: d1, d2 and d3 as described below: d1

=

distance travelled by the vehicle in question while driver in the overtaking vehicle completes the passing manoeuvre (Overtaking Time);

d2

=

distance between the overtaking and opposing vehicles at the point in time at which the overtaking vehicle returns to its designated lane (Safety Time);

d3

=

distance travelled by the opposing vehicle within the above mentioned ‘Perception – reaction’ and overtaking times (Closing Ti me).

In order to establish the values for full overtaking sight distance, it is assumed that the driver making the overtaking manoeuvre commences it at two design speed steps below the designated design speed of the section of highway in question. The overtaking vehicle then accelerates to the designated design speed. During this time frame, the approaching vehicle is assumed to travel towards the overtaking vehicle at the designated design speed. The safety time, d2 is assumed to be 20% of d 3. These assumptions yield the following equation:

      2 √ 2√ 2 0.2   2.05  0.57

….4.6 ….4.7

Where;

42 v V t

= = =

speed in m/s; speed in km/h; time taken to complete the entire manoeuvre.


42

Sight Distance for Multi-Lane Roads

43

The value of, ‘t’ is generally taken as 10 seconds, as it has been established that it is less than this figure in 85% of observed cases [Rogers, 2003].

2.0m

0.15m

1.07m

1.3m

Stopping Sight Distance

Extra distance for large vehicles

Passing Sight Distance

Figure 4.1: Stopping and Passing Sight Distances on a crest c urve

Source: Uganda Road Design manual Vol.1, 2005

Note that in Uganda, the AASHTO standard and NOT the British Standard has been adopted for computation of FOSD (See Uganda Road Design manual Vol.1, 2005). 4.5.4

Sight Distance for Multi-Lane Roads

Divided highways with 4 or more lanes need only be designed for safe stopping sight distance. Undivided highways with 4 lanes have enough opportunities for overtaking within one half of the carriageway. Such roads therefore need only be designed for safe stopping sight distance.

4.5.5

Set-back Distance at Obstructions of Horizontal Curves

On horizontal curves with obstructions on the inside, an important consideration is the lateral clearance so as to obtain the sight distance. It should be noted that: i) Sight distance is measured along the arc of the curve; ii) If the pavement has two or more lanes, sight distance is measured along the arc at the centre of the inner lane. The presence of obstructions adjacent to the highway such as boundary walls, buildings, slopes of cuttings may cons train the limiting radius of the hori zontal alignment. To provide the necessary horizontal sight distances, it may be necessary to set back obstructions. In cases where the obstructions are immovable, it may be necessary to redesign the road alignment in order to meet the safety requirements. It is therefore necessary to estimate the offset clearance necessary to secure the required horizontal distance by considering two cases as in the following sections.

43


43

Set-back Distance at Obstructions of Horizontal Curves a) Required Sight distance, S lies wholly within the length of the curve, L (S

Figure 4.2: Sight Distance Requirements on a horizontal curve with S

44

L)

L

Source: Rogers, 2003

The offset M can be approximated by considering the vehicle truck to be along the chords AC and CB. When the radius of horizontal curvature is large, then it can be assumed that the required sight distance, S , approximates to a straight line. When S lies within the curve length, the minimum offset M from the centreline to the obstruction can be estimated by considering the triangle OAM and ACD. Thus: From triangle OAM, R 2 = x 2 + ( R − M ) 2

(i)

From triangle ACD, 2

⎛ S ⎞ 2 2 ⎜ ⎟ = x + M ⎝ 2 ⎠

(ii)

Solving (i) and (ii),

    8

….4.8

b) Required Sight distance, S lies outside the length of the curve, L (S > L) S is greater than the available length of the curve L and overlaps on the tangents for a distance l on each side.


44

44

Set-back Distance at Obstructions of Horizontal Curves

Figure 4.3: Sight Distance Requirements on a horizontal curve with S

45

L


Assuming a large horizontal radius of curvature and considering triangles ACP and OAP, 2

⎛ S ⎞ 2 2 ⎜ ⎟ = x + M ⎝ 2 ⎠ 2 d 2 = x 2 + ( R − M ) .

(ii)

d 2 = l 2 + R 2

(iii)

(i)

Also,

But

S

2

=

L

2

+ l so that (iii) becomes 2

⎡ S − L ⎤ + R 2 d = ⎢ ⎥ ⎣ 2 ⎦ 2

(iv)

Solving (i), (iii) and (iv),

  28

….4.9

Example A 2-lane 7.3 m single carriageway road has a horizontal curve of radius of 600 m. If the minimum sight stopping distance required is 160 m, calculate in metres the required distance to be kept clear of obstructions if the length of the curve is:

(a)

200 m;

(b)

100 m.


45

45

Horizontal Alignment

46

Solution

From the question, S = 160 m, R = 600 m. (a) The length of the curve 200 m > 160 m. So the required sight distance S lies wholly within the length of the curve. Applying equation (4.8), the required offset

(160)2 M = = 5.33 m 8(600) (b)

The length of the curve 100 m < 160 m. So the required sight distance S lies outside the length of the curve. Applying equation (4.9), the required offset 100 [2(160 ) − 100 ] M = = 4.58 m 8(600 )

4.6

Horizontal Alignment

Horizontal alignment deals with the design of the directional transition of the highway in a horizontal plane. A horizontal alignment consists, in its most basic form, of a horizontal arc and two transition curves forming a curve which joins two straights. In some cases the transition curve may have zero length. The design procedure itself must commence with fixing the position of the two straight lines which the curve will join together. The basic parameter relating these two straight lines is the intersecting angles. Minimum permitted horizontal radii depend on the design speed and the super-elevation of the carriageway, which has a maximum allowable value of 7% in the UK, with designs in most cases using a value of 5%. The relationship between super-elevation, design speed and horizontal curvature is detailed in the following sub section.

4.6.1

Basic Formula for Movement of Vehicles on Curves

When a vehicle is moving on a curved path, it is subjected to an outward force, commonly known as the centrifugal force. In order to resist this force, it is the usual practice to superior-elevate the roadway cross-section. Figure 4.4 shows the forces acting on the vehicle at a super-elevated section.

46

Figure 4.4: Forces acting on a vehicle on a horizontal curve



46

Basic Formula for Movement of Vehicles on Curves Let;

M v V R g N µ α e C

= = = = = = = = = =

47

mass of the vehicle; speed of the vehicle in m/s; speed in km/h; radius of the curve in metres; acceleration due to gravity (=9.81m/s2); normal force; coefficient of lateral friction; angle of super-elevation; rate of super-elevation, normally given as a percentage (= tan α) centrifugal force.

The centrifugal force acting on the vehicle, C =

Mv 2 R

(i)

For equillibrium, resolving forces parrallel to the incline plane Mg sin α + P =

Mv 2 R

cos α .

(ii)

Resolving forces perpendicular to the incline plane P = μ (W cos α + C sin α )

=

μ ( Mg

cos α +

Mv 2 R

sin α ) .

(iii)

Substituting equation (iii) into equation (ii) gives, Mg sin α +

μ ( Mg

cos α +

Mv 2 R

sin α ) =

Mv 2 R

cos α .

Dividing through the above equation by Mg cos α we obtain:

tanα + μ + The term

μ v

μ v

2

gR

tanα =

v

2

gR

.

(iv)

2

gR

tanα is very small and can be ignored leading to the expression:

tan α + μ =

v

2

gR

Or

v2 gR

= e + μ .

(iiv)

Expressing speed as V in km/hr

    127

….4.10

Equation 4.10 above is the basic equation relating the speed of vehicles, the radius of the curve, the super-elevation and coefficient of friction. This equation forms the basis of design of horizontal curves, Equation 4.10 can be rewritten as shown below and is known as the minimum radius equation:

    127  Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

….4.11 47

47

Value of the Coefficient of Lateral Friction, µ

The terms

v2

gR respectively.

and

v2 R

48

are known as the ‘ centrifugal ratio’ and ‘centrifugal acceleration’

If the entire centrifugal force is counteracted by super-elevation, then frictional force will not come into play. In this case, µ = 0 in equation 4.10. The super-elevation then provided is said to be ‘equilibrium super-elevation’. In such a case, case, the pressures on the inner and outer wheels would be equal. Design practice is based on the assumption that at absolute minimum radius the 99 th percenti perc enti le speed spe ed vehicl veh iclee sho should uld not experie expe rience nce more than the maximu max imum m level lev el of v2

centrifugal acceleration acceptable for comfort. Its value is 0.22 g. Thus if

R

= 0.22 g ,

then the total centrifugal acceleration at the design speed (85th percentile speed) should not exceed:

  0.2√ 22  0.156 4.6.2

… . 4.12

Value of the Coefficient of Lateral Friction, µ

The value of the coefficient of lateral friction depends upon a number of factors, chief among them being the vehicle speed, type and condition of roadway surface, and type of and condition of the tyres. AASHTO recommends the values given in Table

Table 4.6: Coefficient of Lateral Friction as Recommended by AASHTO

Design Desi gn S peed (kph) (kph) Maximum Lateral Friction Source: Kadiyali, 2006

50

65

80

100

120

130

0.16

0.15

0.14

0.13

0.12

0.11

A constant value of 0.15 is generally recommended. 4.6.3

Maximum super-elevation Value, e max

If equation 4.10 is to be used for design, it is desirable to know the maximum superelevation that can be permitted. Practice in this regard varies from country to country. The AASHTO practice limits it to 0.12 (12%), whereas the UK practice limits it to 0.07 (7%). In Uganda the value is limited to 0.08 (8%) [MoWH&C, 2005].

4.6.4

Super-elevation Rates

Super-elevation on curves is intended to counteract a part of the centrifugal force, the remaining part being resisted by the lateral friction. Also, super-elevation results in economies in maintenance. This is because skidding and unequal pressures on the wheels of vehicles, which result from high value of sideway force between the tyres and the roadway surface, necessitate frequent attention to the surface.


48

48

Radii of curves for which no super-elevation is required

49

Super-elevation can be provided either to fully counteract the centrifugal force or to counteract a fixed proportion of the centrifugal force. In the former case, the super-elevation needed would be more than 1 in 15 (6.67%) on sharp curves causing inconvenience to slow moving vehicles. Since super-elevation has to be limited to 7% or 8% as per Ugandan practice, maximum friction would have to be relied relied upon when the sharpest possible curve is traversed. When a vehicle negotiates a flat curve, friction would not be developed to the maximum and this would not be a balanced design. It is desirable that the super-elevation should be such that a moderate amount of friction is developed while negotiating flat curves and friction not exceeding the maximum allowable value should be developed at sharp curves. Therefore designing the super-elevations to fully counteract the centrifugal force developed at a fraction of the design speed will provide the necessary balance. The above is achieved as per UK practice by providing full super-elevation for a speed of 67.082% of the design speed such that 45% of the centrifugal force is balanced by superelevation while 55% of the centrifugal force is balanced by friction. Therefore equation 4.10 becomes:

 0.45    0. 6 7082   127  127  282

….4.13

The super-elevation computed from equation 4.13 is restricted to a value of 7% (0.07) or 8% (0.08) as per Ugandan practice.

4.6.5

Radii of curves for which no super-elevation is required

The normal cambered section of a highway can itself be continued on a curve where the super-elevation calculated is less than the camber. From equation 4.13

    282

….4.14

Substituting the values of camber for e in equation 4.14 above, the minimum radius beyond which no super-elevation is required is obtained. In such cases where the radius is greater than those given by the above formula it is desirable to remove the adverse crown in the outer half of the carriageway and super-elevate s uper-elevate at the normal crown slope.

4.6.6

Method of Attainment of Super-elevation

The normal cambered surface on a straight reach of road is changed into a super-elevation surface in two stages. In the first stage, the outer half of the camber is gradually raised until it is level as shown below:

49


49


50

Figure 4.5: Stages involved in attainment of super-elevation

It is desirable to accomplish the raising of the outer-half till it is level before the starting point of the transition curve. The raising of the outer edge should be done in a slope not exceeding 1 in 150 for plain and rolling terrain and 1 in 60 for hilly terrain. In the second stage, any of the three methods given below may be adopted to attain the full super-elevation: a) The surface of the road is rotated about the centre-line of the carriageway, gradually lowering the inner edge and raising the outer edge while keeping the level of the centre-line constant (Figure 4.5 d); b) The surface of the road is rotated about the inner edge, raising the centre and the outer edge (Figure 4.5 e); c) The surface of the road is rotated about the outer edge depressing the centre and the inner edge (Figure 4.5 f); In most circumstances method (a) is generally used a it results in the least distortion of the pavement. Figure 4.6 below shows the method of attaining super-elevation using method (a).

50


50


51

Figure 4.6: Attaining Super-elevation by revolving about the centre line

Example Calculate the super-elevation to be provided for a horizontal curve with a radius of 400m for a design speed of 100kph in plain terrain. Comment on the results. What is the coefficient of lateral friction mobilised if super-elevation is to be restricted to 7%. Solution 1.0

2.0

Data Summary a) Curve Radius, R b) Design Speed, V c) Maximum super-elevation, e

= = =

400m 100kph 7%

Maximum Elevation, e max According to the UK practice, the super-elevation is calculated on the assumption that it should 45% of the centrifugal force developed at 67.082% of the design speed.

Therefore from equation 4.13

 100     282 282400  0.089  8.9% 3.0

Comment on the Result Since, as per UK practice, the maximum super-elevation allowable is 7%, then the computed super-elevation is too high and should be restricted to 7%. The balance of the centrifugal force will be taken care of by the friction which is mobilised. If µ is the coefficient of friction, then from equation 4.10:

   127    100   127   127400 0.07  0.127

51

This is less than the recommended value of 0.15


51

Transition Curves 4.6.7

52

Transition Curves

A transition curve is a curve in which the radius changes continuously along its length and is used for the purpose of connecting a straight with a circular curve, or two circular curves of different radii. a) Need for Transition When a vehicle travelling on a straight course (i.e. R= ) enters a curve of infinite radius, it suddenly subjected to the centrifugal force which causes shock and sway. In order to avoid this, it is customary to provide a transition curve at the beginning of the circular curve, having a radius equal to infinity at the end of the straight and gradually reducing the radius of the circular curve where the curve begins. Incidentally, the transition portion is also used for gradual application of the super-elevation, curve widening and improvement of the general appearance. The transition curve is also used to achieve the following:

∞

i) ii) iii) iv)

They reduce the tendency of vehicular skidding; They minimise passenger discomfort; They provide convenient sections over which super-elevation or pavement widening may be applied; They improve the appearance of the road by avoiding sharp discontinuities in alignment at the end and beginning of circular curves.

b) The Spiral Various forms of curves are suitable for highway transitions, but the most popular and recommended for use in this country is the spiral. It is easy to set out in the field and the rate of acceleration is uniform through the length of transition. Figure 4.7 below shows the main elements of a circular curve provided with spirals for transition at its two ends.

52

Figure 4.7: Main Elements of a Circular Curve Provided with Transitions


52

Transition Curves

53

The following nomenclature applies

φmax θ T R S L I T T1 T2 U

=

Spiral angle

= = = = = = = = = =

Deflection angle Tangent length Radius of circular curve Shift Length of Spiral (or transition curve) Point of intersection Beginning of spiral Beginning of circular curve End of circular curve End of spiral

Some of the important properties of a spiral are given below:

       2 radians  L Shift,S  24R in metres Tangent length,T  Rstanθ2 L2

….4.15 ….4.16 ….4.17 ….4.18

c) Length of Transition The length of the transition should be determined from the following two considerations:

i)

The rate of change of centrifugal acceleration adopted in the design should not cause discomfort to the drivers. If C is the rate of change of acceleration then:

     Where; aT1 aT t

= = =

….4.19 radial acceleration at T1 (= v2/R) radial acceleration at T (= 0) time taken (= L/v)

Substituting the above in equation 4.15 gives

0        

….4.20

From which the length of transition curve, L is given by

1         3.6 .  Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

….4.21 53

53

Curve Widening

54

Where v is speed in m/s and V is speed in Km/hr. The value of C is usually taken as 0.3m/s3. ii)

The rate of change of change of super-elevation should also be such as not to cause higher gradients and unsightly appearances. This could be kept in 1 in 150 for roads in plain and rolling terrain and 1 in 60 for roads in hilly terrain. Since the superelevation can be given by rotating about the centreline, inner or outer edge, the length of the transition will be governed accordingly. In calculating the length of transition, the pavement width should include any widening that may have been provided at the curve. The higher of the values given by the above two methods should be adopted.

4.6.8

Curve Widening

Widening of pavements is needed on curves for the following reasons: a) On curves, the vehicles occupy a greater width because the rear wheels track inside the front wheels (See Figure 4.8)

b) On curves, drivers have difficulty in steering their vehicles to keep to centre line of the lane. c) Drivers have psychological shyness to drive close to the edges of the pavement on curves. From Figure 4.8, considering the triangle OCB, right angled at B,

   2 2  Neglecting m2, since it is small gives;

    2

….4.22

Figure 4.8: Widening on Curves

Assuming a wheel base of 6m for a vehicle corresponding to AASHTO single unit, widening in metres, m is given by:

 18

….4.23 54

Where; R = radius in metres The widening due to psychological reasons is a function of speed and can be assumed to be given by the empirical formula, W p; Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514 806514

54

Curve Widening

  0.1 √ 

55

….4.24

Where; V = speed in kph; R = radius in metres The total widening for pavements, W e is given by the following formula:

    . 2 0.1 √ 

….4.25

Where; n = number of lanes Example A two-lane (7.0m wide) pavement on a National highway in hilly terrain has a curve radius of 250m. The design speed is 80kph, maximum super-elevation is 7%, camber is 2.5%, the relative longitudinal gradient is 0.5% (1 in 200), the angle of deviation is 60 o and the rate of change of radial acceleration is 0.3m/s 3. Determine the following assuming that; a) the t he curve will need to be widened if the curve radius is less than 300m, b) the super-elevation is obtained by rotation about the centre line, and c) the design vehicle is a DV-2 single unit truck with a wheel base of 6.1m. i) The length of transition curve; ii) The tangent length iii) The total length of the curve. Solution 1.0

Data Summary a) Pavement width, W b) Curve Radius, R c) Design Speed, V d) Maximum super-elevation, emax e) Camber, eo f) Relative longitudinal gradient, S g) Angle of deviation, θ h) Rate of change of radial accn., C

= = = = = = = =

7.0m 250m 80kph 7% 2.5% 0.5% 60o 0.3m/s 3

2.0

Sketch drawing Refer to Figure 4.6

3.0 3.1

Transition Length, L Based on the rate of change of centrifugal acceleration, La From equation 4.21, the transition length, L, required for safety and comfort is given by;

1    3.6 .   1 80  3.6 . 0.3250  146.319

3.2

55

Based on the rate of change of super-elevation, Ls


55

Curve Widening

56

Since super-elevation is obtained by rotation about the centre line then the transition length, L, will be given by;

 2   

….

Where; W=pavement width, e = super-elevation, S=longitudinal gradient, and e o =camber Since radius, R < 300m, extra widening, W e, of the carriageway is required.

   6. 1  2. 2250 0.1 √ 80250 250 0.655 Therefore the pavement width will be W

=

7.000 + 0.655

=

7.655m

.... (ii)

Maximum super-elevation, e, is given by

 80   2.822250 250  9.08%

… . 

This value is high and should be restricted to 7% (i.e., e =7%) From which equation (i) becomes

 72.60.555 2.57.0 2.57.0  72.723 Therefore; Adopt L = 146.319m since La > Ls. (i.e. take the greater of the two values) 4.0

Tangent Length, T The tangent length T is given by;

T  R  Stanθ2 L2 Where;

 146.319 L S  24R 24 24250 250  3.57m T  2503.57 2503.57tan602  146.2319  219.558  220m

Therefore;

5.0

56

Total length of the horizontal Curve, L T The total length LT is given by;

L L  2L Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514 806514

56

General Controls for Horizontal Alignment

57

Where; Lc is the length of circular curve, and L is the transition length;

L R θ2φ  angles are in radians 146.319   115.500m L R πθ180 22RL   Rπ60 2 180 2x250   L 115. 5 002 146. 3 19 408.138m

From which;

4.6.9

General Controls for Horizontal Alignment

The following general controls for horizontal alignment should be kept in view in a sound design practice: a) The alignment should be as directional as possible; b) The alignment should be consistent with topography and should generally conform to the natural contours. A line cutting across the contours involves high fills and deep cuts, mars the landscape and is difficult for maintenance; c) The number of curves should, in general, be kept to a minimum; d) The alignment should avoid abrupt turns. Winding alignment consisting of short curves should be avoided, since it is the cause of erratic vehicle operation; e) A sharp curve at the end of along tangent is extremely hazardous and should be avoided. If sharp curvature is unavoidable over a portion of the route selected, it is preferable that this portion of the road be preceded by successive sharper curves. Proper signage, well in advance of a sharp horizontal curve is essential; f) Short curves giving the appearance of kinks should be avoided, especially for small deflection angles. The curves should be sufficiently long to provide a pleasing appearance and smooth driving on important highways. They should be at least 150m long for a deflection angle of 5 degrees, and the minimum length should be increased by 30m for each 1 degree decrease in the deflection angle; g) For a particular design speed, as large a radius as possible should be adopted. The minimum radii should be reserved only for the critical locations; h) The use of sharp curves should be avoided on high fills. In the absence of cut slopes, shrubs, trees, etc., above the roadway, the drivers may have difficulty in estimating the extent of curvature and fail to adjust to the conditions; i) While abrupt reversals in curvature are to be avoided, the use of reverse curves becomes unavoidable in hilly terrain. When they are provided, adequately long transitional curves should be inserted for super-elevation run-off; j) Curves in the same direction separated by short tangents, say 300m -500m long, are called broken-back curves. They should be avoided as they are not pleasing in appearance and are hazardous; k) Compound curves may be used in difficult topography in preference to a broken back arrangement, but they should be used only if it is impossible to fit in a single circular curve. To ensure safe and smooth transition from curve to curve, the radius of the flatter curve should not be disproportional to the radius of the sharper curve. A ration of 2:1 or preferably 1.5:1 should be adopted; l) The horizontal alignment should blend with the vertical harmoniously. General controls for the combination of horizontal and vertical alignments should be followed [Kadiyali, 2006].


57

57

Vertical Alignment 4.7

58

Vertical Alignment

Vertical alignment design refers to the arrangement of tangents and curves which compose the profile of the road. It is composed of a series of straight-line gradients connected by curves, normally parabolic in form. The main aim of vertical alignment is to ensure that a continuously unfolding stretch of the road is presented to motorists so that their anticipation of directional change and future action is instantaneous and correct [Rogers, 2003; O’Flaherty, 2002].

4.7.1

Major Requirements of Vertical Curves

The two main requirements in the design and construction of vertical curves are the provision of: Adequate visibility, and Passenger comfort and safety. In order to provide adequate visibility, oncoming vehicles or any obstructions in the road must be seen clearly and in good time to ensure that vehicles travelling at the design speed can stop or overtake safely. This requirement is achieved by use of sight distances and KValues to be discussed shortly in this chapter. In order to provide passenger comfort, the effect of the radial force on the vehicle traversing a vertical curve must be minimised. In crest curve design this effect could cause the vehicle to leave the road surface (e.g. in hump-back bridges) while in the sag curve the underside of the vehicle would come into contact with the surface, particularly where the gradients are steep and opposed. The result is discomfort and danger to passengers travelling. This can be minimised by:

• Restricting the gradients; which has the effect of reducing the radial force; • Choosing a suitable type and length of curve such that this reduced force is introduced gradually and uniformly as possible [Uren et al, 1989]. 4.7.2

Gradients

The rate of rise or fall of road surface along its length with respect to horizontal distance is termed as gradient [MoWH&C, 2004]. The use of steep gradients in hilly terrain generally results in lower road construction and environmental costs. However, it also adds to road user costs through delays, extra fuel costs and accidents. Gradients of up to about 7% have little effect on the speeds of passenger cars. Nevertheless, the speeds of commercial vehicles are considerably reduced on long hills with gradients in excess of 2%. For short distances, gradients of 5% or 6% may have little detrimental effect on commercial vehicle speeds [O’Flaherty, 2002]. Long, steep, downhill grades are very dangerous and need careful design, preferably with escape roads (side roads that are designed to bring out-of-control vehicles to a safe stop) [MoWH&C, 2004]. The Uganda Road Design manual (2004), suggests maximum gradients as presented in Table 2.16 below: Table 4.7: Maximum Grades as recommended by MoWH&C

Speed (km/h)

Maximum Grade (%) Flat Rolling Mountainous 50 6-8 7-9 9-10 80 4-6 5-7 7-9 100 3-5 4-6 6-8 Source: Uganda Road Design Manual (2004) Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

58

58

Climbing Lanes

59

According to British Standards of road design, a minimum longitudinal gradient of 0.5% is needed to ensure effective drainage of carriageways with kerbs.

4.7.3

Climbing Lanes

The limitation of gradients to a maximum value is not in itself a complete design control, and therefore an additional climbing lane is normally provided on long uphill climbs. The provision of a climbing lane is normally considered when the combination of hill severity and traffic volumes and composition is such that the operational benefits achieved are greater than the additional costs of constructing an additional lane. In Uganda, however, climbing lanes are recommended for use if the design truck speed decreases more than 20 km/h under the truck speed limit, normally 80 km/h in rural conditions. A climbing lane is inserted into the carriageway by means of entry and exit tapers to the left of the continuous lane so that slow moving vehicles have to merge into the faster traffic at the termination point as shown below.

Figure 4.9: Climbing Lane outside the ordinary lane

Source: Uganda Road Design Manual (2004)

4.7.4

Cross falls

A minimum cross fall of 2.5% is normally recommended in the form of either a straight camber extending from one edge to the other or as one sloped from the centre of the carriageway towards both edges. The primary aim of these cross falls is to adequately get rid of surface runoff from the highway pavement.

59

Figure 4.10: Highway Cross falls



59

Vertical Curves 4.7.5

60

Vertical Curves

A vertical curve provides a smooth transition between successive gradients in the road profile. When the algebraic difference in gradients, A is positive the curve is called a crest or summit curve whereas if it is negative the curve is called a s ag or valley curve.

Figure 4.11: Typical Vertical Curves

Source: O’Flaherty, 2002

a) Shape of the Curve

Where the ratio of length of curve to radius is less than 1-10, there is no practical difference between the shapes of a circle, a parabola and an ellipse. Owing to the fact that this condition can be shown to apply in most of the cases met in practice, a parabolic form of vertical curve is therefore used to guide vertical curve design [Bannister et al, 1998].

Figure 4.12: A Simple Symmetrical Parabolic curve

Source: Rogers (2002)

60

b) Equation of a Vertical Curve

According to Thagesen (1996), a simple parabola is recommended when modelling vertical curves. The parabola provides a constant rate of change of curvature, and hence visibility, along its length. The vertical curve is of the form: Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

60

Vertical Curves

           

Let;

Then;

61

…. ….

Where; C1 is a constant. This implies that; At x = 0

  0       100 100              100 100 100

….

At x = L

….

Where; A is the algebraic difference in grade (m - n) Substituting for k in equation (ii), we get

      100 100       200 100

Integrating equation (v) gives;

…. ….

From the above equation, it implies that If x = 0, the y = C 2 = RLPC (i.e. reduced level at PC) Therefore the general equation used determine the reduced level at any point on the vertical curve, RLx is given by;

     100  200

….4.26

Reduced level of crest or sag curve For maxima or minima,

 0           100 100       /   100    200      /   200

….

Substituting the value of x above in equation (vi) gives;

From which;


….4.27 61

61

Vertical Curves

62

c) Sight Distances

The length of curve to be used in any given situation depends on the sight distance. It is the distance of visibility from one side of the curve to the other [Uren et al, 1989]. There are two categories of sight distance namely:

• Stopping sight distance (SSD) ; which is the theoretical forward sight distance required by a driver in order to stop safely and comfortably when faced with an unexpected hazard on the carriageway, and Full overtaking sight distance (FOSD) ; which is the length of visibility required by motorists to enable them to safely and comfortably overtake vehicles ahead of them.

•

When designing vertical curves, it is important to know whether safe overtaking is to be included in the design. If it is to be included, then the FOSD must be incorporated in the design and if it is not then SSD must be incorporated. On single carriageways, it is usually necessary to consider whether to design for overtaking only at crest curves since overtaking is not a problem on dual carriageways and visibility is usually more than adequate on single carriageways [Uren et al, 1989].

Figure 4.13: Sight distance over crest curves when a) S ≤ L and b) when S > L

Source: O’Flaherty (2002)

d) K-Values

In the past it was necessary to use the appropriate sight distance for the road type and design speed in question to calculate the minimum length of the vertical curve required. Nowadays, however, constants which greatly simplify calculations have been provided by the MoWH&C [Uren et al, 1989]. The minimum length of vertical curve L min for any given road is obtained from the formula.

 

….4.28

Where; K R A

= = =

constant obtained from MoWH&C standards (K = R/100) radius of curvature of the curve (in meters) algebraic difference in grade (%)

There are three categories of K-Values for crest curves (SSD and FOSD crest curve values obtained from Table 2.17) and one category of K-Values for sags obtained from Table 2.18. The K-Values obtained are derived from the sight distances as already discussed [MoWH&C, 2004].


62

62

Vertical Crest Curve Design and Sight Distance Requirements

63

Table 4.8: Minimum Radii for Crest Curves as Recommended by MoWH&C

Radius (m) R (R= K * 100) Stopping Overtaking (km/h) d es ir ab le m in im um d es ir ab le “ n o o ver tak in g” centreline markings 50 1100 600 11000 5500 80 4500 3000 32000 15000 100 10000 7000 65000 24000 Source: Uganda Road Design Manual, 2004 Speed

Table 4.9: Minimum Radii for Sag Curves as recommended by MoWH&C

Speed

Radius (m) R

(km/h)

Desirable

Minimum

50

600

400

80

1300

1000

100

2000

1500

Source: Uganda Road Design Manual, 2004

4.7.6

Vertical Crest Curve Design and Sight Distance Requirements

In calculating the minimum lengths of crest curves, two design conditions have to be considered namely: • Where the sight distance is contained within the length of the vertical curve i.e. the sight distance is less than the length of curve. • Where sight distance overlaps onto the tangent sections on either side of the vertical curve. In this case the sight distance extends beyond the vertical curve [TRL, 1993]. Considering the properties of the parabola: For S ≤ L;

 200      

….4.29



For S > L;

 200         2  

….4.30

Where; Lmin S A h1 h2

= = = = =

minimum length of vertical crest curve (m) required sight distance (m) Algebraic difference in gradients Driver eye height (m); taken as 1.05m Object height (m); taken as 0.26m

For full overtaking sight distance, FOSD, h 1 = h 2 = 1.05m. The decision to which equation should be used at a given site can be made by solving either of the equations below;

   8    8


….4.31 63

63

Vertical Sag Curve Design and Sight Distance Requirements

64

If e > h1 then equation (4.29) is used and when e < h 1, equation (4.30) is used [O’Flaherty, 2002].

4.7.7


a) Based on clearance from structures during day time

In certain situations when a road passes beneath structure such as an over pass or bridge on sag curves, the primary design criterion for designing the sag curve may be the provision of necessary clearance in order to maintain the drivers’ line of sight [Rogers, 2003]. The minimum length of sag curve which meets minimum stopping sight distance requirements is given by; When SSD ≤ L Lmin =

2

AS

⎡ h1 + h2 ⎤ ⎢8 D − 8⎜ 2 ⎟ ⎥ ⎝ ⎣ ⎦

....(4.33)

When SSD > L

⎡ h1 + h2 ⎤ ⎥ ⎣ 2 ⎦

2S − 8⎢ Lmin =

A

....(4.34)

Where; h1 h2 L A D

=` = = = =

Drivers eye height (Usually 1.05m) Object height (usually 0.26m) minimum length of sag curve (m) algebraic difference in grades expressed as a decimal. vertical clearance (ideally taken as 5.7m) to the critical edge of the bridge The critical edge is assumed to be directly over the point of intersection of tangents. In practice both equations can be considered valid provided that the critical edge is not more than 60m from the point of intersection [O’Flaherty, 2002]. b) Based on night time Conditions During night time conditions, a critical concern in design of sag curves can be the headlight sight distance, where the length of the highway illuminated by the cars headlight is the governing parameter [Rogers, 2003]. The minimum length of sag curve is thus given by:

For SSD ≤ L Lnight =

AS 2

200(h3 + S tan α )

....(4.35)

For SSD > L L night = 2 S −

200 ( h3 + S tan α ) A

....(4.36)

Where h3 = headlight height (usually 0.6m above the carriageway), α = angle of upward divergence of light beam (usually 1.0 o), and L, A, and S are as defined previously. It should however be noted that the above equations (based on night time conditions) are;


64

64


65

•

Very sensitive to the assumption of a 1degree upward divergence of the light beam; • They erroneously assume that headlights can illuminate an object on the carriageway at long distances and they ignore the fact that many vehicles are driven on dipped lights; and • The effect of headlamps is reduced on horizontal curves [O’Flaherty, 2002]. c) Based on Motorist Comfort The minimum length of vertical sag curve is given by: Lmin =

AV 2

13 a

=

AV 2

390

....(4.37) Where; V = design speed (km/hr), A is the algebraic difference in grade (%), and a = vertical radial acceleration (m/s2) usually taken as 0.3 m/s 2 for comfortable design [O’Flaherty, 2002]. d) Design Speed and Speed Limit Design speed is a measure of road quality. It may be defined as the maximum safe speed that can be maintained at a given section of the road where conditions are so favourable that the design features of the road govern vehicular movements [MoWH&C, 2004]. The selection of design speeds for road sections of a particular classification is primarily influenced by; • Nature of terrain:- whether level, rolling or mountainous; and • Motorist expectations: - in relation to free speed at which it is safe to drive (in rural areas) or legal to drive (in urban areas) [O’Flaherty, 2002].

Speed limit on the other hand is the maximum allowable speed on a road. The normal speed limit on rural roads in Uganda is 80km/hr and that in trading centres, towns and cities is 50km/hr. Speed limits may be reduce but not increased by local speed limits shown on regulatory traffic signs [MoWH&C, 2004]. In a nutshell, the design speed should not be lower than the speed limit and should be preferably 10km/hr higher than the speed limit. Short rural sections with design speeds lower than the speed limit should be treated with warning signs and no overtaking markings [MoWH&C, 2004]. e) Length of Vertical Curve to be used Normally the value for minimum length of curve obtained from the K-Value is not used. A greater value is instead chosen. This may be due to the necessity to fit the curve into particular site conditions and the necessity to fit the vertical alignment of the road to the horizontal alignment (a process known as phasing of vertical and horizontal alignment) [Uren et al, 1989]. f) Phasing of the Vertical and Horizontal Alignment Phasing is usually done when designing new roads or improving existing alignments and follows the procedure below; • Designing or redesigning the horizontal alignment; • Taking reduced levels at regular intervals along the proposed centreline and plotting a longitudinal profile; • Superimposing chosen gradients on the longitudinal section, altering their percentage as necessary to try to balance out any cut and fill in addition to trying to get the vertical tangent points to coincide with those of the horizontal curve. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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65

General Controls for Vertical Curve Alignment

66

It is this last point that often gives the length of vertical curve in order to avoid the creation of optical illusions in the vertical plane [Uren et al, 1989].

g) Setting-Out Data In setting out a vertical curve on ground, the objective is to place large pegs at the required intervals along the line of the proposed roadway and to nail a cross-piece to each peg at a certain height (usually 1.0m), above the proposed road level. These pegs are called profiles and the erection of these profiles is the standard method of setting out proposed levels on any construction site. The following information is required for any setting out calculations; the length of the curve (which is dependent on the gradient of the straights and site distance) and the gradients of slopes together with one change point preferably a point of vertical intersection [Irvine, 1998].

4.7.8


The following general controls for vertical alignment should be kept in view while designing the vertical profile of a highway: a) The grade line selected should be smooth with gradual changes, consistent with the class of highway and terrain. Numerous breaks and short lengths of grades should be avoided; b) The ‘roller-coaster’ or ‘hidden type’ of profile should be avoided as it is hazardous and aesthetically unpleasant; c) Undulating grade line, involving substantial lengths of momentum grades, should be appraised for their effect upon traffic operation. Such profiles permit heavy trucks to operate at higher overall speeds than when an upgrade is not preceded by a down grade, but may encourage excessive speeds of trucks with consequent hazard to traffic; d) A broken-back grade line (two vertical curves in the same direction separated by short section of tangent grade) should generally be avoided; e) On long continuous grades, it may be preferable to place the steepest grades at the bottom and flatten the grades near the top. Alternatively, long grades may be broken by short intervals of flatter grades; f) Intersections on grades should be avoided as far as possible. Where unavoidable, the approach gradients and the gradient through the intersections should be flattened to the maximum possible extent. Vertical Curve Examples Question one The elevation of an intersection of rising gradient of 1.5% and a falling gradient of 1.0% on a proposed road is 93.600m AOD. Given that the K-Value for this particular road is 55, the through stationing of the intersection point is 0 + 671.340 and the vertical curve is to have equal tangent length. Calculate: a) The through stationing of the tangent points of the vertical curve if the minimum required length is to be used. b) The elevations of the tangent points and the elevations at exact 20m multiples of through stationing along the curve. c) The position and level of the highest point on the curve.


66

66

General Controls for Vertical Curve Alignment Solution 1.0 Data Summary a) Grades; Initial, m Final, n b) Point of Intersection Stationing of PVI Elevation of PVI c) K-value

= =

1.5% -1.0%

= = =

0 + 671.340 93.600m AOD 55

67

2.0 Sketch Drawing

3.0 Length of Vertical Curve, L L = KA Where; A = m–n = Therefore; L = 55(2.5) = 137.500m

(+1.5%) – (-1.0%)

=

2.5%

4.0 Stationing and Elevation of PVC and PVT Stationing of PVC = Stationing of PVI – 0.5L = (0+671.340) – 0.5(137.500) = 0+602.590

Elevation of PVC

= = =

Elevation of PVI – m.L/200 93.600 – 1.5(137.500)/200 92.570m AOD

Stationing of PVI =

Stationing of PVI + 0.5L = (0+671.340) + 0.5(137.500) = 0+740.090

Elevation of PVI

= = =

Elevation of PVI – n.L/200 93.600 – 1.0(137.500)/200 92.910m AOD

5.0 Table of Results

     100  200  92.5700.015x0.000091

67

From which the table below is derived Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

67

General Controls for Vertical Curve Alignment Stationing Sta. 0+602.590 (PVC) 0+620.000 0+640.000 0+660.000 0+680.000 0+700.000 0+720.000 0+740.000 0+740.090 (PVI)

68

Chord Length Curve Length Elevation x (m) Elev. 0.000 0.000 92.570 17.410 17.410 92.804 20.000 37.410 93.004 20.000 57.410 93.131 20.000 77.410 93.186 20.000 97.410 93.168 20.000 117.410 93.077 20.000 137.410 92.913 0.090 137.500 92.912

Checks: Xmax = L = 137.500m RLLast = RLPVT = 921,912m AOD 6.0 Position and Level of the Highest Point on the Curve Since;



its OK its OK

     100  200     x  0;  x  82.5m  100 100 Therefore;

This means that the highest point is located 82.5m from PVC i.e. at station (0+602.590) + 82.5 = 0+685.090 The elevation of the highest point is located at x = 82.5m, therefore from the above equation

82.5 2.5 82.5  93.189m AOD . 92.570 1.5 100 200

Question Two An equal tangent vertical curve is to be constructed between grades of -2.0% (initial) and +1.0% (final). The PVI (Point of vertical intersection) is at station 11 + 000.000 and elevation 420.000m AOD. Due to a street crossing, the elevation of the roadway at station 11 + 071.000 must be at elevation 421.500m. Design the curve assuming it has a shape of the form; y = ax 2 + bx + c . Solution 1.0 Data Summary a) Type of vertical curve b) Grades; Initial, m Final, n c) Point of Intersection, PVI Stationing Elevation d) Point of Interest Stationing Elevation

: = =

Equal tangent -2.0% +1.0%

= =

11+000.000 420.000m AOD

= =

11+710.000 421.500m AOD 68


68


69

2.0 Sketch Drawing

Note: There is need to determine, L such that station 11+071.000 is at elevation 421.500m AOD 3.0 Solving the Parabolic Equation for constants a, b and c The parabolic equation is of the form;

 bxc  2 b m  b  0.02  2 0b  100 n  a   nm  1.02.0  0.015  2 b  100 200L 200L L    4200.50.02   0.0L15  0.02x 4200.01L At PVC; x = 0

At PVT; x = L

The elevation at PVC; c = elevation at PVI + 0.5mL

….i ….ii ….iii ….iv ….v

Equations (iii), (iv) and (v) in equation (i) gives;

4.0

….vi

Determination of the Length of Vertical Curve, L based on the ‘Point of interest’ The point of interest (Sta. 11+071.000) is 71m from PVI (Sta.11+000.000). Hence, from PVC, this point is located at:

  0.5 71   0.0L15 0.5 71 0.02 0.5 71  4200.01L  421.500 0.00375  1.855L75.615  0 4 0.0037575.615 1 . 8 55 1 . 8 55   20. 0 0375  44.825,449.842 Equation (vii) in (vi) for L gives;

….vii

Multiply through by L and simplify to obtain

Solving for L gives;

44.825m is not feasible since the point of interest is 71m beyond PVI, therefore L = 449.842. This means that the point of interest is located x = 0.5(449.842) +71 = 295.921m from PVC.


69

69

Cross-Sectional Elements 5.0

70

Stationing and Elevation of PVC and PVT Stationing of PVC = Stationing of PVI – 0.5L = (11+000.000) – 0.5(449.842) = 10+775.079

Elevation of PVC

= = =

Elevation of PVI + m.L/200 420 + 2.0(449.842)/200 424.498m AOD

Stationing of PVT

= = =

Stationing of PVI + 0.5L (11+000.000) + 0.5(449.842) 11+224.921

Elevation of PVT

= = =

Elevation of PVI + n.L/200 420 + 1.0(449.842)/200 422.249m AOD

Exercise A 150m long equal tangent vertical curve connecting grades of +1.2% (initial) and -1.08% (final) crosses a one-meter diameter pipe at right angles. The pipe is located at station 11 + 025.000 and its centerline is at elevation 1091.6m. The PVI of the vertical curve is at station 11 + 000.000 and elevation 1095.2m. Using offsets determine the depth, below the surface of the curve, to the top of the pipe and determine the station of the highest point on the curve.

4.8

Cross-Sectional Elements

4.8.1

General

The cross-sectional elements of a highway design pertain to those features which deal with its width. They embrace aspects such as road reserve width, carriageway width, central reservation (median), shoulders, camber, side slopes, horizontal and vertical clearances etc. carriageway support strip shoulder lateral clearance fill or embankment

traffic lane

camber %

right-of-way boundary

traffic lane

camber %

shoulder lateral clearance

edge strip for road markings

cut

catch drain

back or outer slope fore or inner table drain slope

natural terrain road prism

support strip

roadside area

verge

safety zone

verge road reserve

Figure 4.14: Single Carriageway Cross-section Elements


70


70

Road Reserve

71

divider kerbed footway

edge strip

traffic lanes outer

edge strip

median

edge strip

traffic lanes

edge strip

separate footway/ cycleway

inner camber %

outer hard shoulder

camber %

inner hard shoulder

outer hard shoulder

Figure 4.15: Dual Carriageway Cross-section Elements


4.8.2

Road Reserve

The road reserve or right-of-way width is the width of land secured and preserved in public interest for road development purposes. The road reserve should be adequate to accommodate all the elements that make up the cross-section of the highway and may reasonably provide for future development. In order to prevent ribbon development along highways, it is sometimes necessary to establish ‘control lines’ and ‘building lines’. A ‘control line’ is a line which represents the nearest limits of future uncontrolled activity in relation to a road. This signifies that though building activity is not totally banned between the building line and the control line, the nature of buildings permitted here is controlled. A ‘building line’ on the other hand is a line on either side of the road between which no building activity is permitted at all.

4.8.3

Carriageway Width

The term “carriageway” is used here to cover the traffic lanes, any auxiliary lanes, and the shoulders [MoWH&C, 2004]. The width of traffic lanes governs the safety and convenience of traffic and has a profound influence on the capacity of a road. The factors that influence capacity of a carriageway are: a) The design volume, i.e. the greater the traffic volume the wider the carriageway and, normally, the greater the number of lanes; b) Vehicle dimensions, i.e. heavy commercial vehicles require wider carriageways to ensure adequate clearances when passing each other; c) The design speeds, i.e. vehicles travelling at high speed, especially commercial vehicles, require wider carriageways to ensure safe clearances between passing vehicles; d) The road classification, i.e. the higher the road classification the greater the level of service (and width of carriageway) expected. Internationally, it is generally accepted that lane widths should normally be at least 3.5m, although narrower lanes are often used for economic or environmental reasons on both rural and urban roads. However, increasing the lane width up to 3.65m on two lane two way rural roads decreases accident rates [O’Flaherty, 2002]. 71 4.8.4

Central Reservation (Median) Strip

A central reservation strip is the longitudinal space separating dual carriageways. The functions of the median strip are: Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

71

Shoulders

72

a) To separate the opposing streams of traffic; b) To minimise head-light glare; c) To include space for safe operation of crossing and turning vehicles at intersections at grade; d) To provide a stopping area in case of emergencies. The central reservations on high-speed heavily trafficked rural roads in the United States are typically 15m to 30m. In Europe they tend to be much narrower (say 4 – 10m) and to be used with safety barriers. Those in Britain are normally 4.5m wide, and include a crash barrier. In urban areas they can be as narrow as 1m, but 3m is preferred so that a crossing pedestrian pushing a pram or wheelchair has space to wait in safety [O’Flaherty, 2002]. On severely restricted arterial streets, where a narrow separator of 0.6 – 1.2m is feasible, it may be desirable to have few, if any, openings in median except at intersections.

4.8.5

Shoulders

A shoulder is a portion of the roadway adjacent to the carriageway and is intended for accommodation of stopped vehicles, emergency use and lateral support of base and surface courses. The width of the shoulder should be adequate for giving working space around a stopped vehicle. American practice recommends a 3m width for high type facility and a width of 1.2m -2.4m for low type facilities. UK practice for rural roads recommends widths ranging from 1.2m to 3.65m depending upon the road type and nature of kerb treatment.

4.8.6

Laybys and bus bays

When economic considerations do not favour the construction of shoulders on rural roads, laybys should be provided instead, at spacings that are appropriate to the traffic volume. Thus, for well trafficked and lightly trafficked single carriageways, it is British practice to provide 2.5m and 3m wide by 30m long laybys at 1.5km and 5.8km intervals, respectively, on either side of the carriageway, while 3m wide by 100m long laybys are provided at approximately 1km intervals on each side of dual carriageways. Laybys should be located at sites with good visibility and provided with tapered hard-strips at either end to assist in the safe deceleration and acceleration of vehicles using them. Full bus bays (3.25m by at least 12m, plus 20m end tapers) may be provided at bus stops in urban areas; however, the appropriateness of this provision is dependent on the traffic volumes on the road in question.

4.8.7

Kerbs

A kerb (as termed as curb) is a vertical or sloping member along the edge of a pavement or shoulder, forming part of gutter, strengthening or protecting the edge, and clearly defining the edge to vehicle operators. Its functions are: a) b) c) d) e)

To facilitate and control drainage; To strengthen and protect the pavement edge; To delineate the pavement edge; To present a more finished appearance; To assist in the orderly development of the roadside.


72

72

Camber

73

Kerbs are classified as ‘barrier’ or ‘mountable’. Barrier kerbs are designed to discourage vehicles from leaving the pavement. The face may be vertical or sloping and the height may range from 15cm to 25cm. Mountable kerbs are those which can be easily crossed by vehicles if required. They are used at medians and channelizing islands.

4.8.8

Camber

Camber, also known as cross fall, facilitates drainage of the pavement laterally. The pavement can have a crown or a high point in the middle with slopes downwards towards both edges. This is favoured on two-lane roads and wider undivided roads. On divided roads, the individual carriageways may be centrally crowned separately or a unidirectional slope may be provided across the entire carriageway width. The amount of camber to be provided depends upon the smoothness of the surface and the intensity of rainfall. In the UK, a value of 2.5% is generally adopted for design. A cross fall for the shoulders should be generally steeper than for the pavement by about 0.3 – 0.5% to facilitate quick drainage. The UK practice is to provide 5% slope on the shoulder [Kadiyali, 2006].

4.8.9

Side slope

According to O’Flaherty (2002), soil mechanics analysis enables the accurate determination of maximum slopes at which embankments or cuts can safely stand. However, these maximum values are not always used, especially on low embankments not protected by safety fences. The slopes of embankments and cut sections depend upon the type of soil and the height of embankment or depth of cuttings. A flatter slope is conducive for erosion control, but is costly. Flatter slopes of embankments promote safety of traffic. Ordinarily, 1.5:1 to 2:1 in mild slope conditions and 2:1 to 3:1 in overwhelming slope conditions w ill be adequate.

4.9

Intersection Design and Capacity

4.9.1

General

An intersection is defined as the general area where two or more highways join or cross, within which are included the roadway and roadside design features which facilitate orderly traffic movements in that area. An intersection leg is that part of any one of the highways radiating from an intersection which is outside of the area of the intersection. The importance of intersection design stems from the fact that efficiency of operation, safety, speed, cost of operation and capacity are directly governed by the design. Since an intersection involves conflicts between traffic in different directions, its scientific design can control accidents and delay and can lead to orderly movement of traffic. Intersections represent potentially dangerous locations from the point of view of traffic safety. It is believed that well over half the fatal and serious road accidents in built-up areas occur at junctions [Kadiyali, 2006]. The following principles should be considered in a good design: a) The number of intersections should be kept to a minimum. If necessary, some minor roads may be connected with each other before joining a major road;


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At-grade and Grade Separated Junctions

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b) The geometric layout should be so selected that hazardous movements by drivers are eliminated. This can be achieved by various techniques such as channelizing and staggering; c) The design should permit the driver to discern quickly either from the layout or from traffic signs about which path he/she should follow and the actions of merging and diverging. This can be achieved by good layout, traffic islands, signs and carriageway markings. Good visibility improves safety; d) The layout should follow the natural vehicle paths. Smoothness, in contrast to abrupt and sharp corners, should guide minor streams of traffic into stopping or slowing down positions; e) The number of conflict points should be minimised by separating some of the many cutting, merging or diverging movements; f) Vehicles that are forced to wait in order to cross a traffic stream should be provided with adequate space at the junction.

4.9.2

At-grade and Grade Separated Junctions

An intersection where all roadways join or cross at the same level is known as an at-grade intersection. An intersection layout which permits crossing manoeuvres at different levels is known as a grade separated intersection. The choice between an at-grade and grade separated intersection at a particular site depends upon various factors such as traffic, economy, safety, aesthetics, delay etc. Grade-separated junctions generally are more expensive initially, and are justified in certain situations. These are: a) On high type facilities such as expressways, freeways and motorways; b) Certain at-grade intersections which have reached the maximum capacity and where it is not possible to improve the capacity further by retaining the at-grade crossing; c) At certain locations which have a proven record of bad accident history when functioning as at grade junctions; d) At junctions where the traffic volume is heavy and the delays and economic loss caused justify the provision of grade-separation; e) At certain specific topographical situations where it is logical to provide a gradeseparated structure rather than an at-grade intersection, which may involve considerable earthwork or acquisition of land.

4.9.3

Basic Forms of At-grade Intersections

Intersections can be divided into the basic forms shown in Figure 4.16 below. From a design aspect these intersections can also be divided according to whether they are controlled, priority controlled (stop, Give Way), space-sharing (i.e. roundabouts), time sharing (i.e. traffic-signal controlled), or grade-separated (including interchanges) [O’Flaherty, 2002].

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74

Overview of the Design Process

75

Figure 4.16: Basic Intersection Forms


4.9.4

Overview of the Design Process

The at-grade intersection design process involves data collection of both traffic and site conditions, the preparation of preliminary designs from which a layout is selected, and the development of the final design using appropriate design standards. Traffic data gathered for design purposes normally include peak period volumes, turning movements and composition for the design year, vehicle operating speeds on the intersecting roads (these are needed for sight-distance/ speed-change lane design) pedestrian and bicycle movements (these affect the layout/traffic control design), public transport needs (e.g. bus priority measures and bus stop locations affect the layout/traffic control design), special needs of oversize vehicles (the selected design may have to cope with the occasional heavily loaded commercial vehicle with a wide turning path), accident experience (if an existing intersection is being upgraded), and parking practices (especially in built-up areas). Site data collected typically include topography, land usage, and related physical features (natural and manufactured), public and private utility services (above and below ground), items of special interest (e.g. environmental, cultural and historical features), horizontal and vertical alignments of intersecting roads (existing and future), sight distances (and physical features which limit them), and adjacent (necessary) accesses.

The preliminary design phase is essentially an iterative one. It involves preparing a number of possible intersection layouts and generally examining each in terms of its operating characteristics (especially safety and capacity), ease of construction and likely capital cost, and environmental and local impacts that might affect the design selection. The most promising of the rough layouts are then selected for further development and analysis (including road user and vehicle operating costs, if appropriate), refined and examined in greater detail until that considered most suitable for the intersection is selected for detailed design and preparation of final construction plans and specifications [O’Flaherty, 2002]. 75 4.9.5

At-grade Intersection Types (from a design perspective)

Different at-grade junction (intersection) types will be appropriate under different circumstances depending on traffic flows, speeds, and site limitations. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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a)

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An Access According to MoWH&C (2005), an access is defined as the intersection of an unclassified road with a classified road and shall generally be provided within the road reserve boundary of the classified road. Access roads are used to connect pro perties etc. to the road network. Accident risk increases with the frequency of access roads, so they should, as far as possible, be discouraged on higher classes of roads. The lay out and location of the access must satisfy the visibility requirement for "stop” conditions given in Figure 4.17 below.

Figure 4.17: Typical Access Layout showing Visibility Requirements


b) A Junction or an Intersection A junction is the intersection of two or more classified roads on the same surface / at grade. At grade intersections can be classified in to two main intersection categories based on the type of control used. For each category, there are a number of intersection types as shown below.

Table 4.10: Types of At-grade Intersections as recommended by MoWH&C

Intersection category

Priority intersection

Control intersection

Traffic control Major road

Minor road

Priority

Stop or give way sign

Traffic signals or give way sign

Intersection types

A Unchannelised T-intersection B Partly Channelised T-intersection C Channelised T-intersection D E

Roundabout Signalised intersection


i)

Priority Intersections Priority intersections will be adequate in most rural situations. Three types of T intersections are given below:

76 Unchannelised T-Intersection (A) The unchannelised design is suitable for intersections where there is a very small amount of turning traffic . It is the simplest design and has no traffic islands (see Figure 4.18). Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Partly channelised T-Intersection (B) The partly channelised design is for intersections with a moderate volume of turning traffic. It has a traffic island in the minor road arm. In urban areas, the traffic island would normally be kerbed in order to provide a refuge for pedestrians crossing the road. Channelised T-Intersection (C) The fully channelised design is for intersections with a high volume of turning traffic or high –speeds. It has traffic islands in both the minor road and the main road.

Unchannelised

Partly channelised

Channelised

Figure 4.18: Typical T-Intersections


The crossroads form of priority intersection must not be used . It has a very high number of conflict points, and has a much higher accident risk than any other kind of intersection. Existing crossroads should, where possible, be converted to a staggered intersection, or roundabout, or be controlled by traffic signals [MoWH&C, 2005]. ii)

Control Intersections Control intersections are mostly used in towns and trading centres. However, roundabouts can be used in rural areas in intersections between major roads or other intersections with high traffic volumes. There are two types of control intersections:

Roundabout (D) Roundabouts are controlled by the rule that all entry traffic must give way to circulating traffic. The ratio of minor road incoming traffic to the total incoming traffic should preferably be at least 10 to 15%. Roundabouts can be of normal size, i.e. with central island radius 10 m or more, or small size, i.e. with central island radius less than 10 m (see Figure 4.18). Signalised Intersection (E) Signalised intersections have conflicts separated by traffic signals. No conflicts are allowed between straight through traffic movements. Typical design of control intersections is shown in Figure 8.3.


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77


Roundabout

78

Signalised intersection

Figure 4.19: Typical Designs for Control Intersections


c)

Design Requirements The design of at-grade junctions must take account of the following basic requirements:

• safety • operational comfort • capacity • economy i) Safety and Operational Comfort A junction is considered safe when it is perceptible, comprehensible and manoeuvrable. These three requirements can generally be met by complying with the following guidelines.

Perception • The junction should be sited so that the major road approaches are readily visible; • Early widening of the junction approaches; • The use of traffic islands in the minor road to emphasize a “yield” or “stop” requirement. • The use of early and eye-catching traffic signs; • Optical guidance by landscaping and the use of road furniture, especially where a junction must be located on a crest curve; • The provision of visibility splays which ensure unobstructed sight lines to the left and right along the major road; • The angle of intersection of the major and minor roads should be between 70 and 110 degrees; • The use of single lane approaches is preferred on the minor road in order to avoid mutual sight obstruction from two vehicles waiting next to each other to turn or cross the major road. Comprehension • The right of way should follow naturally and logically from the junction layout; • The types of junctions used throughout the whole road network should be as much as possible similar; Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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•

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The provision of optical guidance by the use of clearly visible kerbs, traffic islands, road markings, road signs and other road furniture.

Manoeuvrability • A1l traffic lanes should be of adequate width for the appropriate vehicle turning characteristics. To accommodate truck traffic, turning radii shall be 15 meters minimum; • The edges of traffic lanes should be clearly indicated by road markings; • Traffic islands and kerbs should not conflict with the natural vehicle paths. ii) Capacity The operation of uncontrolled junctions depends principally upon the frequency of gaps which naturally occur between vehicles in the main road flow. These gaps should be of sufficient duration to permit vehicles from the minor road to merge with, or cross, the major road flow. In consequence junctions are limited in capacity, but this capacity may be optimized by, for example, channelisation or the separation of manoeuvres. iii) Economy An economical junction design generally results from a minimization of the construction, maintenance and operational costs.

Delay can be an important operational factor and the saving in time otherwise lost may justify a more expensive, even grade separated, junction. Loss of lives, personal injuries and damage to vehicles caused by junction-accidents are considered as operational "costs" and should be taken into account. The optimum economic return may often be obtained by a phased construction, for example by constructing initially an at-grade junction which may later become grade separated [MoWH&C, 2005].

d) Selection of Intersection Type i) General These selection guidelines provided by MoWH&C mainly deal with traffic safety. The ministry recommends that other important impacts such as capacity/road user costs, environmental issues, investment and maintenance costs should also be taken into consideration.

The selection is divided into two steps; selection of intersection category (priority or control) and selection of intersection type. It is based on the following assumptions: Priority intersections can be safe and give sufficient capacity for certain traffic volumes • and speed limits;

•

If a priority intersection is not sufficient for safety and capacity, the major road traffic must also be controlled. Depending on location, traffic conditions and speed limits, different types of priority or control intersection should be selected.

ii) Selection of Intersection Category Based on Safety The selection of intersection category should mainly be based on safety. The selection can be made by using diagrams with the relationships between the safety levels and the average annual daily approaching traffic volumes (AADT in veh/day) based on accident statistics. The diagrams shown in Figure 4.20 are for T-intersections on 2-lane roads with 50, 80 and 100 km/h speed limit. The diagrams are, as already stated, based on general European Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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experience on relationships between speed, safety and traffic flows. They are judged reasonable to be used in Uganda until sufficient local research is available. Minor road approaching AADT, Q3 veh/day 6000

50 km/h

Q3

Q1

Minor road approaching AADT, Q3 veh/day 3000

Select control intersection

Select control intersection

Q2

4000

80 km/h Q3 Q1

2000

Q2

Consider control intersection

Consider control intersection

2000

1000 Select priority intersection

Select priority intersection

5000 10000 Major road approaching AADT, Q1+Q2 veh/day


Minor road approaching AADT, Q3 veh/day

100 km/h

3000

2000

Select control intersection Consider control intersection

Q3 Q1

Q2

1000 Select priority intersection


Figure 4.20: Selection of Intersection Category based on Safety


Based on Capacity The selection of intersection category based on safety should be checked for capacity. It can be made by using diagrams with the relationships between the capacity and the approaching traffic volumes during the design hour (DHV in pcu/design hour). The diagrams shown in Figure 4.21 are for T-intersections on 2-lane roads with 50, 80 and 100 km/h speed limit. The desired level refers to a degree of saturation (actual traffic flow/capacity) of 0.5. The acceptable level refers to a degree of saturation of 0.7. The diagrams are based on Swedish capacity studies with findings similar to other European countries. It is judged reasonable to be used in Uganda until sufficient Ugandan research is available. Minor road approaching DHV, Q3 pcu/design hour

400

50km/h Acceptable Control or gr ade-separated intersection needed

Minor ro ad approaching DHV, Q3 pcu/design hour

80km/h 400

Control or grade-separated intersection needed

Acceptable

Desired

Desired

200

200 Q3

Q3 Q1

Q2

Q1

0

Q2

0

0 0 1500 500 1000 500 1000 1500 Major road approaching DHV,Q1+Q2 pcu/design ho ur Major road approaching DHV,Q1+Q2 pcu/design hour

80


80


81

Minor ro ad approaching DHV, Q3 pcu/design hour 400

100km/h Acceptable Control or grade-separated intersection needed

200 Desire d Q3 Q1

0

0

Q2

500

1000

1500

Major road approaching DHV,Q1+Q2 pcu/desig n ho ur Figure 4.21: Selection of Intersection Category based on Capacity


iii) Selection of Intersection Type Priority intersections The selection of priority intersection type should mainly be based on safety. The selection can be made by using diagrams with the relationships between the safety levels and the average annual daily approaching traffic volumes (AADT in veh/day) based on accident statistics. The diagrams shown in Figure 4.22 are for T-intersections on 2-lane roads with 50, 80 and 100 km/h speed limit. Crossroads should be avoided. The number of right turners should obviously also impact the decision. The diagrams are based on general European findings on safety effects of right turn lanes. It is judged reasonable to be used in Uganda until sufficient Ugandan statistics are available. Note however they are only a starting point for determining the most appropriate form of intersection.

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Minor road approaching AADT, Q3 veh/day 6000

50 km/h

Q3 Q1

Select channelised T-intersection

Q2

4000

2000

Select unchannelised or partly channelised T-intersection

5000 10000 Major road approaching AADT, Q1+Q2 veh/day Minor road approaching AADT, Q3 veh/day 3000

80 km/h

Q3 Q1

Q2

2000 Select channelised T-intersection

1000 Select u nchannelised or partly channelised T-intersection

5000 10000 Major road approaching AADT, Q1+Q2 veh/day Minor road approaching AADT, Q3 veh/day

100 km /h

1500

Q3

1000

Q1

Q2

Select channelised T-intersection

500 Select unchannelised or partly channelised T-intersection


Figure 4.22: Selection of Priority Intersection type based on Safety


Partly channelised T-intersections should normally be used if needed to facilitate pedestrian crossings and also if the minor road island is needed to improve the visibility of the intersection. Control intersections Roundabouts are suitable for almost all situations, provided there is enough space. Roundabouts have been found to be safer than signalised intersections, and are suitable for both low and medium traffic flows. At very high traffic volumes they tend to become blocked due to drivers failing to obey the priority rules. Well-designed roundabouts slow traffic down, which can be useful at the entry to a built-up area, or where there is a significant change in road standard, such as the change from a dual carriageway to a single carriageway.


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Capacity of a T-Junction

83

Traffic signals are the favoured option in the larger urban areas. Co-ordinated networks of signals (Area Traffic Control) can bring major improvements in traffic flow and a significant reduction in delays and stoppages. However, they must be demand-responsive, in order to get the maximum capacity from each intersection. Observance of traffic signals by Ugandan drivers is reasonably good, and could be improved through enforcement campaigns. For some traffic distributions, for example high traffic volumes on the major road, the total delay can be shorter in a signalised intersection than in a roundabout. The diagram in Figure 8-8 shows the traffic conditions for which signalised intersections are most suited, based on Kenyan and UK experience. Minor r oad appro aching AADT, veh/day 15 000

10 000 Interchange needed Rounda bout

5 000 Consider Sign alised Intersection

0 0

10 000

20 000

30 000

Major road approaching AADT, veh/day Figure 4.23: Selection of Control Intersection Type


If a signalised intersection is considered due to planning conditions or traffic volumes, a capacity analysis and economic analysis should be made. This should include road construction and maintenance costs, accident costs, travel time costs, vehicle operating costs and environmental costs [MoWH&C, 2005].

4.9.6


The capacity of a T junction is primarily dependent upon the ratio of the flows on the major and minor roads, the critical (minimum) gap in the main road traffic stream acceptable to entering traffic and the maximum delay acceptable to minor road vehicles. As traffic builds up on the main road, headways between vehicles decline, fewer acceptable gaps become available, and delays to vehicles on the minor road increase accordingly, theoretically to infinity. Field measurements on single carriageway roads indicate that the critical time gaps accepted by minor road vehicles at the head of a queue average about 3 seconds for left turn merging with, and 4 to 5 seconds for right turn cutting off, the traffic stream in the nearside lane of the main road. Empirical research has resulted in predictive capacity equations for Tintersections, which were derived from traffic flow measurements and from certain broad features of junction layout. A T-intersection has six separate traffic streams (see Figure 4.24 below), of which the through streams on the major road (C-A and A-C) and the left-turn stream off the major road (A-B) are generally assumed to be priority streams and to suffer no delays from other t raffic, Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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while the two minor road streams (B-A and B-C) and the major road right-turn stream (C-B) incur delays due to their need to give way to higher priority streams. Predictive capacity equations for the three non-priority streams are as follows:

  62714 0.364  0.114   0.229  0.520   … .4.38    7450.364  0.144  ….4.39   7450.364  ….4.40   10.0345 ….4.41 Where;

  1 0.094  3.6510.0009 120 1 0.0061 0.006 150  4.42

  10.094 3.6510.0009  120    10.094 3.6510.0009 120 

….4.43 ….4.44

Figure 4.24: Selection of Control Intersection Type


• • • • • •

The superscript s (e.g. q sB-A) denotes the flow from the saturated stream i.e. one in which there is stable queuing. The geometric parameters w B-A and wB-C denote the average widths of each of the minor road approach lanes for waiting vehicles in streams B-A and B-C respectively, measured over a distance 20m upstream from the Give Way line; wC-B denotes the average width of the right-turn (central) lane on the major road, or 2.1m if there is no explicit provision for right turners in stream C-B. The parameters VrB-A and V lB-C denote right and left visibility distances, respectively, available from the road; VrC-B is the visibility available to right-turning vehicles waiting to turn right from the major road; W is the average major road carriageway width at the intersection; in the case of ghost or raised islands, W excludes the width of the central (turning) lane;


84

84

Design Reference Flow (DRF)

•

85

WCR is the average width of the central reserve lane at the intersection on a dual carriageway road.

All capacities and flows are in passenger car units per hour (pcu/hr) and distances are in meters. One heavy vehicle is considered equivalent to two (2) pcu for calculation purposes. Capacities are always positive or zero; if the right-hand side of any equation is negative, the capacity is taken as zero. The ranges within which the geometric data are considered valid are as follows: w = 2.05-4.70m, V r = 17 – 250m, V l = 22 – 250m, W CR = 1.2 - 9m (dual carriageway sites only), W = 6.4 - 20m.

4.9.7

Design Reference Flow (DRF)

One of the methodologies used to assess the adequacy of the capacity available to a non priority traffic stream is the ratio of the design reference flow (DRF) to the capacity called the reference flow to capacity (RFC) ratio. For the satisfactory operation of any given approach lane it is generally considered that reference flow to capacity ratio should not exceed 0.85. DRF value considers the function of the road. The 200 th highest hourly flow in the design year is used on recreational roads, the 50 th highest hourly flow on interurban roads and the 30th highest hourly flow in the design year on urban roads. It would be economically and/or environmentally undesirable to design for the highest hours in the design year. For an existing intersection the DRF values are often determined from manual counts (including classifications and turning movements) of the existing flows which are grossed up to the design year using appropriate factors.

4.9.8

Delay

An estimate of the total 24 hour delay due to congestion, D 24x, at an existing T-intersection can be estimated from the empirically derived equation

 8

….4.45

Where; D3 = Total intersection delay (h) during the peak three hours, and P 3 = Ratio of flow in the peak three hours to the 24-hour flow. The above formula assumes that delays are inflicted only on minor road vehicles, which have to yield priority to the major road streams.

T-Junction Example A new industrial complex is planned to be sited adjacent to an existing priority intersection. The width of the main carriageway is 8m. The width of the carriageway for traffic movements B-A, B-C and C-B are 3, 3 and 2m respectively. The visibility distances at the drivers’ eye height for the junction are: V rB-A = 60m, V lB-A = 75m, VrB-C = 60m, V rC-B = 60m. The width of the central reservation is 2m wide. The design flows (in pcu/hr) are represented in the figure below.

85


85

Delay

86

You are required to determine the following: i) Calculate the capacities of the turning movements; q sB-A, q sB-C, q sC-B, for the priority intersection shown in the figure above. ii) Asses the arms of the junctions and advise on which arms have sufficient capacity and which ones do not.

Solution 1.0 Summary of Design Data W = 8m VrB-A = 60m WB-A = 3m VlB-A = 75m WB-C = 3m VrB-C = 60m WC-B = 2m VrC-B = 60m

qA-C = 800pcu/hr qA-B = 500pcu/hr qC-A = 800pcu/hr qC-B = 400pcu/hr

qB-A = 100pcu/hr qB-C = 400pcu/hr qC-B = 400pcu/hr WCR = 2m

2.0 Capacities of Turning Movements

  62714 0.364  0.114   0.229  0.520   . .    7450.364  0.144  ..    7450.364   ..   10.0345 .. 10.03458 0.7240  110.094   3.65 10.0009  120 1 0.0061 0.006  150  .. 0.0943 3.651 0.000960 1201 0.0061 0.00675150  0.4885   10.094 ..  3.6 510.0009  120  10.09433.6510.000960120  0.8882 10.0009  120      10.09423.65 10.094 3.6510.000960120 ..  0.7993 Where;

Substituting the above values in equation (i), (ii) and (iii), the required turning movement capacities can then be obtained as shown below;

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  0.4885 6271420.72400.3648000.1145000.2298000.520400  59/ Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Rotary Intersections (Roundabouts)

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  0.8882 7450.72400.3648000.144500  428/   07993 7450.3640.7240800500  322/ 3.0 Assessment of Junctions Arms The method used to assess the adequacy of the capacity available to a non priority stream is the design reference flow (DRF) to capacity ratio called RFC (i.e. Reference Flow to Capacity ratio). For satisfactory operation of any given approach lane, it is generally considered that RFC should not exceed 0.85. The critically affected arms are:

Arm B-A

       1.69  0.85     Arm B-C

    0.93  0.85     Arm C-B

   1.24  0.85     Based on the reference flow capacity ratios obtained, it is apparent that all the arms have exceeded their capacities and therefore need to be redesigned.

4.9.9


A roundabout is a form of channelization intersection in which vehicles are guided onto a one-way circulatory road about a central island. Entry to the intersection is controlled by Give Way markings and priority is now given to vehicles circulating (clockwise in Uganda) in the round about. The main objective of roundabout design is to secure the safe interchange of traffic between crossing traffic streams with the minimum delay. The operating efficiency of a roundabout depends upon entering drivers accepting headway gaps in the circulating traffic stream. Traffic streams merge and diverge at small angles and low relative speeds. For this reason, accidents between vehicles in roundabouts rarely have fatal consequences [O’Flaherty, 2002]. a) General Usage of Roundabouts Roundabouts are most effective as at grade intersections in urban or rural areas that have all or a number of the following characteristics:

• • • • • •

High proportions and/or volumes of right turning traffic; Priority is not given to traffic from any particular road; Presence of accidents involving crossing or turning movements; Traffic on the minor roads is delayed by the use of ‘Stop’ or ‘Give Way’ signs; Where they cause less overall delay to vehicles than traffic signals; Where there is a marked change in road standard e.g. from a dual to a single carriageway road.

Roundabout intersections are not appropriate at the following sites: Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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• • • • •

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Where there is inadequate space or unfavourable topography that limits a good geometric design; Where traffic flows are unbalanced, e.g. at major/ minor T-Intersections; Where they follow a downhill approach, the approach should be at least a 2% grade and should be flattened at least 30m to the intersection. Where there are heavy volumes of vehicular traffic and where there is heavy cyclist and pedestrian traffic. Between traffic controlled signal intersections which could cause queing back into the roundabout exits.

b) Types of Roundabouts In Uganda there are two types of roundabouts namely:

i)

ii)

Normal roundabouts with a centre island radius greater than or equal to 10m. The central island radius should normally be between 10m and 25m otherwise it becomes difficult to control speeds for a radius bigger than the above range and puts pedestrians and cyclists at risk. The width of the circulating carriageway depends on whether it is one or two lane. Small roundabouts with a central island less than 10m. The inner central island radius should be at least 2m.

c) Design Features of Roundabouts • For small roundabouts, the central island should be approximately 1/3 of the inscribed circle diameter (1/3D); • At larger sites the proportion should be >1/3 to limit the circulatory width to a maximum of 15m; • The circulatory width around the roundabout should be constant at about 1.0 to1.2 times the highest entry width subject to the above maximum of 15m; • Steep downhill gradients should be avoided at roundabout approaches; • The frequent occurrence of roundabouts on high speed rural roads should be avoided; • Mini roundabouts must only be used at existing junctions where there are space limitations and where the 85 percentile approach speed on all approaches is less than 50km/hr; • Entries should be flares. Single and two lane approaches should become 3 and 4 lanes respectively at the give way line; • The entry flare taper should be approximately 1 in 3. Each lane should be 2.5m to 3.5m wide at the give way line dependent on site conditions. The taper width at the Give Way line should never be less than 3m. The best entry angle is approximately 30 degrees. Lanes may be tapered to 2m width (minimum) on the roundabout approaches; • The entry width of an approach arm at a roundabout is one of the major factors apart from approach carriageway half width that affects capacity. Flares on the approaches to roundabouts should be designed in such a way that maximum entry widths are not greater than 10.5m on single carriageway roads and 15m on dual carriageway approach roads. A typical flare length on a rural road is 25m. The length can be as low as 5m on urban roads; • Pedestrian crossing places (including zebras) should normally be within the flared approach but as far from the Give Way line as pedestrian convenience will allow. This reduces the road width to be crossed by pedestrians. A central refuge should always be provided wherever possible. A deflection island may fulfil this function but should be at least 1.2m wide; Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Pedestrian guard rail should be used, where necessary to control haphazard pedestrian crossing of the traffic streams. It also improves safety.

d) Capacity of Roundabouts The capacity of a roundabout as a whole is a function of the capacities of the individual entry arms. The capacity of each arm is defined as the maximum inflow when the traffic flow at the entry is sufficient to cause continuous queuing in its approach road.

The main factors influencing entry capacity are the approach half width, and the width and flare of the entry, while the entry angle and radius also have small but significant effects. The predictive equation used with all types of single at-grade roundabouts is

 0         

Where;

Qe = Qc = k = F = f c = 60)/10].

….4. 4 6 ….4.47

saturation or capacity entry flow (pcu/h); circulating flow across the entry (pcu/h); 1 - 0.00347(ϕ-30) – 0.978[(1/r)-0.05]; 303x2; where; x2 = v + (e – v)/(1+2S) and S = 1.6(e –v)/l’; 0.210tD(1 + 0.2x2) where tD = 1 + 0.5/(1 +M) and M = exp[(D-

The symbols e, v, l’, S, D, ϕ and r are described in Table 4.11. Q e and Qc are in pcu/h, and one heavy goods vehicle is assumed equivalent to 2 pcu for computation purposes. Table 4.11: The Limits of the Parameters used in Roundabout Capacity Equation Geometric Paramete r S ymbol Unit Prac tical Limits Entry width e m 4 - 15 Approach half-Width v m 2 -7 .3 Average effective flare length l' m 1 - 100 Sharpness of flare S 0 - 2.9 Inscribed circle diameter D m 15 - 100 Entry angle ϕ deg 10 - 60 Entry radius r m 6 - 100 Source: O’Flaherty, 2002

From the roundabout equation above, entry capacity decreases as circulation flow increases. The sharpness of flare, S is a measure of the rate at which extra width is developed in the entry flare. Small ‘S’ values correspond to long gradual flares and big ones to short severe ones. The angle ϕ acts as an alternative for the conflict angle between the entering and circulating traffic streams. The entry radius, r is measured as the minimum radius of curvature of the nearest kerbline at entry. e) Design Reference Flow (DRF) When designing a roundabout intersection, the entry angle for each arm of a trial layout is compared with the hourly flow for the design (DRF). The reference flow to capacity ratio (RFC) is an indicator of the likely performance of an intersection under the future year traffic loading. If an RFC ratio of 0.85 occurs, it can be expected that queuing will automatically be avoided in the design year peak hour in five out of six cases.


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Roundabout Example The table below shows measured turning movements in the AM peak period as recorded in a traffic survey at a four arm roundabout. The survey was carried out in 2005. The expected rate of traffic growth is 2%. It is assumed that funding will be readily available and that if any redesign and reconstruction is needed, the roundabout will be reopened to traffic in the same year the survey was carried out. The roundabout is being assessed for capacity to carry peak flows in 2019. The geometric parameters for arms A and B are as shown below: Geometric Parameter Symbol Unit Entry width e m Approach half-Width v m Average effective flare length l' m Sharpnes s of flare S Inscribed circle diameter D m Entry angle ϕ deg Entry radius r m

Arm A 14.0 8.0 40.0 30.0 30.0 40.0

Arm B 9.0 4.5 40.0 30.0 40.0 30.0

The base year traffic survey carried out in 2005 revealed the following traffic flows in pcu/hr.

From (Origin)

A B C D

To (Destination) A B C 220 450 200 320 550 250 100 420 220

D 210 450 320 50

The general layout of the roundabout is shown below Determine the following; • The design flows for the year 2019 • The approach capacity of arms A and B of the roundabout. • Establish which of the two arms still has capacity and which one does not.

Solution 1.0 Summary of Design Data a) Traffic growth rate, r b) Design life, Y [= (2019-2005)+1] c) Geometric parameters of Arm A and B

= =


2% 15yrs as shown in the table

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2.0 Sketch Drawing As shown in the diagram above 3.0 Traffic Assessment 3.1 Design Flow, DF

  1.125  1.1251 Where; P r Y DF DRF

= = = = =

present flows (in pcu/hr); traffic growth rate (in %); design life (in years); Design Flow (a modification of the future traffic flow); Design Reference Flow.

The design flows, DF in 2019 are presented in the table below using the above formulae

From (Origin)

A B C D

To (Destination) A B C 0 333 681 303 0 485 833 379 0 151 636 333

D 318 681 485 76

3.2 Entry Capacity, Qe

0       

The parameters k, F, f c, and Q c are determined as follows a)

Values of k

b)

k  10.00347300.9781r  0.05  :  10.003473030 0.978140 0.05  1.0245  :  10.0034740300.978130 0.05  0.9816 F  303 148 94.5 1. 6 1. 6 S 1.6 ev  S   0. 2 40, S      l 40 40  0.180  ev  x    8  148  12.054 x v 12S 120.24 94.5  7.809  x    4.5  12 0.180 Values of F

Where;


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   :  303 12. 0 54  3652. 3 62 :  3037.809  2366.127 c) Values of f c

f   0.210 10.2x          0.0498 M  exp 60   M  e  0. 0 498, M  e     10 0.5   t    1 0.5   1.476, t    1.476 t  1 1 10.0498  : :  0.0.2210101.1.44767610.2 10.27.12.809054  0.1.794057

Where;

And;

Therefore;

d) Circulating Capacity Q c Arm A: Qc = = = Arm B:

Qc

= = =

QBB + QCC + QDD + QCB + QDB + QDC

0076379636333 1424 / 0076333681318 1408 /

QAA + QCC + QDD + QDC + QAC + QAD

Finally, the entry capacity, Q e for;

 :  1.0245 3652.3621.0571424 2200pcu/hr  :  0.9816 2366.1270.7941408 1225 pcu/hr

3.3 Approach Capacity, Q Arm A: Q

Arm B:

Q

= = =

QAA + QAB + QAC + QAD

= = =

QBA + QBB + QBC + QBD

01332/h 333681318 303 0485681 1469/h

3.4 Capacity Check, RFC For sufficient capacity;

RFC  QQ  0.85


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332  0.61  0.85       Arm A: RFC  QQ 12200 469  1.20  0.85       Arm B: RFC  QQ 1 1225 4.0 Conclusion Arm C has a RFC ratio of 61% which is less than 70%, implying that queuing on this arm will be avoided for 39 out of 40 peak hours. Arm D, on the other hand, has a RFC ratio of 120% which is far greater than 85%, implying that queuing will occur on this arm of the roundabout in all the peak hours.

4.10

References

1. Banister, A. and Baker, R, 1998, Surveying, 7th Edition, Longman limited, Singapore. 2. Irvine W, H, 1998, Surveying for construction, 4th Edition, Patson press, Great Britain. 3. Kadiyali, L.R., 2006. Principles and Practices of Highway Engineering (including th Expressways and Airport Engineering), 4 Edition. Khanna Publishers, New Delhi. 4. Ministry of Works, Housing and Communications, 2004. Draft Road Design Manual. 5. Ministry of Works, Housing and Communications, 2005. Road Design Manual Vol.1,Geometric Design Manual, Republic of Uganda, Kampala. 6. O’Flaherty C.A., 2002. Highways: The Location, Design, Construction and Maintenance of th Pavements. 4 Edition, Oxford, Butterworth Heinemann. 7. Rogers, Martin 2003, Highway Engineering, Oxford, Blackwell Publishing Ltd. 8. Thagesen, B., 1996. Highway and Traffic Engineering in Developing Countries. 1 st Edition. E & FN Spon Publishers, London, Uk. 9. Transport Research Laboratory, 1988, A Guide to Geometric Design, Overseas Road Note 6 , Crowthorne, England. 10. Uren, J, and Price, W.F, 1989, Surveying for Engineers, 2 nd Edition, Macmillan Publishers, Hong Kong.

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5.0

Design of Flexible Pavements

5.1

Introduction

The highway pavement is a structure consisting of superimposed layers of selected and processed material whose function is to distribute the applied wheel loads to the subgrade. This is to ensure that the stresses transmitted to the subgrade do not exceed its support capacity. Road traffic is carried by the pavement, which in engineering terms is a horizontal structure supported by in situ natural material. In order to support this structure, existing records must be examined and sub surface explorations conducted. The engineering properties of the local rock and soil are established, particularly with respect to strength, stiffness, durability, susceptibility to moisture, and propensity to shrink and swell over time. The relevant properties are determined by either field tests, by empirical estimates based on soil type, or by laboratory measurements. The material is tested in its weakest expected condition, usually at its highest moisture content. Probable performance under traffic is then determined. Soils unsuitable for the final pavement are identified for removal, suitable replacement materials are earmarked, the maximum slopes for embankments and cuttings are established, the degree of compaction to be achieved during construction is determined, and drainage needs are specified. If the road is in cut, the subgrade will consist of the in situ soil. If it is constructed on fill, the top layers of the embankment structure are collectively termed the subgrade [TRL, 1993]. The pavement designer must develop the most economical combination of layers that will guarantee adequate dispersion of the incident wheel stresses so that each layer in the pavement does not become overstressed during the design life of the highway. The major variables in design of a highway pavement are: • The thickness of each layer in the pavement; • The material contained within each layer of the pavement; • The type of vehicles in the traffic stream; • The volume of traffic predicted to use the highway over its design life; • The strength of the underlying subgrade [Rogers, 2003]. Pavements are called either flexible or rigid depending on their relative flexural stiffness.

5.2

Types of Pavements

5.2.1

Flexible Pavements

These pavements are rather flexible in their structural action under loading. They are surfaced with bituminous or asphalt materials. Flexible pavements consist of several layers of materials and rely on the combination of layers to transmit load to the subgrade. As a result of this action, flexible pavements distribute load over a small area of subgrade.


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Rigid Pavements 5.2.2

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Rigid Pavements

Rigid pavements are made of Portland Cement Concrete (PCC). The concrete slab ranges in thickness from 6 to 14 inches. These types of pavements are called rigid because they are substantially stiffer than flexible pavements due to PCC’s high stiffness. As a result of this stiffness, rigid pavements tend to distribute load over a relatively w ide area of subgrade. The concrete slab that comprises a rigid pavement supplies most of its structural capacity. In deciding whether to use flexible or rigid pavements, Engineers take into account lifetime costs, riding characteristics, traffic disruptions due to maintenance, ease and cost of repair, and the effect of climatic conditions. Often there is little to choose between rigid and flexible pavements. 5.3

Elements of a Flexible Pavement and their significance

A flexible pavement is built up of layers namely; surfacing courses, roadbase, sub-base, capping layer and subgrade [Kadiyali, 2000].

5.3.1

Surfacing

The surfacing forms the topmost layer of the pavement. It usually consists of a bituminous surface dressing or a layer of premixed bituminous material. It is comparatively thin, but resists abrasion and the impacts caused by wheel loads and the effects of weather condition [Bindra, 1999]. The functions of this layer are; provision of a safe and comfortable riding surface to traffic, taking up wear and tear stresses caused by traffic, provide a water tight surface against infiltration of water, provide a hard surface which can withstand tyre pressure. Where premixed materials are laid in two layers, these are known as the wearing course and the base course (or binder course) as shown in Figure 5.1 [TRL, 1993]. Wearing Course Base Course or Binder Course

Surfacing

Roadbase

Sub-base

Subgrade

Figure 5.1: Definition of Pavement layers

Source: TRL (1993)

5.3.2

Roadbase

The roadbase is the main load-spreading layer of the pavement. It is structurally the most important layer of a flexible pavement. It distributes the applied wheel load to the subgrade in such a way that the bearing capacity of the subgrade soil is not exceeded. This layer requires higher quality material often obtained by stabilizing sub-base materials. It will normally consist of crushed stone or gravel, or of gravely soils, decomposed rock, sands and sand-clays stabilised with cement, lime or bitumen [TRL, 1993]. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Subbase 5.3.3

96

Subbase

This is the secondary load-spreading layer underlying the roadbase. It will normally consist of a material of lower quality than that used in the roadbase such as unprocessed natural gravel, gravel-sand, or gravel-sand-clay. It may or may not be present as a separate layer since its presence is justified by the insufficiency of the subgrade or reliability [TRL, 1993]. Major uses include: • Distribution of stresses to the subgrade; as a result the sub base material must be stronger than the subgrade material; • Acts as a drainage layer in case of poor subgrade. A good drainage layer should be able to drain very fast if water is logged, but also must be able to retain some moisture in times of extreme drought; • Serving as a separating layer preventing contamination of the roadbase by the subgrade material; • Under wet conditions; it has an important role to play in protecting the subgrade from damage by construction traffic; • Preventing capillary attraction effect. The sub-base is omitted when the subgrade is a hard intact rock or if it is granular and has a CBR greater than 30% and has no high water table [TRL, 1993].

5.3.4

Capping Layer (Selected or Improved Subgrade)

A capping layer may consist of better quality subgrade material brought in from somewhere else or from existing subgrade material improved by mechanical or chemical stabilisation. It is usually justified where weak soils are encountered [TRL, 1993].

5.3.5

Subgrade

This is the top surface of a road bed on which the pavement structure and shoulders including kerbs are constructed. Generally the top soil portion up to 0.5m of the embankment or cut-section is referred to as the subgrade [Bindra, 1999]. It may be undisturbed local material or may be soil excavated elsewhere and placed as fill. The loads on the pavement are ultimately received by the subgrade layer; it is therefore, essential that the layer should not be over-stressed. The top part of the layer requires preparation to receive layers on top either by stabilizing it adequately and therefore reduce required pavement thickness or designing and constructing a sufficiently thick pavement to suit subgrade strength. The subgrade strength depends on the type of material, Moisture content, dry density, internal structure of the soil particles, and type and mode of stress applied [TRL, 1993]. The major factors that influence pavement thickness are; design wheel load, strength of subgrade (and other pavement materials), climatic and environmental factors [Singh, 2001].

5.4

The Pavement Design Process

The overall process of designing a road consists of the following steps: • Surveying possible routes which are part of the feasibility study process; • Assessing traffic; • Measuring subgrade strength; • Selecting pavement materials; • Selecting the type of pavement structure to use including drainage system. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Traffic Assessment

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The three main steps to be followed in designing a new road pavement are discussed below.

5.4.1

Traffic Assessment

The first step involves estimating the amount of traffic and the cumulative standard axles that will use the road over its design life. In this step, other sub-activities include: measurement of traffic volume by class; measurement of axle loads; choosing the design life and Calculation of the total traffic. The thickness of the pavement greatly depends on the design wheel load. In design of a pavement, knowledge of the maximum wheel load is more important than gross weight of vehicles. Heavier loads require thicker pavements provided other design factors remain constant [Gupta, 1999]. During structural design, emphasis is placed on commercial and heavy goods vehicles whose axle weight is greater than 1,500 kg. It is these classes of vehicle that are most damaging to the pavement. Their volume becomes critical in design [TRL, 1993].

5.4.2

Subgrade Assessment

The next step involves assessment of the strength of the subgrade soil . The sub-activities involved in this step include: Assignment of climatic a regime, testing of soils, definition of uniform sections, and designing of earth works. Properties of the subgrade soil are important in designing the depth of the pavement. Weak subgrade material requires higher thickness to protect it from traffic loads. Pavement deformation mainly depends on the subgrade properties and drainage. During design and construction, proper drainage has to be maintained in order to control pavement deformation. Climatic factors are important here because rainfall affects the moisture of the subgrade and pavement layers. The daily and seasonal variations of rainfall are important in the design and performance of the pavement. Where the water table is close to the formation level of the roads, adjustments in the design of the pavement layer thicknesses are necessary. According to Kadiyali (2000) and Arora (2000), the heights of embankments and the depth of water table below the embankment affect the performance of an embankment and must be examined. Some of the key tests in the design of the subgrade include the Compaction test, the Dynamic Cone Penetrometer test and the California Bearing Ratio (CBR) test. 5.4.3

Material Selection

The last step in pavement design involves the selection of the most economical combination of pavement materials and layer thickness that will provide satisfactory service over the design life of the pavement. Materials together with their grading determine the stress distribution characteristics. Their durability under adverse weather conditions should be considered [TRL, 1993]. See Figure 5.2for a summary of the pavement design process.

5.5

Approaches to Design

Arora (2000) classifies the various approaches of pavement design into empirical and semiempirical methods. Empirical methods include; Group index method, CBR method (or thickness design method) whereas semi - empirical methods include AASHTO method, Triaxial test, Nottingham method, California Resistance Value Test, McLeod method and Banister method. In Uganda, the AASHTO and Thickness design methods are most commonly applied. These methods will be looked at in more detail during the assessment of subgrade strength. The Group index method is limited as it considers only the particle distribution of the soil and its atterberg limits. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

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Highway Design Standards

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Figure 5.2: Summary of the Pavement Design Process

Source: TRL (1993)

5.6

Highway Design Standards

98

In Uganda, design of flexible pavements has been based on a number of design standards that include the TRL, Overseas Road Note 31 (1993), Uganda Road Design Manual (1994) which has been updated, the Kenya Road Design Manual and the American Association of State Highways and Transportation Officials (AASHTO) interim guides for design of Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

98

Uganda Road Design Manual

99

pavement structures 1972-1986. The latest version of the AASHTO design guide was printed in 1993. The above design guides have been adapted to suit most materials and climatic conditions found in developing countries. The AASHTO design equation in the design guide 1972-1986 was also modified through research done by the World Bank to suit conditions in developing countries. It is important for engineers to exercise judgement in the use of a given design standard to ensure that they come up with an economical solution for a pavement design. Use of local materials has to always be taken into considerations. Sometimes, more than one design standard is used for the purposes of comparing one pavement design with another so that the comparison guides the engineer in selecting the most economical option.

5.6.1

Uganda Road Design Manual

The Uganda Road Design Manual November 1994 has incorporated the pavement design guide prepared for SATCC countries. The SATCC design guide was developed for Southern Africa Transport and Communication Commission for use in Angola, Zambia, Botswana, Zimbabwe, Mozambique, Malawi, Swaziland, Lesotho, and Tanzania [Thagesen, 1996]. Th e method follows the AASHTO design concept as set forth in AASHTO interim guides for design of pavement structures 1972-1986 published by the American Association of State Highways and Transport Officials. The pavement strength required for a given combination of subgrade bearing capacity, traffic load, service level and climate is expressed by means of the subgrade structural number. Layer coefficients, according to the position in the s tructure, are given to determine the structural number of the pavement. For each type of pavement, the thickness of the base and sub base layers are determined so that the required structural number is satisfied [Uganda Road Design Manual, 1994].

5.6.2

Kenya Road Design Manual

The materials and pavement design in the Kenya Road Design Manual sets forth the standards for structural design of new bitumen surfaced roads in Kenya. The Kenya Road Design Manual includes design of gravel wearing course on unpaved roads.

5.6.3

TRL Road Note 31

The British Transportation and Road Research Laboratory (TRRL) published the first version of Road Note 31 in 1962 and subsequently revised it in 1976 and 1977. The Road Note 31 has in 1993 undergone a comprehensive revision by the transport research laboratory (TRL) and now includes the structural catalogue where a layer thickness can be selected for a whole range of common pavement combinations. The guidelines are based on an empirical method taking into account the organisation’s vast experience in understanding the behaviour of road building materials and their interactions in composite pavements.

5.7

The AASHTO Approach to Pavement Design

5.7.1

The AASHTO Design Equation

The total required structural number (SN) for the entire pavement is as below:

Log D  9.36Log SN 1 0.20LogR10.372S3.0 Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

99

….5.1 99

Regional Adjustment

100

Where; DT = Design Traffic in 80kN e.s.a.; SN = Total Required Structural Number; R = Regional Factor; S = Soil Support Value. The above equation assumes a terminal serviceability index of 1.5. This equation represents the relationship between the weighted structural number and the design traffic. The design traffic has been grouped into classes as shown in Table 5.4. For low traffic volumes less than 0.5 million equivalent standard axles, reference is made to TRL Overseas Road Note 31 for design thicknesses [Ruhweza, 2005].

5.7.2

Regional Adjustment

A regional factor of 1.0 was assumed for areas with rainfall most of the year creating a permanently saturated condition (12 wet months) of the subgrade and unbound pavement layers. The required structural number for this condition was entered into the charts as SN W. A regional factor of 0.1 was assumed for very arid climates (0 wet months) where the pavement structure and the subbase never reach a saturated condition. The required structural number for this condition was entered into the design charts as SN D. Based on research carried out by the transportation department of the World Bank in connection with the development of the HDM III model, a method for weighing SN W and SND was developed to obtain the Design Structural Number ‘DSN’ taking the actual wet and dry periods into account. The modified formula for weighing of the structural number in accordance with the applicable seasonal conditions (rainfall) assumes the form:

DSN  n  SNSNnW . 12 SN. 12 SNW. Where; DSN SND, SNw nD, nw = 12);

= = =

Note: SND and SNw 5.7.3

….5.2

Design Structure Number; The structure number for dry and wet condition respectively; Number of wet and dry months respectively during one year (n D+ n w

are indicated on the design charts

Design Tables Table 5.1: Subgrade Classes Clas s CB R Rang e ( % ) S0 ˂ 2 S1 2-7 S2 8 - 14 S3 12 - 20 S4 18 - 30 S5 > 30 Source: AASHTO, 1993


100

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Design Tables

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Table 5.2: Traffic Groups Group Description 1 Private Cars and Small Pick-Ups 2 Light Goods Vehicles, e.g. Land Rovers, Minibuses 3 2-Axle and Tandem Axle Rigid Trucks 4 Rigid Trucks with Drawbar Trailers 5 Articulated Units with Semi-Trailers 6 Buses Source: AASHTO, 1993 Table 5.3: Average Vehicle Equivalence Factors, C i Group

3 4 5

Equivalence Factor for α =4.0

Des cription

2-Axle and Tandem Axle Rigid Trucks Rigid Trucks with Drawbar Trailers Articulated Units with Semi-Trailers

2.0 6.0 6.0

6 Buses Source: AASHTO, 1993

1.0

Table 5.4: Traffic Classes Traffic Class

Cum.No. Of standard

esa per day in

6

axles x 10

T0 Very Heavy T1 Heavy T2 Medium T3 Light T4 Very Light Source: AASHTO, 1993

2

year one

> 20 8 - 20 2.5 - 8 0.5 - 2.5 0.15 - 0.5

Maximum ADT in year one

> 2500 1000 - 2500 300 - 1000 60 - 300 20 - 60

for Heavy vehicles x 10 5 10 15 20 250 125 83 63 100 50 33 25 30 15 10 7.5 6 3 2 1.5

25 50 20 6 1.2

Table 5.5: Determination of DSN for different Subgrade and Traffic Classes Nw S2

T4 T3 T2 T1

0 27.0 59.0 69.0 80.0

S3

T4 T3 T2 T1

24.0 54.0 64.0 74.0

24.5 55.0 65.2 75.3

25.0 56.1 66.5 76.7

25.5 57.3 67.8 78.3

26.1 58.6 69.3 79.9

26.7 60.0 71.0 81.7

27.4 61.6 72.7 83.7

28.2 63.3 74.7 85.8

29.1 65.2 76.9 88.2

30.1 67.4 79.3 90.8

31.2 69.8 82.1 93.8

32.5 72.7 85.3 97.1

34.0 76.0 89.0 101.0

S4

T4

21.0

21.4

21.8

22.2

22.7

23.2

23.8

24.5

25.2

25.9

26.8

27.8

29.0

T3 T2 T1

49.0 59.0 68.0

50.0 60.1 69.2

51.0 61.3 70.6

52.1 62.5 72.0

53.3 63.9 73.6

54.6 65.4 75.3

56.1 67.1 77.2

57.7 68.9 79.2

59.6 70.9 81.5

61.6 73.1 84.0

64.0 75.7 86.9

66.7 78.6 90.2

70.0 82.0 94.0

T4 T3 T2

12.0 29.0 37.0

12.3 29.7 37.8

12.6 30.4 38.8

13.0 31.3 39.8

13.4 32.2 40.9

13.8 33.2 42.1

14.4 34.4 43.5

15.0 35.7 45.1

15.7 37.3 46.9

16.5 39.1 49.1

17.6 41.4 51.7

19.0 44.2 54.9

21.0 48.0 59.0

T1

46.0

47.0

48.1

49.2

50.5

51.9

53.5

55.3

57.4

59.7

62.5

65.8

70.0

S5

1 27.6 60.1 70.3 81.4

2 28.2 61.3 71.7 83.0

3 29.0 62.5 73.1 84.6

4 29.7 63.9 74.8 86.4

5 30.6 65.4 76.5 88.3

6 31.6 67.1 78.4 90.4

7 32.7 68.9 80.6 92.7

8 33.9 70.9 82.9 95.2

9 35.4 73.1 85.6 98.1

10 37.2 75.7 88.6 101.2

11 39.3 78.6 92.0 104.9

12 42.0 82.0 96.0 109.0

Source: AASHTO, 1993


101

101

Steps involved in the AASHTO method of Design

102

Table 5.6: Layer Coefficients Layer/Material Surfacing Surface dressing

Layer Coefficient

a1 = 0.20 a1 = 0.35

Asphalt concrete Base Bitumen Macadam

a2 = 0.20 a2 = 0.12

Natural or Crushed Gravel Crushed Stone on: Natural Gravel Subbase Stabilised Subbase Cement Treated Gravel: Type A, 3.5 ≤ UCS (MPa) ˂ 5.0 Type B, 2.0 ≤ UCS (MPa) ˂ 3.5

a2 = 0.14 a2 = 0.18 a2 = 0.18 a2 = 0.14

Subbase Natural Gravel, CBR ≥ 30% Cement Treated Material:

a3 = 0.11 a3 = 0.16 a3 = 0.12

Type B, 2.0 ≤ UCS (MPa) ˂ 5.0 Type C, 0.7 ≤ UCS (MPa) ˂ 2.0 Source: AASHTO, 1993

Table 5.7: Compacted Thickness Ranges Layer Material Type Surfacing Asphalt concrete Surface dressing

Base Natural and Crushed Gravel Crushed Stone Cement Treated Gravel: Type B Type A Bituminous Dense Graded Macadam Bituminous Semi-Dense Macadam Subbase Natural Gravel Cement or Lime treated Material, Type C Cement Treated Gravel, Type B Source: AASHTO, 1993

5.7.4

Min (mm)

Max (mm)

30 10

100 30

125 125

200 200

125 125 70

175 175 150

70

150

100

250

100 100

200 200


The steps followed in designing following the AASHTO method are as follows: a) Determination of the Subgrade strength Class The study of the alignment soil enables homogeneous sections to be defined in terms of the design CBR Value. This is the CBR value of the subgrade. For each homogeneous section, the strength class of the subgrade is determined as indicated in Table 5.1. Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

102

102


103

b) Determination of the Traffic Class The study of the initial traffic flows and axle load distribution, the choice of the design period and the estimation of the traffic growth rate permits the calculation of the cumulative number of standard axles to be carried by the road. This acts as the design criteria for structural bearing capacity. The design equivalent number of standard axles is derived from the chosen traffic expressed as the average annual daily traffic in vehicles per day, the traffic growth rate, the vehicle fleet characteristics and the traffic composition. The contribution of the axle load from private cars and light goods vehicles is ignored in the design of the equivalent number of standard axles. The axle loading of a mixture of vehicle types is converted to a number of equivalent standard axles using equivalence factors obtained using the formula below:

 P C  80

Where; C = P = α =

….5.3 80kN equivalence factor; Load of a single axle (in kN); Influence coefficient (may be taken as 4.0 for most instances).

It is better to measure actual axle loads using a mobile weighbridge for medium to heavily trafficked roads. For purposes of feasibility studies or where it may not be possible to obtain actual axle loads especially in lightly trafficked roads, the equivalence factors in Table 5.3 may be applied. The average daily traffic from equivalent standard axles is obtained using the formula below:



T  VC

….5.4



Where;

Vi Ci

= =

average daily number of each type of commercial vehicle; appropriate equivalence factor.

For all commercial vehicles having the same growth rate, a cumulative number of standard axles during the design period are calculated using the formula below:

1 1r D  365T   r 

….5.5

Where; DT Td r Y

= = = =

Design traffic as cumulative number of 80kN esa; Average daily number of esa in the first year after opening; average growth rate for the design period in percent per annum; Design period in years.

To obtain the traffic class, Table 5.4 is used.

103

Equation 5.5 can be rewritten as shown below:

D  365T .G.Y10 in msa Kyambogo University | P. O. Box 1, Kyambogo Uganda CE323 – Highway Engineering 1, Lecture Notes. © FOE- 2010. E-mail: [email protected] . Mobile No.: (256) 701 806514

….5.6 103


104

Where; G is referred to as the growth factor and is given by:

1 1r G   Y.r 

….5.7

c) Selection of Possible Types of Pavements Knowledge of the types and characteristics of the available pavement materials as well as of the climates, allows selecting one type of pavement. d) Calculation of the Required Structural Number For the estimated number of wet and dry months (n W and nD) and the wet and dry structural numbers (SNW and SND) taken from the appropriate design chart, the weighted structural number is calculated using equation 5.2. SNW and SND are obtained from design charts 1 to 8. e) Determining the Thickness of the Surfacing, Base and Subbase courses The thickness of the surfacing, base and subbase layers are determined so that the following equation is satisfied.

DSN a ha ha h

….5.8

Where; DSN = Weighted structural number for the entire pavement; a1,a2,a3 = layer coefficients representing the surfacing, base and subbase course respectively. h1,h2,h3= actual thicknesses in mm of surface, base and subbase courses respectively Note: The layer coefficients assumed are in Table 5.6. For different types of materials considered, guide values for the minimum and maximum practical thicknesses of a layer are given in Table 5.7 for effective compaction [Ruhweza, 2005].

Pavement Design Example The Kampala – Gayaza road is in a state of failure and is due for reconstruction. The following facts have already been gathered about the project road: a) The road is located in a region that has a rainy season with a total span of 5 months; b) The subgrade soil is a good quality gravel with a soaked CBR in the range of 20 – 30%; c) The subbase material will be cement treated Type C; d) The most economical material for the roadbase will be crushed stone e) The most suitable surfacing material will be Asphalt Concrete (AC); Traffic counts and axle load surveys have shown that the initial (unidirectional) daily number of commercial vehicles will be as follows: : 140 veh/day; a) 2-Axle and Tandem trucks : 30 veh/day; b) Trucks with drawbar trailer : 16 veh/day; c) Articulated Units : 40 veh/day. d) Buses The economic study has recommended a 15 year design life and forecasts a constant annual traffic growth rate of 2.5%. Design the flexible pavement using the AASHTO approach.


104

104

Steps involved in the AASHTO method of Design Solution 1.0 Design Information (a) Number of wet months in the region, nW (b) Subgrade CBR (c) Traffic growth rate, r (d) Design life, Y (e) Construction Materials: Surfacing Material Roadbase material subbase Subbase material 2.0

3.0

= = = =

105

5 20 – 30% 2.5% 15yrs

Asphalt Concrete (AC) Crushed stone on stabilised Cement treated Type C

Determination of subgrade strength, S From Table 5.1, the given CBR range of (20 – 30%) falls in the range 18% < CBR < 30% implying that the subgrade strength class is S4. Determination of cumulative design traffic, T



     365....  10   

Where;

a)

Unidirectional traffic Flow, V The directional split is 100% (i.e. unidirectional) F = 100% of the traffic volume for each vehicle class e.g. for 2-Axle and Tandem Trucks; F = 100% x 140 = 140 Veh/day

b)

Wear factor, W From equation 5.3

.   ,    80  e.g. for 2-Axle and Tandem Trucks; C = 2.0 (from Table 5.3) since no axle loads were provided. c)

Growth Factor, G According to AASHTO growth factor equation;

1 1r G   Y.r  1 10.025 G   150.025   1.1955

105


105

Steps involved in the AASHTO method of Design d)

106

Table of results Vehicle Class

V (Veh/day) 140 2-Axle and TandemTrucks 30 Trucks with Drawbar Traile r 16 Articulated Units 40 Buses

C (esa) 2 6 6 1

G

1.1955 1.1955 1.1955 1.1955

Cumulative Des ign Traffic, D T (in msa)

Y (years) 15 15 15 15

DT (msa) 1.833 1.178 0.628 0.262 3.901

From Table 5.4 a cumulative design traffic of 3.901 msa corresponds to a traffic class of T2 i.e. 2.5 < T (in msa) < 8.0. 4.0

Required Design Structural Number, DSN The DSN is given by:

DSN  n  SNSNnW . 12SN. 12SNW.  5  59827  .  65.4 12 59. 12 82. 5.0

Layer Thicknesses based on the Actual Design Structural Number, DSN a The Actual Design Structural Number, DSNa is given by

DSN a ha ha h From design chart no. 6, for a subgrade strength class S4 and Traffic Class of T2 (i.e. S4 – T2) corresponds to an asphalt surfacing thickness, h 1 of 50mm. And from Table 5.6 a1 = 0.35, a 2 = 0.18 and a3 = 0.12. Therefore;

DSN  0.35 50 0.18h 0.12h  By trial and error with guidance from Table 5.7, let’s try h 2 = 200mm and h 3 = 200mm. From which;

DSN  0.35 50 0.18200 0.12200  77.5 Since DSNa (= 77.5) > DSN (= 65.4), it implies that the design thicknesses of the layers are acceptable. 6.0

Conclusion The pavement should therefore be composed of the following layer thicknesses : 50mm a) Surfacing material : 200mm b) Roadbase : 200mm c) Subbase


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