Rural Road Design
A Guide to the Geometric
Design of Rural Roads
Rural Road Design
Rural Road Design: A Guide to the Geometric Design of Rural Roads
© Austroads Inc 2003
NAASRA Guides: Austroads Guides First published 1955 Seventh Edition 1989 Second Edition 1961 Reprinted 1991 Third Edition 1967 Reprinted 1993 Reprinted 1967 Reprinted 1997 Reprinted 1968 Reprinted 1999 Fourth Edition 1970 Eighth Edition 2003 Fifth Edition 1973
Sixth Edition 1980
This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part may be reproduced by any process without the written permission of Austroads. National Library of Australia Cataloguing-in-publication data:
Rural Road Design: A Guide to the Geometric Design of Rural Roads ISBN 0 85588 606 4
Austroads Project No. T&E.D.C.019 Austroads Publication No. AP-G1/03
Standards Australia and Standards New Zealand Handbook No. HB152:2002
Project Manager
John Cunningham, VicRoads
Prepared by
Arup Group
Published by Austroads Incorporated Level 9, Robell House
SYDNEY 2002
R u r a l R o a d D e s i g n
A G u i d e t o t h e G e o m e t r i c D e s i g n o f R u ra l R o a d s
Austroads is the association of Australian and New Zealand road transport and traffic authorities whose purpose is to contribute to the achievement of improved Australian and New Zealand transport related outcomes by:
● developing and promoting best practice for the safe and effective management and use of the road system ● providing professional support and advice to member
organisations and national and international bodies ● acting as a common vehicle for national and international
action
● fulfilling the role of the Australian Transport Council’s Road Modal Group
● undertaking performance assessment and development of Australian and New Zealand standards
● developing and managing the National Strategic Research Program for roads and their use.
Within this ambit, Austroads aims to provide strategic direction for the integrated development, management and operation of the Australian and New Zealand road system — through the promotion of national uniformity and harmony, elimination of unnecessary duplication, and the identification and application of world best practice.
Austroads membership comprises the six State and two Territory road transport and traffic authorities and the Commonwealth Department of Transport and Regional Services in Australia, the Australian Local Government Association and Transit New Zealand. It is governed by a council consisting of the chief executive officer (or an alternative senior executive officer) of each of its eleven member organisations:
● Roads and Traffic Authority New South Wales ● Roads Corporation Victoria
● Department of Main Roads Queensland ● Main Roads Western Australia
● Transport South Australia
● Department of Infrastructure, Energy and Resources Tasmania
● Department of Infrastructure, Planning and Environment Northern Territory
● Department of Urban Services Australian Capital Territory ● Commonwealth Department of Transport and
Regional Services
● Australian Local Government Association ● Transit New Zealand
The success of Austroads is derived from the synergies of interest and participation of member organisations and others in the road industry.
A U S T R O A D S I N C O R P O R AT E D
A U S T R O A D S M E M B E R S H I P
The Austroads Reference group for the guide: Members
Project Manager John Cunningham, Manager VicRoads Design, Victoria Technical Editor Dennis Maxwell, VicRoads, Victoria
Michael Brauer/Peter Ellis Roads and Traffic Authority, New South Wales John Byrden VicRoads, Victoria
Dennis Davis Transit New Zealand
Geoff Clarke Commonwealth Department of Transport and Regional Services Tony Gill Department of Urban Services, Australian Capital Territory Geoff Glynn Municipal Association of Victoria
Rob Grove Main Roads, Western Australia
Arthur Hall Department of Main Roads, Queensland
Fritz Nabholtz Department of Infrastructure, Planning and Environment, Northern Territory Graeme Nichols Department of Infrastructure, Energy and Resources, Tasmania
Richard Saunders Department of Transport South Australia
Project Research and Writer ARUP Group
A U S T R O A D S R E F E R E N C E G R O U P
This guide represents the combined experience and international best practices of Austroads member agencies and industry experts in the area of geometric design of rural roads. The Guide has been prepared as the common design tool for Australia and New Zealand. For a more detailed explanation of specific matters, which may vary from place to place, designers should check with the relevant road authority. It has been the aim of the Consultant and the Reference group to validate all tables, figures and graphs included in the Guide. The validation took the form of developed formulae, laboratory test results, field observations or references.
In some cases the designer has been provided with a range of desirable and absolute values. A design can be produced which may take into account the design topography, the safety of the occupants and the design parameters. Care should be taken to ensure the combined use of absolute values does not create an inappropriate design. Each circumstance should be individually evaluated based on local conditions by experienced personnel.
This document does not cover the geometric design of unsealed roads. The designer is directed to the ARRB document “Unsealed Roads Manual – Guidelines to Good Practice, 1993”. The document referred to will provide the practical and basic aspects for the maintenance design and construction of unsealed roads.
P R E FA C E
This is the eighth edition of the Geometric Design of Rural Roads. The guide was last revised in 1989.
This revision of Rural Road Design: Guide to the Geometric Design of Rural Roads follows the 2002 release of Urban
Road Design: Guide to the Geometric Design of Major Urban Roads.
TOPIC PAGE NO.
FOREWORD v
PREFACE vii
GLOSSARY OF TERMS xii
PART 1 INTRODUCTION 1 1. A BALANCED APPROACH 1 1.1 General 1 1.2 Design Standards 1 1.3 Speed Concept 1 1.3.1 General 1
1.3.2 High Speed Roads 2
1.3.3 Intermediate Speed Roads 2
1.3.4 Low Speed Roads 3
1.3.5 85th Percentile Speed 3
2. ROAD FUNCTIONAL CLASSES 3
3. DESIGN APPROACH 4
3.1 General 4
3.2 The Driver’s View 4
3.3 Co-ordination of Horizontal and Vertical Alignment 4
3.3.1 General 4
3.3.2 Curvilinear Design 5
3.3.3 Combined Horizontal and
Vertical Alignment 5
PART 2 FUNDAMENTAL DESIGN CONSIDERATIONS 8
4. TRAFFIC VOLUME & TRAFFIC COMPOSITION 8
5. DESIGN VEHICLE 8
6. ENVIRONMENTAL CONSIDERATIONS 9 6.1 Traffic Related Intrusion 9
6.1.1 Visuals 9
PART 3 DESIGN INPUTS 13
7. SPEED, USED FOR GEOMETRIC DESIGN 13
7.1 Introduction 13
7.2 Explanation of Terminology 13 7.2.1 Vehicle Speed on Roads 13
7.2.2 Operating Speed 14
7.2.3 Operating Speed of Trucks 14 7.2.4 Section Operating Speed 14
7.2.5 Design Value 14
7.3 Estimating Operating Speeds on Rural Roads 14
7.3.1 General 14
7.3.1.1 Driver Behaviour 14 7.3.1.2 Road Characteristics 14 7.3.1.3 Vehicle Characteristics 14 7.3.2 Operating Speed Estimation Model 14 7.3.3 Acceleration On Straights Graph 17 7.3.4 Deceleration On Curves Graph 17 7.3.5 Section Operating Speeds 17 7.3.5.1 Length Of Road to be included in
The Study 17
7.3.5.2 Identification of Sections 19 7.3.6 Estimating Speed on a Section of Road 21 7.3.6.2 Step 2 – Estimate Speed at Point C 21 7.3.6.3 Step 3 – Estimate Speed at Point D 21 7.3.6.4 Step 4 – Estimate Speed at Point E 21 7.3.6.5 Step 5 – Estimate of Speed at Point F 21 7.3.6.6 Step 6 – Estimate of Speed at Point G 21 7.3.6.7 Step 7 – Estimate of Speed at
Point H and I 21
7.3.7 Effects Of Grades 21
7.3.8 Effect of Cross-Section 23 7.3.9 Effect of Pavement Condition 23 7.3.10 Use of Operating Speed in the Design
8.3.3 Car to Road Object Stopping
Sight Distance 27
8.3.4 Truck to Road Object Stopping
Sight Distance 27
8.4 Overtaking Sight Distance 30
8.4.1 General 30
8.4.2 Overtaking Model 30
8.4.3 Determination of Overtaking Provision 30 8.4.4 Determination of Percentage of
Road Providing Overtaking 31 8.5 Manoeuvre Sight Distance 33
8.5.1 Derivation 33
8.6 Headlight Sight Distance 33 8.7 Horizontal Curve Perception Distance 34
PART 4 GEOMETRIC DESIGN GUIDELINES 35
9. HORIZONTAL ALIGNMENT 35
9.1 General 35
9.2 Movement on a Circular Path 35
9.3 Horizontal Curves 35
9.3.1 Types of Horizontal Curves 35
9.3.1.1 Reverse Curves 35
9.3.1.2 Compound Curves 35 9.3.1.3 Broken Back Curves 35 9.3.1.4 Transition Curves 35
9.4 Side Friction Factor 36
9.5 Minimum Radii Values For Horizontal Curves 37 9.5.1 Minimum Radius Values 37 9.5.2 On Steep Down Grades 38 9.6 Horizontal Alignment Design Procedure 38
9.7 Superelevation 39
9.7.1 Maximum Values of Superelevation 42 9.7.2 Minimum Values of Superelevation 42 9.7.3 Application of Superelevation 42 9.7.4 Length of Superelevation Development 42 9.7.4.1 Rate of Rotation 43
9.7.4.2 Relative Grade 43
9.7.4.3 Design Superelevation
Development Lengths 44 9.7.5 Positioning Of Superelevation Runoff 44 9.7.5.1 Without Transitions 44 9.7.5.2 With Transitions 46 9.7.6 Superelevation on Bridges 48 9.8 Curves With Adverse Crossfall 48 9.9 Minimum Horizontal Curve Length 48 9.10 Pavement Widening on Horizontal Curves 48
9.11 Sight Distance on Horizontal Curves 51 9.11.1 Benching for Visibility on
Horizontal Curves 51
9.11.2 Other Restrictions to Visibility 51 9.12 Curvilinear Alignment Design in Flat Terrain 52
9.12.1 Introduction 52
9.12.2 Theoretical Considerations 52 9.12.3 Advantages of Curvilinear Alignment 52
9.13 Bridge Considerations 53
10. VERTICAL ALIGNMENT 54
10.1 Introduction 54
10.2 Grades 54
10.2.1 General 54
10.2.2 Vehicle Operation on Grades 54
10.2.3 Maximum Grades 55
10.2.4 Length of Steep Grades 55 10.2.5 Steep Grade Considerations 55
10.2.6 Minimum Grades 56
10.3 Vertical Curves 56
10.3.1 General 56
10.3.2 Forms and Types of Curve 56 10.3.3 Crest Vertical Curves 56 10.3.3.1 Appearance 56 10.3.3.2 Sight Distance Criteria (Crest) 57 10.3.4 Sag Vertical Curves 57 10.3.4.1 Appearance and Comfort 57 10.3.4.2 Sight Distance Criteria (Sag) 58 10.3.5 Reverse/Compound/Broken Back
Vertical Curves 58
11. CROSS SECTION 60
11.1 General 60
11.2 Traffic Lane Width 60
11.3 Traveled Way 61
11.3.1 Single Carriageways 61 11.3.2 Divided Carriageways 62 11.3.2.1 Independent Design of Carriageways 63 11.3.2.2 Superelevation Issues 63 11.3.2.3 Transitions Between Divided and
Undivided Carriageways 63 11.4 Pavement Crossfall and its Considerations 63
11.5 Shoulder 65 11.5.1 Function 65 11.5.2 Width 65 11.5.3 Shoulder Sealing 66 11.5.4 Crossfalls 67 11.6 Verge 67 11.7 Batters 67
11.7.1 Benches 69 11.7.2 Batter Rounding 69 11.8 Medians 69 11.9 Roadside Drains 72 11.9.1 Table Drains 72 11.9.2 Catch Drains 72 11.9.3 Median Drains 72 11.10 Noise Barriers 72 11.11 Right of Way 72 11.12 Widths of Bridges 72
PART 5 OTHER DESIGN CONSIDERATIONS 75
12. PRINCIPAL FACTORS 75 12.1 Financial Level 75 12.2 Safety 75 12.3 Energy 75 12.4 Stage Construction 75 13. AUXILIARY LANES 75 13.1 General 75
13.2 Types of Auxiliary Lanes 75
13.3 Speed Change Lanes 76
13.3.1 Acceleration Lanes 76 13.3.2 Deceleration Lanes 76 13.4 Overtaking Lanes/Climbing Lanes 76
13.4.1 Overtaking Lanes 76 13.4.1.1 Overtaking Demand 76 13.4.1.2 Overtaking Opportunities 76 13.4.1.3 Warrants 79 13.4.1.4 Length 79 13.4.1.5 Location 80 13.4.1.6 Spacing 80 13.4.1.7 Improvement Strategy For
Overtaking Lanes 81
13.4.2 Climbing Lanes 81
13.4.2.1 General 81 13.4.2.2 Warrants 81 13.4.2.3 Length 83
13.5 Slow Vehicle Turnouts 83
13.5.1 Partial Climbing Lanes 83
13.7.4.2 Escape Exits 89
13.7.4.3 Spacing 89 13.7.4.4 Summary of Design Considerations 90 13.7.5 Brake Check and Brake Rest Areas 90 13.8 Geometry of Auxiliary Lanes 90 13.8.1 Starting and Termination Points 90
13.8.2 Tapers 91 13.8.3 Cross Section 91 13.8.3.1 Pavement Width 91 13.8.3.2 Shoulder Width 91 13.8.3.3 Crossfall 91 13.8.3.4 Lane Configurations 91 13.8.4 Line marking and Signing 92 13.8.4.1 Signs 92 13.8.4.2 Linemarking 92
14. VEHICLE STOPPING AREAS 92
14.1 General 92
14.2 Service Facilities 92
14.2.1 Rest Areas 92
14.2.1.1 Major Rest Areas 93 14.2.1.2 Basic Rest Areas 93
14.2.1.3 Other Areas 94
14.2.2 Location of Vehicle Stopping Areas 95 14.2.3 Heavy Vehicle Considerations 95
15. COMMUNITY CONSULTATION 96 16. DRAINAGE 96 16.1 General 96 16.2 Flood Estimation 96 16.3 Rational Method 97 16.4 Design Considerations 98 16.5 Water Quality 99 17. ROADSIDE SAFETY 100 17.1 Safety Objectives 100 17.2 On-Road Safety 100 17.2.1 Intersections 100 17.2.2 Mid Block 101 17.3 Recovery Area 101 17.3.1 Clear Zone 101
17.3.2 Existing Hazards Within a Clear Zone 102
REFERENCES 113
APPENDICES 117
Appendix A – Characteristics of the Euler Spiral
(Clothoid) 117 Appendix B – Vertical Curve Formulae 119 Appendix C – Derivation of Sight Distance
Requirements at Railway Level Crossings 121
1. General 121
2. Case 1: Sight Distance Required for
Give Way Control 121
3. Case 1(i): Decelerate and Safely Stop at the Stop or Holding Line 122 4. Case 1(ii): Proceed and Clear the
Crossingwith an Adequate
Safety Margin 122
5. Case 2: Sight Distance Required
AADT Annual Average Daily Traffic is calculated by counting the number of vehicles passing a roadside observation point in a year and dividing this number by 365.
Abutment An end support of a bridge or similar structure.
Acceleration Lane An auxiliary lane used to allow vehicles to increase speed without interfering with the main traffic stream. They are often used on the departure side of intersections.
Access The driveway by which vehicles and/or pedestrians enter and/or leave property adjacent to a road.
Adverse Crossfall A slope on a curved pavement that generates forces detracting from the ability of a vehicle to maintain a circular path.
Alignment The geometric form of the centreline (or other reference line) of a carriageway in both the horizontal and vertical directions.
Alignment Co-ordination A road design technique in which various rules are applied to ensure that
(coordinated alignment) the combination of horizontal and vertical alignment is both safe and aesthetically pleasing.
Aquaplaning Full dynamic aquaplaning occurs when a tyre is completely separated from the road surface by a film of water.
Arrester Bed An arrester bed is a safe and efficient facility used to deliberately decelerate and stop vehicles by transferring their kinetic energy through the displacement of aggregate in a gravel bed.
Arterial Road A road that predominantly carries through traffic from one region to another, forming the principal avenue of communication for traffic movements.
Auxiliary Lane The portion of the carriageway adjoining the through traffic lanes for speed change, or for other purposes supplementary to the through traffic movement.
Average Recurrence Interval (ARI) The Average Recurrence interval (ARI) is the average interval of time during which an event will be equalled or exceeded once. It should be based on a lengthy period of records of the event. Statistically it is the inverse of the Average Exceedence Probability. The term replaces recurrence interval.
Batter The uniform side slope of walls, banks, cuttings or embankments, expressed as a ratio of 1 vertical on x horizontal as distinct from grade.
Batter rounding Curvature that is applied to improve the stability and appearance of the road at the intersection of the extension of the road crossfall and/or existing surface (hinge point), with the batter slope of an embankment or cutting.
Barrier An obstruction placed to prevent vehicle access to a particular area.
Barrier Kerb A kerb with a profile and height sufficient to prevent or discourage vehicles
G L O S S A R Y O F T E R M S
A
Bunching Grouping of vehicles travelling in the same direction with restricted speed caused by the slow moving head of the bunch and limited overtaking opportunities.
Bus Bay An auxiliary lane of limited length at a bus stop or terminus usually indented into the shoulder or verge.
Carriageway That portion of a road or bridge devoted particularly to the use of vehicles, inclusive of the shoulders and auxiliary lanes.
Catch drain A surface channel constructed along the high side of a road or embankment, outside the batter to intercept surface water.
Catchment Area The area that will contribute to the discharge of a stream after rainfall at the point under consideration.
Channelised Intersection An intersection provided with channelised islands.
Centreline The basic line that defines the axis or alignment of the centre of a road or other works.
Clear Zone An area adjacent to the traffic lane that should be kept free from features potentially hazardous to errant vehicles.
Clearance The space between a stationary and/or moving object.
Climbing Lane A special case of an overtaking lane located on a rising grade.
Coefficient of Run-off The ratio of the amount of water that runs off a catchment area to the amount that falls on the catchment.
Compound Curve A curve consisting of two of more arcs of different radii curving in the same direction and having a common tangent point or being joined by a transition curve.
Crossfall The slope, measured at right angles to the alignment, of the surface of any part of a carriageway.
Cross Section The transverse elements of the longitudinal elements.
Crown The highest point on the cross section of a carriageway with two-way crossfall.
Curvilinear Alignment The alignment is a continuous curve with constant, gradual and smooth changes of direction.
Cycle Lane A paved area adjacent to and flush with the traffic lane pavement, for the movement of cyclists. A lane designated for the exclusive use of cyclists.
Deceleration Lane An auxiliary lane provided to allow vehicles to decrease speed.
Deck The bridge floor directly carrying traffic loads.
Design Life The period during which the quality of a structure (eg riding quality of a pavement) is expected to remain acceptable.
Design Speed A speed fixed for the design and correlation of those geometric features of a carriageway that influence vehicle operation. Design speed should not be less than the operating speed.
Design Traffic The predicted cumulative traffic at the design year, expressed in terms of vehicles.
Design Vehicle A hypothetical road vehicle whose mass, dimensions and operating characteristics are used to determine geometric requirements.
Design Year The predicted year in which the design traffic would be reached.
Discharge The volumetric rate of water flow.
Divided Road (divided carriageway) A road with a separate carriageway for each direction of travel created by placing
some physical obstruction, such as a median or barrier, between the opposing traffic directions.
C
Drainage The natural or artificial means for the interception and removal of surface or subsurface water.
Ease Section of rounding.
Footpath A public way reserved for the movement of pedestrians and manually propelled vehicles. A separate facility for pedestrians remote from the road carriageway. It may also be the paved part of the “footpath” used by pedestrians.
Footway Pedestrian facility on a bridge.
Formation The surface of the finished earthworks, excluding cut or fill batters.
Frangible Term is used to describe roadside furniture designed to collapse on impact. The severity of potential injuries to the occupants of an impacting vehicle is reduced, compared to those that could occur if the furniture was unyielding.
Freeway A divided highway for through traffic with no access for traffic between interchanges and with grade separation at some interchanges.
Grade The rate of longitudinal rise (or fall) of a carriageway with respect to the horizontal, expressed as a percentage.
Grade Separation The separation of road, rail or other traffic so that crossing movements, which would otherwise conflict, are at different elevations.
Hinge Point The point in the cross-section of a road at which the extended batter line would intersect the extended verge line.
Horizontal Alignment The bringing together of the straights and curves in the plan view of a carriageway.
Horizontal Curve A curve in the plan view of a carriageway.
Intensity of Rainfall The rainfall in a unit of time.
Interchange A grade separation of two or more roads with one or more interconnecting carriageways.
Intermediate Sight Distance The ISD is equal to 2 x stopping distance for the operating speed.
Intersection A place at which two or more roads meet.
Intersection Angle 1. The angle between two intersecting roads.
2. The angles between the centrelines of two intersecting carriageways.
Intersection (at-grade) An intersection where carriageways cross at a common level.
Intersection Leg Any one of the carriageways radiating from and forming part of an intersection.
K Value The length required for a 1% change of grade on a parabolic vertical curve.
Kerb A raised border of rigid material formed at the edge of a carriageway.
G L O S S A R Y O F T E R M S
( c o n t ’ d )
H
I
J, K
E
F
G
Limiting Curve Speed Standard The curve speed at which f just equals f max, Vs.
Line of Sight The direct line of uninterrupted view between a driver and an object of specified height above the carriageway in the lane of travel.
Longitudinal Friction Factor The friction between vehicle tyres and the road pavement under locked wheel braking conditions, measured in the longitudinal direction.
Longitudinal Section A vertical section, usually with an exaggerated vertical scale, showing the existing and design levels along a road design line, or another specified line.
Median A strip of road, not normally intended for use by traffic, which separates carriageways for traffic in opposite directions.
Median Island A short length of median serving a localised purpose in an otherwise undivided road.
Median Lane The traffic lane nearest the median.
Median Opening A gap in a median provided for crossing and turning traffic.
Minimum Turning Path The path of a designated point on a vehicle making its sharpest turn.
Minimum Turning Radius The radius of the minimum turning path of the outside of the outer front tyre of a vehicle.
Motorway A divided highway for through traffic with no access for traffic between interchanges and with grade separation at some interchanges.
Multiple Combination Vehicles The full range of truck, prime mover and semi trailers and road trains.
Normal Cross Section The cross section of the carriageway where it is not affected by superelevation or widening.
Off-tracking The radial offset between the path traced by the centre of the front axle and the centre of the effective rear axle.
One-way Road A road or street on which all vehicular traffic travels in the same direction.
Operating Speed The 85th percentile speed of cars at a time when traffic volumes are low and will allow a free choice of speed within the road alignment.
Overtaking The manoeuvre in which a vehicle moves from a position behind to a position in front of another vehicle travelling in the same direction.
Overtaking Distance The distance required for one vehicle to overtake another vehicle.
Overtaking Lane An auxiliary lane provided to allow for slower vehicles to be overtaken. It is line-marked so that all traffic is initially directed into the left-hand lane, with the inner lane being used to overtake.
Overtaking Zone A section of road on which at least 70 per cent of drivers will be prepared to carry out overtaking manoeuvres subject to availability of adequate gaps in the opposing direction.
Passing The manoeuvre by which a vehicle moves from a position behind to in front of another vehicle, which is stationary or travelling at crawl speeds.
Passing Bay A very short auxiliary lane (of the order of 100 m) that allows a slow vehicle to pull aside to allow a following vehicle to pass.
Pavement That portion of a road designed for the support of, and to form the running surface for, vehicular traffic.
Perception Distance The sight distance required accessing the curvature of horizontal curves on approach.
Property Line The boundary between a road reserve and the adjacent land.
Rainfall Intensity The rate of rainfall (mm/hr).
Rate of Rotation The rate of rotation required achieving a suitable distance to uniformly rotate the
N
O
P
Q, R
M
crossfall from normal to full superelevation. The usual value adopted is 0.025 rad/sec; 0.035 rad/sec is the maximum value.
Reaction Distance The distance travelled during the reaction time.
Reaction Time The time between the driver’s reception of stimulus and taking appropriate action.
Re-alignment An alteration to the control line of a road that may affect only its vertical alignment but, more usually, alters its horizontal alignment.
A method of widening a road reservation.
Reverse Curve A section of road alignment consisting of two curves turning in opposite directions and having a common tangent point or being joined by a short length of tangent.
Residual Median The remnant area of the median adjacent to right turn lanes.
Road Furniture A general term covering all signs, streetlights and protective devices for the control, guidance and safety of traffic, and the convenience of road users.
Roadside Safety Barrier A device erected parallel to the road to retain vehicles that are out of control.
Road (way) A route trafficable by motor vehicles; in law, the public right-of-way between boundaries of adjoining property.
Roundabout An intersection where all traffic travels in one direction around a central island.
Run-off That part of the rainfall on a catchment which flows as surface discharge past a specified point.
Sag Curve A concave vertical curve in the longitudinal profile of a road.
Section Operating Speed The 85th percentile speed of cars traversing a section of road alignment.
Semi-Mountable Kerb A kerb designed so that it can be driven across in emergency or on special occasions without damage to the vehicle.
Shared Path A paved area particularly designed (with appropriate dimensions, alignment and signing) for the movement of cyclists and pedestrians.
Shoulder The portion of formed carriageway that is adjacent to the traffic lane and flush with the surface of the pavement.
Sideways Friction Coefficient The ratio of the resistance to side ways motion of the tyre of a vehicle (on a specified pavement) and the normal force on that wheel due to the vehicle mass.
Sight Distance Approach Sight Distance (ASD)
The distance required for a driver to perceive marking or hazards on the road surface approaching an intersection and to stop.
Car Stopping Distance (SSD)
G L O S S A R Y O F T E R M S
( c o n t ’ d )
The clear area required for a truck driver to perceive a train approaching an uncontrolled railway crossing and to stop the truck
Safe Intersection Sight Distance (SISD)
The distance required for a driver in a major road to observe a vehicle entering from a side road, and to stop before colliding with it.
Sight Distance Through Underpass
The distance required for a truck driver to see beneath a bridge located across the main road, to perceive any hazard on the road ahead, and to stop.
Stopping Sight Distance
The sight distance required by an average driver (car or truck depending on design requirements), travelling at a given speed, to react and stop before striking an object on the road.
Truck Stopping Sight Distance
The distance required for a truck driver to perceive a hazard, react and brake to a stop.
For design purposes, the braking of an unladen vehicle in wet weather conditions without locking the wheels is assumed.
Sight Triangle The area of land between two intersecting roadways over which vehicles on both roadways are visible to each driver.
Skid Resistance The frictional relationship between a pavement surface and vehicle tyres during braking or cornering manoeuvres. Normally measured on wet surfaces, it varies with the speed and the value of ‘slip’ adopted.
Slope 1. The inclination of a surface with respect to the horizontal, expressed as rise or fall in a certain longitudinal distance.
2. An inclined surface.
Speed 85th Percentile Speed
The speed at which 85 percent of car drivers will travel slower and 15 percent will travel faster.
Operating Speed of Trucks
The 85th percentile speed of trucks measured at a time when traffic volumes are low.
Section Operating Speed
The value at which vehicle speeds on a series of curves tend to stabilise, are related to the range of radii on the curves.
Speed-change Lane A subdivision of auxiliary lanes, which cover those lanes used primarily for the acceleration or deceleration of vehicles. It is usual to refer to the lane by its actual purpose (eg. deceleration lane).
Sub-arterial Road Road connecting arterial roads to areas of development, and carrying traffic directly from one part of a region to another.
Superelevation A slope on a curved pavement selected so as to enhance forces assisting a vehicle to maintain a circular path.
Superelevation Development The length over which the crossfalls on a carriageway are gradually changed from normal crossfall to full superelevation crossfall.
Superelevation Runoff That part of superelevation development that goes from flat crossfall to full superelevation crossfall (on the outside of the curve, when there are segments rotating either side of the axis of rotation).
Swept Path The area bounded by lines traced by the extremities of the bodywork of a vehicle while turning.
Swept Width The radial distance between the innermost and outermost turning paths of a vehicle.
Table drain The side drain of a road adjacent to the shoulder, having its invert lower than the pavement base and being part of the formation.
Tangent Runout The length of roadway required to accomplish the change in crossfall from a
T
normal crown section to a flat crossfall at the same rate as the superelevation runoff.
Terrain Topography of the land.
Level Terrain
Is that condition where road sight distance, as governed by both horizontal and vertical restrictions, are generally long or could be made to be so without construction difficulty or major expense.
Undulating Terrain
Is that condition where road sight distance is occasionally governed by both horizontal and vertical restrictions with some construction difficulty and major expense but with only minor speed reduction.
Rolling Terrain
Is that condition where the natural slopes consistently rise above and fall below the road grade and where occasional steep slopes offer some restriction to normal horizontal and vertical roadway alignment.
The steeper grades cause trucks to reduce speed below those of passenger cars.
Mountainous Terrain
Is that condition where longitudinal and transverse changes in the elevation of the ground with respect to the road are abrupt and where benching and side hill excavation are frequently required to obtain acceptable horizontal and vertical alignment. Mountainous terrain causes some trucks to operate at crawl speeds.
Time of Concentration The shortest time necessary for all points on a catchment area to contribute simultaneously to run-off at a specified point.
Traffic A generic term covering all vehicles, people, and animals using a road.
Traffic Control Signal A device that, by means of changing coloured lights, regulates the movement of traffic.
Traffic Island A defined area, usually at an intersection, from which vehicular traffic is excluded. It is used to control vehicular movements and as a pedestrian refuge.
Transition Transition length for increasing or decreasing the number of lanes.
Traffic Lane A portion of the carriageway allocated for the use of a single line of vehicles.
Traffic Sign A sign to regulate traffic and warn or guide drivers.
Transition Curve A curve of varying radius to model the path of a vehicle entering or leaving a horizontal circular curve.
Transition Length for alignment The distance within which the alignment is changed in approach from straight to a horizontal curve of constant radius.
Transition Length for crossfall The distance required rotating the pavement crossfall from normal to that appropriate to the curve. Also called superelevation development length.
1 .
A B A L A N C E D A P P R O A C H
1 . 1 G e n e ra l
Roads will continue to be an important part of our transport system for the foreseeable future by providing for the safe and operationally efficient movement of people and goods. A balanced approach towards road planning and design can improve road safety and public amenity, and reduce the effect of noise, vibration, pollution and visual intrusion on the areas through which a road passes. The objectives of new and existing road networks should be carefully considered to achieve the desired balance and must take into account the available resources to achieve them.
In every situation designers will be faced with competing demands from different sections of the community as they endeavour to design safe, operationally efficient roads. The various chapters in this publication provide a guide to practitioners on the standards that can be achieved within social, environmental, economic and other constraints using best local and overseas practice.
1 . 2 D e s i g n S t a n d a rd s
Geometric road design standards are used as an aid to achieving consistent and operationally effective road designs. Rapid expansion and improvement to road networks precipitated the need for standards to:
● maintain a degree of uniformity, particularly across administrative boundaries;
● enable satisfactory designs to be produced, even where there was not a high degree of expertise; and
● ensure that road funds were not miss-spent, through inappropriate designs, or through inadequate provision for future traffic growth or for current operations.
Prior to the 6th edition of this guide, many of the standards
adopted in Australia were based heavily on those used in the USA and other developed countries. However, with the 6th
edition, standards that were more appropriate for Australia were promoted. There were two aspects to these new standards:
● Technical – relating to safety and efficiency of traffic operations and particularly to alignment design. Experience has shown that rigid adherence to the earlier standards did not always ensure a safe, operationally efficient road; and
● Costs of desirable road construction projects almost always exceed the total of funds that can be made available. In this situation, each upward increment in design standards
to which a road project is built, results (due to the slightly increased cost) in the deferment of other projects to enable the higher cost project to be funded. Improved provision for future traffic results in greater deficiencies on the balance of the road system with respect to present traffic. The more constrained the financial situation, the more these tradeoffs become evident.
There are three distinct stages in the development of a country’s road system. The importance of geometric standards depends very much on the stage reached.
● Stage 1 – Basic Network. The establishment of a basic
network so that transport links exist where they are required. The roads must be trafficable. Geometric standards are relatively unimportant except as they affect matters like drainage and gradient;
● Stage 2 – Increasing Capacity. Improving the road’s ability
to carry increasing volumes of traffic. This includes structural strength, but geometric standards assume greater importance; and
● Stage 3 – Quality of Service. Building operational safety,
efficiency and convenience into the network, as embodied in a concept of ‘quality of service’. Alignment standards become important, and cross section standards need to be more generous to accommodate significant volumes of high-speed traffic.
The development of the Australian and New Zealand road network is a mixture of increasing the network capacity and providing for an improved quality of service. Parts of the more remote areas still have road development problems associated with the establishment of a basic network. Many of the imported geometric standards that were used prior to the 6th
edition related to the quality of service that a road provides. Problems arose through their inappropriate application in areas where a basic network was still being developed. The main problem now for geometric standards is that there are many areas where the road system exhibits all three stages of development. In these areas motorists are more likely to be influenced by the geometry of the ‘Stage 2’ roads. Hence, they are likely to be more demanding of the standard of geometry on ‘Stage 1’ roads.
1 . 3 S p e e d C o n ce p t
1 . 3 . 1
G e n e ra l
When assessing the major roles that a road should fulfil and the standard of this provision, engineering judgement will be required. Identified problems or concerns need to be carefully considered and a range of alternative solutions examined before deciding upon a particular course of action. Judgement of what is considered “acceptable” for the road in question will involve a balance between such issues as traffic capacity, the environment, speed, safety and road user comfort. It is
I N T R O D U C T I O N
1
important to determine which of the various demands should be given priority, taking into account function and operating conditions of the road and its relationship with other roads in the adjacent network.
Use of the traditional design speed concept as a criterion for alignment consistency on rural roads was introduced in the USA in the 1930s in response to increasing numbers of accidents at horizontal curves. This concept was developed as a mechanism for designing rural road alignments permitting the majority of drivers to operate uniformly at their desired speed. However, as identified by researchers in various countries, the concept has not always produced safe and consistent alignments.
Various speed studies in Australia, New Zealand and overseas have shown that on roads designed for speeds less than 100 km/h the 85th percentile driver exceeds the design speed by up to 20 km/h. The revised design procedure in Guide to the
Geometric Design of Rural Roads (NAASRA, 1989) incorporated
considerations of operating speeds to improve alignment consistency. The guide had four basic speed parameters: ● desired speed;
● speed environment;
● design speed; and
● limiting curve speed standard.
There was some uncertainty in the application of the NAASRA (1989) design parameters because:
● different interpretations were given to the term speed environment;
● designers were reluctant accept the predicted speeds on some low radii;
● no clear instructions were available on the use of the design curves;
● results obtained by different designers were not consistent; and
● very long lengths of relatively straight road were required for vehicles to reach the speed environment.
In spite of these problems, the basic procedure provided appropriate outcomes. However, in order to make the procedures more transparent, there is a need for a more specific method for determining speeds on straight and horizontal curves.
From observations of driver behavior in hilly terrain, it was
1 . 3 . 2
H i g h S p e e d R o a d s
These are roads with design speeds in excess of 100 km/h. On these high-speed roads operating speeds are not constrained by the geometry of the road but by a number of other factors, which include:
● The degree of risk the drivers are prepared to accept; ● Speed limits and the level of policing of these limits; and ● Vehicle performance.
Roads with design speeds of 110 km/h and 130 km/h are likely to have similar operating speeds.
McLean (Ref. 71) noted that drivers generally wish to travel at around 100 km/h to 110 km/h. On roads designed for lower speeds, drivers tend to “overdrive” the road. Conversely on roads designed for higher speeds, drivers adopt an operating speed of 100 km/h to 110 km/h.
1 . 3 . 4
L ow S p e e d R o a d s
These are roads having many curves with radii less than 150 m. Operating speeds on the curves vary from 50 km/h to 70 km/h. These roads are only used when difficult terrain and costs preclude the adoption of higher speeds. The alignments provided in these circumstances could be expected to produce a high degree of driver alertness, so those lower standards are both expected and acceptable. The most pragmatic approach to the design of individual elements in such constrained situations is to provide the best that appears practicable, and to check that it is within the absolute minimum standards for the predicted 85th percentile speed. Innovative, non-standard treatments will often be required when these standards cannot be met.
On roads with speed limits less than 100 km/h, the operating speed of vehicles will be determined by the geometric constraints of the road on the imposed speed limits and the corresponding operating speeds refer Section 7.2 and Figure 7.1.
1 . 3 . 5 8 5 t h
Pe rce n t i le
S p e e d
The term “eighty fifth percentile speed” indicates that 85 percent of car drivers will travel at or below this speed and 15 percent will travel faster. In effect, this means that designs based on the 85th percentile speed will cater for the majority of drivers. For design purposes, the 15% of drivers who exceed this speed are considered to be aware of the increased risk they are taking and are expected to maintain a higher level of alertness, effectively reducing their reaction times.
2 .
R O A D F U N C T I O N A L C L A S S E S
Roads fall into a hierarchy of functional classes ranging from major arterial to local access. Austroads has defined a system of functional classification for rural roads (see Table 2.1). Functional classes are not always clear-cut since almost all roads have some degree of local importance.
Rural roads of higher functional class generally cater for a higher (though normally still modest) proportion of longer length journeys, and it may be appropriate to select higher design standards for such roads so that the quality of service is more appropriate to the longer trip’s duration. However designers must be aware of placing too much importance on functional class alone where traffic volumes are low. Further discussion on functional classification of roads is given in Ref. 22.
3 .
D E S I G N A P P R O A C H
The road, therefore, must be considered at all stages of design as a three-dimensional structure that should be safe, functional and economical but also aesthetically pleasing.
3 . 2 T h e D r i ve r ’s V i ew
The driver sees a foreshortened and, thus, distorted view of the road, and unfavourable combinations of horizontal and vertical curves can result in apparent discontinuities in the alignment, even though the horizontal and vertical designs each comply separately with the provisions of their individual design requirements. Such combinations can mask from the driver a change in horizontal alignment or even a sag curve deep enough to conceal a significant hazard (the hidden dip problem). Only the consideration of the road as a three dimensional entity can reveal such deficiencies, and good design practice requires the elimination of all avoidable hazards even though some additional expense may be incurred. The removal of hazards is not, however, the only benefit, as the improved safety and performance potential is invariably accompanied by significantly enhanced amenity.
Not only is the driver’s view constantly changing, but the duration of his view of successive elements of the road is also varying. Features situated in long, low sag curves remain in view for a considerable length of time whereas other features at or near an abrupt crest or on a tight curve are in view only fleetingly. It follows then that important features such as intersections are most favourably located on long sag curves. Visual cues to the driver from peripheral areas must be given adequate attention. While the designer views the whole road layout at once, and is aware of all changes in alignment, the driver sees much less at any one time. The driver’s inherently restricted view can be further limited at night, or in other times of poor visibility. The designer must, therefore, provide the driver with as many clues as possible as to what lies ahead, but must make sure that the roadside conditions do not convey messages which are ambiguous or misleading.
3 . 3 C o - o rd i n a t i o n o f H o r i z o n t a l a n d
Ve r t i c a l A l i g n m e n t
3 . 3 . 1
G e n e ra l
It has been shown that the operation of a road is influenced partly by the nature of the terrain and partly by the horizontal alignment. It follows, therefore, that if the
Table 2.1 Austroads Functional Rural Road Classification
A R T E R I A L R OA D S
C l a s s 1
Those roads, which form the principal avenue for communications between major regions, including direct connections between capital cities.
C l a s s 2
Those roads, not being Class 1, whose main function is to form the principal avenue of communication for movements between:
● A capital city and adjoining states and their capital cities; or
● A capital city and key towns; or ● Key towns.
C l a s s 3
Those roads, not being Class 1 or 2, whose main function is to form an avenue of communication for movements: ● Between important centres and the Class 1 and Class
2 roads and/or key towns; or ● Between important centres; or
● Of an arterial nature within a town in a rural area.
LO CA L R OA D S
C l a s s 4
Those roads, not being Class 1, 2 or 3, whose main function is to provide access to abutting property (including property within a town in a rural area).
C l a s s 5
Those roads, which provide almost exclusively for one activity or function, which cannot be assigned to Classes 1 to 4.
ensure adequate sight distances to potential hazards on the road and, where such sections merge into more constrained alignment sections, such transition must be accomplished gradually rather than suddenly.
In flat open terrain, long straight road sections are common, but generally there is advantage in avoiding excessive lengths of straight road. A gentle curvilinear design, as discussed in Section 9, always helps to keep the operating conditions ‘under control’ and at the same time, affords scope for far more sympathetic fitting of the road to terrain. The increased flexibility of this approach enables more pleasing designs to be produced at no extra cost; economies in earthworks can often be achieved by fitting the road more closely to the terrain. In addition, safety is enhanced by making the driver more aware of his speed, by allowing him to make better assessments of the distances and speeds of other vehicles, by reducing headlight or sun glare in appropriate circumstances and by reducing boredom and fatigue. Even in flat country curvilinear designs can be used. Radii must be very large, so that all of the benefits of a curving alignment are achieved. Estimation of speed of oncoming vehicles is not significantly improved over a straight alignment when radius exceeds about 5,000m to 10,000m. It is the opinion of experienced designers, however, that sufficient benefits do still remain to make the exercise worthwhile.
3 . 3 . 2
C u r v i l i n e a r D e s i g n
Curvilinear design is most readily applicable to divided roads with their less stringent sight distance requirements but the principles are just as relevant to single carriageway roads provided care is taken to ensure adequate overtaking opportunities are available.
Very large radius curves can provide overtaking opportunities and, as mentioned above, retain at least some of the benefits of curvilinear alignment. If the topography is such that ‘natural’ curvature precludes the provision of overtaking sight distance, then the provision of overtaking zones may produce an economical as well as an aesthetic solution.
Figure 3.1 illustrates basic examples of the method and benefits of proper fitting of the road to the terrain and of proper coordination of horizontal and vertical elements. In addition, there are some examples of poor design form, with indications of appropriate remedial measures. These latter examples are typical of the results likely if the designer does not consider the vertical and horizontal views simultaneously; particularly if a ‘minimum’ vertical standard is superimposed on a relatively unrestricted horizontal regime.
The diagrams are not intended to be comprehensive, but serve merely to demonstrate the general concepts that should (or should not) be followed. In all cases, recognition of the deficiency is sufficient to indicate the appropriate remedy, and the recognition of the deficiency is dependent only on the designer taking a three-dimensional, rather than a two-dimensional view of the problem.
Specific rules are not appropriate to good design, as each particular project has its own peculiar problems and constraints. However, some benefit can be obtained from a consideration of what combinations of horizontal and vertical elements are most likely to produce satisfactory results, and
visualising the schemes in these dimensions using whatever aids are available.
3 . 3 . 3
C o m b i n e d H o r i z o n t a l a n d
Ve r t i c a l A l i g n m e n t
The most pleasing three-dimensional result is achieved if the horizontal and vertical curvature is kept in phase, as
this relates most closely to naturally occurring forms. Where possible, the vertical curves should be contained within the horizontal curves. This enhances the appearance in sag curves by reducing the three-dimensional rate of change of direction, and improves the safety of crest curves by indicating the direction of curvature before the road disappears over the crest. Thus, the best appearance requires the scale of the vertical and horizontal movements to be comparable: a small
movement in one direction should not be combined with a large movement in the other.
Drainage structures in sag curves that are combined with horizontal curves require careful design if a disjointed or kinked appearance is to be avoided. Culverts should introduce little aesthetic difficulty if they are contained within embankments and are made sufficiently long to accommodate full road formation widths.
Bridges built on combined horizontal and vertical curvature can present considerable aesthetic problems, especially if reduced formation widths are used. Particular care should be devoted to the design of the bridge kerbs and railings, as well as to the location and transitioning of approach guard fences. In general, the more generous the curvature, the more pleasing and safer will be the result.
Horizontal curves combined with crests have less influence on the appearance of a road than those combined with sags. Nevertheless, the effect on safety can be much greater, as the crest can obscure the direction and severity of the horizontal curve. Minimum radius horizontal curves, therefore, should not be combined with crest vertical curves.
4 .
T R A F F I C V O L U M E & T R A F F I C
C O M P O S I T I O N
Guide to Traffic Engineering Practice Part 2 (ref 15) provides details of highway capacity analysis. The Highway Capacity Manual Transportation Research Board, HCM 2000, provides a collection of state-of-the-art techniques for estimating the capacity and determining the level of service for transportation facilities, including intersections and roadways as well as facilities for transit, bicycles and pedestrians (Ref 93). Whilst a summary of key principles and issues is provided here, these references should be consulted for more detailed consideration of capacity issues.
Level of Service (LOS) is defined as a qualitative measure describing operational conditions within a traffic stream as perceived by drivers and/or passengers. A level of service definition generally describes these conditions in terms of factors such as speed and travel time, freedom to manoeuvre, traffic interruptions, comfort and convenience and safety. Level of Service A provides the best traffic conditions with no restrictions on desired travel speed or overtaking. Level of Service B to D describes progressively worse traffic conditions. Level of Service E occurs when traffic conditions are at or close to capacity, and there is virtually no freedom to select desired speeds or to manoeuvre within the traffic stream. Flow is unstable and minor disturbances within the traffic stream will cause breakdown of flow.
The service flow rate is defined as the maximum hourly rate at which vehicles can reasonably be expected to traverse a uniform section of a lane or roadway during a given time period under the prevailing traffic and control conditions while maintaining a designated level of service. The service flow rate for LOS E therefore is taken as the capacity of a lane or roadway.
Capacity of rural road sections is influenced by the following key characteristics:
Designers need to consider future traffic demands for a road section to determine the required cross sectional configuration. A design period of 20 years is to be considered in determining capacity requirements. Consideration should be given to the staged construction or widening of roads over this period. Design requirements for rural roads are typically assessed by reference to forecasts of AADT. Design hour volumes may be derived by consideration of the flow pattern across hours of the year. A 30thhighest hourly volume is often adopted as a
design volume. In areas of high peak demands, such as recreational routes, special consideration may be required. Research (Ref. 57) has suggested an alternative specification of the design volume according to the percentage of traffic for which a selected level of service is to be exceeded (eg. provide LOS D or better for 85% of all traffic).
In addition to capacity considerations traffic volume and composition is a key input to the structural design of pavement, culverts and bridges. Truck volumes are a critical input.
5 .
D E S I G N V E H I C L E
The physical and operating characteristics of vehicles using major rural roads are controls in geometric design. The design vehicle is a hypothetical vehicle whose dimensions and operating characteristics are used to establish lane width, intersection layout and road geometry. This chapter discusses the design vehicle for mid-block sections.
Historically, three general classes of vehicles have been selected for design purposes, namely:
● Design prime mover and semi-trailer (19.0 m); ● Design single unit truck/bus (12.5 m); and ● Design car (5.0 m).
These three vehicle types are the basic design vehicles for most road and traffic design situations.
The 19.0m prime mover and semi-trailer is to be used as
F U N D A M E N TA L D E S I G N
C O N S I D E R AT I O N S
2
operating in accordance with normal traffic regulations. Larger vehicles (33 metre B-triple and 30 metre super B-double) and those operating under restricted access conditions may also be catered for, but this will usually involve encroachment into other traffic lanes. This may cause some inconvenience to other road users, but may be acceptable where there is a low frequency of occurrence together with the effect of special conditions associated with the permit.
Where the route is designated for the use of special vehicles that fall outside the three general classes (other freight efficient vehicles, over-length buses, type 1 or 2 road trains), or where regular use of the route by these vehicles could reasonably be expected (access to industrial areas, bus routes), the design should satisfy the needs of such vehicles. The operation of these vehicles should not be compromised by having to encroach into other traffic lanes.
The geometric design should be checked for B-doubles and special vehicles where the need is demonstrated and at the areas where problems are most likely to occur. Most arterial rural roads are likely to have some B-double operation even if they are not specific B-double routes. Table 5.1 describes the provisions that need to be made for trucks. These can also be used for special vehicles. Design guidelines for the various geometric issues in the table are discussed in subsequent sections.
Guide to Traffic Engineering Practice, Part 5 – Intersections at Grade (Ref. 18) provides detailed guidance on intersection design.
6 .
ENVIRONMENTAL CONSIDERATIONS
6 . 1 Tra f f i c R e l a t e d I n t r u s i o n
The various impacts of roads in the rural environment are of growing concern to individuals and communities. It is important to fully consider the impact of these issues in any road design. Reduction of adverse environmental impact should be one of the main objectives of any road project. New rural roads should not only be constructed to link major rural centres, but also to bypass areas sensitive to traffic impacts. Good design should aim to ensure that sensitive environments are not disturbed.
The careful design of rural roads can incorporate the means to ameliorate the environmental intrusion of road infrastructure and associated traffic. In particular, consideration should be given to visual amenity through the use of landscaping and creativity with structures and noise barriers. At the design stage, measures to address safety and access issues for all road users will reduce the impact of road projects. Traffic related intrusions perceived by people include:
● Visual; ● Noise; ● Vibration; ● Air pollution; ● Erosion;
● Risk of accidents and intimidation (Chapter 17); ● Deterioration of water quality (Chapter 16);
● Adverse effect on environmentally sensitive areas; and ● Clearing.
6 . 1 . 1 V i s u a l
The visual intrusion of a road project can have a dramatic effect on abutting individual residents and communities. The visual amenity of a project can be greatly enhanced by the design of creative and functional landscaping. The expense of visual landscaping can be shared by the other functions that the landscaping will aid, such as soil erosion control, replacement vegetation and amenity.
6 . 1 . 2 N o i s e
The potential for noise disturbance to individuals and communities resulting from traffic use of road networks is high. Concern regarding the adverse effects of noise in the environment has resulted in strict noise regulations being developed and enforced by relevant authorities.
Factors affecting noise levels that should be considered by designers include:
● Number, speed, type and condition of vehicles; ● Road surface type, condition and gradient;
● Distance of the noise sensitive land use from the road (particularly intersections);
● Shielding (natural/built) between the road and noise sensitive area;
● Type of terrain (reflective/absorptive) between the road and noise sensitive area; and
● Meteorological conditions (prevailing winds).
Methods available to the road designer to reduce the impact of noise from traffic include:
● Where possible, locating the route away from noise sensitive areas;
● Using pavement surfaces that have been developed for reduced tyre/surface noise (eg. open graded friction course asphalt);
● Using geometric design features that encourage the smoother flow of traffic, such as flatter grades and the elimination of at-grade intersections;
● Locating the road in a cutting or a tunnel where the effects of noise are constrained except at the ends. Cuttings, tunnels and retaining walls could be fitted with noise absorptive cladding; and
● Providing shielding with landscape features such as earth mounds with appropriate plantings, or with noise attenuation barriers. These barriers may be an architectural feature or designed to blend into the surroundings. Transparent barriers can be used to maintain views. The required height, location and material type of barriers should be based on acoustic modelling. Cross-sectional detail to provide for noise barriers is shown on Figure 11.7.
LOCATION
Intersections
Table 5.1: Provision for Trucks
PROVISION FOR TRUCKS
Provide for the swept paths of trucks. Refer to Design
Vehicles and Turning Path Templates. (Ref. 36). Roadside
obstructions shall be located 600 mm clear of the swept path that is travelled when the vehicle’s wheels are in the tray of the kerb and channel.
Provide truck stopping sight distance shown on Table 8.3(b) (lateral sight distance restrictions are often critical, particularly at intersections in hilly terrain or near bridge piers).
Provide truck stopping sight distance (refer to Table 8.3(b)) for intersections on or near crest vertical curves.
Provide truck stopping sight distance (refer to Table 8.3(b)) to allow large/special vehicles to turn safely into each road. Vehicle stability should be considered for turning movements by providing radii appropriate for the turning speeds and providing a uniform rate of change for crossfall.
Provide stopping sight distance to railway crossings, speed change areas and merge areas such as lane drops.
Horizontal curves As far as possible, avoid locating features that are likely to require large/special vehicles to break on curves, such as intersections where the major road is on a low radius curve. Note that the extra braking distances required on horizontal curves are not compensated by higher driver eye height.
Reverse curves Provide a straight 0.6V metre long or transition curves between reverse curves to allow for the spiral tracking of trucks. Where deceleration is required on the approaches to a lower radius curve, sufficient distance must be provided to enable drivers to react and decelerate.
Compound curves If deceleration is likely to be required, allow sufficient distance for drivers to react and decelerate. However, the use of compound curves is not desirable.
Transition curves Provide transition curves wherever possible. However, any transition should involve a shift of >0.25m.
Grades Provide sufficient signs to warn drivers of steep downhill grades.
6 . 1 . 3
V i b ra t i o n
Vibration from traffic on rural roads is very unlikely to be significant and action to ameliorate the intrusion will not usually be necessary.
However, where vibration is an issue, the airborne sound pressure issue can be mitigated through noise attenuation or window design.
6 . 1 . 4
A i r Po l l u t i o n
Motor vehicles have an adverse effect on air quality. This results from the discharge into the air of reactive and non-reactive pollutants. The amount of vehicle emissions is dependent on traffic volume, composition of traffic, traffic flow characteristics and road geometry. The impact of the adverse effect of the emissions is dependent on topography, meteorological and atmospheric conditions and the distance of the receptor from the road. On rural roads this intrusion has minimal effect and need not be considered further.
6 . 1 . 5
E ro s i o n
The construction of rural roads can rapidly disturb the environment, leaving extensive scars on the landscape. The cooperation of road engineers, soil conservationists and all personnel involved is essential to reduce the impact of road construction on the environment. The large areas cleared by earthmoving equipment during road construction are a potential soil erosion hazard. Areas, that do not have a cover of grass to slow and reduce water runoff, are subject to excessive water flows and can result in severe loss of soil. Erosion on construction sites can affect adjacent properties and cause the sedimentation of private and public lands, streams, water storage dams, rivers, harbours and lakes. Sediment can destroy vegetation and the natural habitat of native fauna. Soil erosion and sediment can pose a serious threat to the safety, stability and durability of the road itself. These problems can be greatly reduced if adequate planning is undertaken during design and control measures are implemented for each stage of construction. An erosion management plan, which has been developed by all responsible agencies and authorities, shall be the corner stone of rural road projects. The best results will be achieved when an erosion management plan, developed by agreement between the responsible agencies, is in place and a suitably qualified person is engaged to manage and control its implementation.
Specific control measures may include:
● Training construction personnel to understand and implement the control measures;
● Developing culvert and drainage works prior to major construction;
● Minimising disturbance of natural vegetation cover, particularly adjacent to drainage lines;
● Stockpiling topsoil for later respreading to assist the revegetation of areas disturbed during construction; ● Building sedimentation traps;
● Using earth banks to divert water from disturbed areas; ● Lining drains to prevent scouring and gollying;
● Establishing vegetation using suitable plant species; and ● Implementing an appropriate post-construction
maintenance program.
The advantages of a properly managed erosion management plan are:
● Greatly reduced erosion repair costs;
● Marked decrease in down-time following wet weather, resulting in substantial financial benefits;
● Significant improvements in catchment protection and a more acceptable environment adjacent to the site; ● Increased safety.
The cost of erosion and sediment control is likely to be a fraction of the total project costs, but the aesthetic and general benefits of implementing control measures are far greater.
6 . 1 . 6
E n v i ro n m e n t a l ly S e n s i t i ve A re a s
A proposed rural road may highlight other environmental issues either within or close to the road reserve, such as: ● Native flora and fauna;
● Cultural heritage (indigenous and non-indigenous); and ● Water quality.
The construction, use and maintenance of the road must be sensitive to these issues. For example, it is important to retain significant areas of remnant native vegetation, including grasses, in and adjacent to the road reserve. Road design, construction works and maintenance activity should all aim to reduce impact on native flora and fauna habitat.
Identifying and managing any potential impact on sites of historical or archaeological interest should involve a qualified archaeologist and representatives of relevant local Aboriginal Land Councils and heritage bodies. If required, a program for archaeological monitoring should be developed in consultation with the road authority to determine the most appropriate construction methods to avoid or reduce disturbance to the site.
Designers should also consider the influence of social issues when planning and designing rural roads (see Section 15 Community Consultation).
Runoff from the road surface contains pollutants, which can be detrimental to the receiving waters. When AADT is greater than 30,000, the amount of resultant pollutants is very high and the runoff from rural roads should be considered for treatment over the full length of the project. When AADT is less than 30,000, lengths of a project traversing sensitive receiving environments should be considered for treatment to improve the runoff water quality (Ref Section 16.5).
6 . 1 . 7 C le a r i n g
The clearing of all forms of vegetation should be kept to a minimum within the works area. Cleared areas rob soil of the natural protection from erosion, which vegetation provides. Close attention is to be given to determining the extent of clearing when preparing the erosion protection strategy plan for a project.
6.2 Environmental Related Intrusion
6 . 2 . 1
S n ow a n d I ce
Snow and ice can pose a traffic hazard that may require maintenance action and signage to accommodate the safe passage of vehicles.
6 . 2 . 2 F lo o d s
Many areas are inundated with flood waters that over-top the
special crossings or fences to limit intrusion. Cattle underpasses or overpasses can be installed to allow for the safe movement of stock. In the case of natural animals the special crossings and fences may be installed to provide a safe crossing for migratory reasons.
6 . 3
R e fe re n ce s
Guidelines prepared by Austroads (Ref. 31) establish a range of procedures to evaluate environmental impacts and summarise the legislation and operation of Australian Federal and State procedures for use when assessing major road projects. The impacts that need to be addressed to meet the objectives of ecologically sustainable development strategy are described in another Austroads publication (Ref. 35). This strategy is a key document to assist road planners and designers. Further consideration of these issues is set out in Ref. 32 and 39 and various environmental protection policies or guidelines prepared by local environmental authorities.
