.:;;~ I. THE DEPARTMENT §a OF TRANSPORT TRUNK ROADS AND THE GENERATION OF TRAFFIC The Standing Advisory Committee on Trunk Road Assessment Chairman: Mr DA Wood QC A ':::HttBUi_~ I~ THE DEPARTMENT ~a OF TRANSPORT ~ THE STANDING ADVISORY COMMITTEE ON TRUNK ROAD ASSESSMENT TRUNK ROADS AND THE GENERATION OF TRAFFIC Chairman: Mr DA Wood QC December 1994 London: HMSO © Crown copyright 1994 Applications for reproduction should be made to HMSO First published 1994 ISB~ 0 11 551613 1 A O::::::m!1i...... THE STANDING ADVISORY COMMITTEE I~ ON TRUNK ROAD ASSESSMENT THE DEPARTMENT 2 MARSHAM STREET LONDON SWIP 3EB OF TRANSPORT AIJ~ CHAIRMAN: MR DEREK WOOD QC My Ref: Rt Han John MacGregor aBE MP Secretary of state for Transport Your Ref: J 6 May 1994 Sir This Committee was invited to consider the question of whether new or improved roads generate e~tra traffic over and above the growth in traffic which would be expected in the absence of any improvement to the road network. We now have pleasure in submitting our Report. We have consulted a large number of bodies and individuals, and we have carefully weighed the very substantial volume of evidence submitted to us. We have also had the benefit of extensive discussions wi th your Department. We would like to place on record our very great appreciation of all the help given to us in the course of our work. Yours faithfully, ~~ DA Wood / RH stewart Chairman Vice Chairman D Coombe PB Goodwin ~u. ~ ~.................,'rs..-....~ ......_-- p~ fu·~~~ Dtt~ 7 P J Mackie MEG Taylor CONTENTS Chapter Page EXECUTIVE SUMMARY PART I INTRODUCTION AND DEFINITION OF THE PROBLEM 1 Introduction to the Report 1 Terms of Reference 1 Method of Working 1 Form of the Report 2 Miss Lee's 2 Conventions 3 Acknowledgements 3 2 Identifying the Nature of the Problem 5 Introduction 5 'Generated' and 'Induced' Traffic 7 By What Processes Might Road Improvements Induce More Traffic? 8 Responses Connected with the Total Volume of Activities 8 Responses Connected with the Location of Activities 9 Responses Connected with the Timing of Activities 10 Responses Connected with the Mode of Transport Used 10 Responses Connected with the Co-ordination of Activities by Different Individuals 11 Responses Connected with the Route Chosen 11 Responses Connected with the Effects of the Other Responses 11 Conclusion 11 Four Key Questions 12 The Burden of Proof 12 The Structure of the Rest of our Report 12 References 12 3 Defining the Problem in More Technical Terms 15 Introduction 15 The Importance of Behavioural Responses 15 Generalised Cost and the Elasticity of Demand 16 Formal Definition of these Processes in Transport Models 16 Modelling Land-Use Changes 17 Trip Generation 18 Trip Distribution 18 Mode Choice 18 Journey Scheduling 18 Assignment of Traffic 19 Activity Models 19 Economic Models of Demand Elasticity 19 Trunk Roads and the Generation of Traffic Contents Combinations and Hierarchies of Modelled Effects 19 Our Definition of Induced Traffic 20 Induced Traffic in the Context of a Fixed Trip Pattern 22 Induced Traffic in the Context of Variable Trip Patterns 24 References 25 PART II THE EVIDENCE OF INDUCED TRAFFIC 4 General Evidence on Whether Road Capacity Influences the Amount of Traffic 29 Introduction: Limits to the Possibility of Proof 29 The Common Sense Argument and Observations of Traffic Growth 31 Evidence on Professional Confidence in the Current Assumptions 34 Attitude Surveys of Drivers 35 Evidence on Other Influences on the Amount of Traffic 37 Implications of the Research Findings 45 Conclusion 47 References 47 5 Evidence from Traffic Counts on Improved Roads 51 Introduction 51 Review of Traffic Growth on the M25 51 Literature Reviews 54 Department of Transport Monitoring of Forecasts and Observed Traffic 55 Detailed Analyses of Traffic Flows on Specific Schemes 67 Pells' Literature Review: Results for M62, York Northern Bypass and Severn Bridge 69 London Trunk Road Schemes 70 Schemes in the Greater Manchester Area 80 Amsterdam Orbital Motorway 83 Research in Progress 85 General Conclusion on Evidence from Traffic Counts 85 References 85 PART III THE IMPLICATIONS OF INDUCED TRAFFIC 6 The Department's Current Procedures for Traffic Forecasting 89 Introduction 89 The Overall Framework 89 Scheme Appraisal Models 90 The Area Covered by a Scheme Appraisal Model 90 The Form of Scheme Appraisal Models 91 Principles of Forecasting Traffic 93 The National Planning Data Files 93 The National Road Traffic Forecasts (NRTFs) 94 The National Submodels 95 Trunk Roads and the Generation of Traffic Contents The National Forecast Adjustment Factor (NFAF) 95 Forecasts for Scheme Appraisal 96 Other Models Used for Forecasting 98 Regional Highway Traffic Models 99 Variable Trip Matrix Appraisal of Trunk Road Schemes 100 Constraints on Traffic Growth in Urban Areas 100 Models Used for Motorway Widening Schemes 100 References 101 7 The Role of Traffic Forecasts in the Department's Design and Assessment Procedures 103 Introduction 103 Selection of Geometric Standard 103 Traffic Flows for Geometric Design 104 Traffic Flows for Pavement Design 104 Traffic Flows for Environmental Appraisal 106 Traffic Flows for Safety and Economic Evaluations 108 References 108 8 The Department's Approach to the Economic Evaluation of Schemes 111 Introduction 111 The Role of Economic Evaluation 111 Principles of Economic Evaluation 112 Safety Appraisal 120 The COBA Program 120 How COBA is Used 121 References 122 9 Implications of Induced Traffic for Economic Evaluation 123 Introduction 123 The Robustness of the Traffic Benefits to the Fixed Demand Assumption 123 The Commercial Benefits of Road Investment 129 The Department's Appraisal Practice 131 References 133 10 Implications from Experience of Transportation Modelling 135 Introduction 135 Theoretical Model Based on a Single Link 137 Network-Based Modelling of Cardiff 140 Four-Stage Modelling of Belfast 146 Elasticity Modelling of Belfast, West London and Norwich 149 Strategic Transport Demand Modelling of Bristol 152 Land-Use/Transport Interaction Modelling in Leeds, Dortmund and Bilbao 155 Trunk Roads and the Generation of Traffic Contents Land-Use/Transport Interaction Modelling of a Hypothetical New Motorway 156 Land-Use/Transport Interaction Modelling in Norwich 157 Modelling of the Effects of Changes in Road Conditions on Freight Distribution 158 Concluding Remarks 159 References 161 PART IV CONCLUSIONS FROM THE EVIDENCE AND THE IMPLICATIONS OF INDUCED TRAFFIC 11 Conclusions from the Evidence and the Implications of Induced Traffic 165 Introduction 165 Is Induced Traffic a Real Phenomenon? 165 Does Induced Traffic Matter? 168 When and Where Does Induced Traffic Matter Most? 170 References 171 PART V THE WAY FORWARD 12 The Need for Change 175 Introduction 175 The Current Planning Process for Trunk Roads 175 OUf Concerns about the Current Approach to the Planning and Appraisal of the Trunk Road Network 176 The Current Appraisal Hierarchy 178 Deficiencies in Current Appraisal Practice 180 The National Road Traffic Forecasts 180 Traffic Forecasts for Regions and Interurban Corridors 182 Traffic Forecasts for Urban Areas and Conurbations 182 Traffic Forecasts in Peri-Urban Areas 183 Traffic Forecasts for Scheme Appraisal 183 Forecasts of Land-Use Data for Input to the Traffic Forecasting Process 184 The Key Issues 184 References 185 13 Our Recommendations for Change 187 Introduction 187 Basic Research 187 Our Recommended Approach 187 National Road Traffic Forecasts 188 Regional and Interurban Corridor Traffic Forecasts 188 Urban Area or Conurbation Traffic Forecasts 190 Peri-Urban Scheme Appraisal 192 Scheme Appraisal 192 Trunk Roads and the Generation of Traffic Contents Land-Use Data and Land-Use/Transport Interaction Models 193 Appraisal Strategy 194 Appraisal Methodology 194 14 Interim Procedures 197 Introduction 197 National Road Traffic Forecasts 197 Regional, Interurban Corridor, Urban Area, Conurbation and Peri-Urban Traffic Forecasts 197 Individual Schemes 200 Economic Evaluation 201 References 202 PART VI MAIN CONCLUSIONS AND SUMMARY OF RECOMMENDATIONS 15 Main Conclusions and Summary of Recommendations 205 Introduction 205 Our Answers to the First Three Questions 205 Our Answer to the Fourth Question 206 Annexes I Membership of the Standing Advisory Committee on Trunk Road Assessment 213 II Letter to Consultees (June 1992) 215 III List of Those Who Provided Evidence 225 IV Consultants' Studies 229 V The Main Stages in the Planning and Construction of a Trunk Road 231 VI Miss Lees letter to the Rt Hon. Dr Brian Mawhinney MP 235 Trunk Roads and the Generation of Traffic Contents Tables Page 4.1 Driver Responses to Changes in Congestion 36 5.1 Comparison of Forecast and Observed Traffic Flows on Trunk Road Schemes 56 5.2 Barnstaple Bypass A39, Two-Way Average Annual Daily Traffic Flows, pcu/day 67 5.3 Results of Roadside Interviews of Drivers Using York Northern Bypass (1988) 69 5.4 Westway (M40) Traffic Flows, 1970, veh/day 72 5.5 Traffic Counts (MIl Corridor) 24-hour Two-Way Flow, veh/day 73 5.6 Traffic Counts (A316 Corridor) 24-hour Two-Way Flows, veh/day 74 5.7 Traffic Counts (Lower Thames Screenline) 12-hour Two-Way Flows, veh/day 75 5.8 Traffic Counts (Lower Thames Screenline) 24-hour Two-Way Flows, veh/day 75 5.9 Traffic Counts M25/River Lea Screenline, 12-hour Two-Way Flows, veh/day 76 5.10 Traffic Counts, Western Screenline, Rochester Way Relief Road (A2), 18-hour Two-Way Flow, pcu/day 78 5.11 Traffic Counts, Eastern Screenline, Rochester Way Relief Road (A2), 18-hour Two-Way Flow,