the relationship between traffic congestion and road accidents

253 Pages·2016·4.26 MB·English

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The relationship between
tra(cid:30)c congestion and road
accidents: an econometric
approach using GIS
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• A Doctoral Thesis. Submitted in partial ful(cid:28)llment of the requirements
for the award of Doctor of Philosophy of Loughborough University.
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THE RELATIONSHIP BETWEEN TRAFFIC
CONGESTION AND ROAD ACCIDENTS: AN

ECONOMETRIC APPROACH USING GIS

By

Chao Wang

A Doctoral Thesis
Submitted in partial fulfilment of the requirements
for the award of
Doctor of Philosophy of Loughborough University

February 2010

© by Chao Wang 2010
ii

ABSTRACT
Both traffic congestion and road accidents impose a burden on society, and it is
therefore important for transport policy makers to reduce their impact. An ideal
scenario would be that traffic congestion and accidents are reduced simultaneously,
however, this may not be possible since it has been speculated that increased traffic
congestion may be beneficial in terms of road safety. This is based on the premise that
there would be fewer fatal accidents and the accidents that occurred would tend to be
less severe due to the low average speed when congestion is present. If this is confirmed
then it poses a potential dilemma for transport policy makers: the benefit of reducing
congestion might be off-set by more severe accidents. It is therefore important to fully
understand the relationship between traffic congestion and road accidents while
controlling for other factors affecting road traffic accidents.
The relationship between traffic congestion and road accidents appears to be an under
researched area. Previous studies often lack a suitable congestion measurement and an
appropriate econometric model using real-world data. This thesis aims to explore the
relationship between traffic congestion and road accidents by using an econometric and
GIS approach. The analysis is based on the data from the M25 motorway and its
surrounding major roads for the period 2003-2007. A series of econometric models
have been employed to investigate the effect of traffic congestion on both accident
frequency (such as classical Negative Binomial and Bayesian spatial models) and
accident severity (such as ordered logit and mixed logit models). The Bayesian spatial
model and the mixed logit model are the best models estimated for accident frequency
and accident severity analyses respectively. The model estimation results suggest that
traffic congestion is positively associated with the frequency of fatal and serious injury
accidents and negatively (i.e. inversely) associated with the severity of accidents that
have occurred. Traffic congestion is found to have little impact on the frequency of
slight injury accidents. Other contributing factors have also been controlled for and
produced results consistent with previous studies. It is concluded that traffic congestion
overall has a negative impact on road safety. This may be partially due to higher speed
variance among vehicles within and between lanes and erratic driving behaviour in the
presence of congestion.
Abstract iii

The results indicate that mobility and safety can be improved simultaneously, and
therefore there is significant additional benefit of reducing traffic congestion in terms of
road safety. Several policy implications have been identified in order to optimise the
traffic flow and improve driving behaviour, which would be beneficial to both
congestion and accident reduction. This includes: reinforcing electronic warning signs
and the Active Traffic Management, enforcing “average speed” on a stretch of a
roadway and introducing minimum speed limits in the UK.
This thesis contributes to knowledge in terms of the relationship between traffic
congestion and road accidents, showing that mobility and safety can be improved
simultaneously. A new hypothesis is proposed that traffic congestion on major roads
may increase the occurrence of serious injury accidents. This thesis also proposes a new
map-matching technique so as to assign accidents to the correct road segments, and
shows how a two-stage modelling process which combines both accident frequency and
severity models can be used in site ranking with the objective of identifying hazardous
accident hotspots for further safety examination and treatment.
Key words: Traffic congestion, road accidents, GIS, M25 motorway, site ranking,
accident hotspots, spatial econometrics, full Bayesian hierarchical models, ordered and
nominal response models, two-stage mixed multivariate models
iv

ACKNOWLEDGEMENTS
I would like to take this opportunity to thank everyone who has helped me in
completing this thesis.
First and foremost, I would like to express my extreme gratitude to my supervisors, Dr
Mohammed A Quddus and Professor Stephen Ison, for their continuous support,
concern and guidance throughout my study. It was a great pleasure to work with them
and I have learned a great deal from them, in terms of both academic and everyday life.
I would also like to thank the UK Highways Agency for the data provided and several
constructive meetings with respect to my research. A special thank is extended to
Rupesh Mehta, who was a member of staff at the Highways Agency and provided
excellent coordination between the Highways Agency and Loughborough University.
I am also very grateful to my friends for their support during my study. I would
especially like to thank Weiyun Li for her encouragement, understanding and concern
in the last year of my research.
Finally, I would like thank my parents, Zhongyu Wang and Bing Li, for their constant
encouragement, support and love. I would not have completed my research and this
thesis without their endless love.
v

TABLE OF CONTENTS
ABSTRACT .................................................................................................................... II
ACKNOWLEDGEMENTS ........................................................................................ IV
TABLE OF CONTENTS .............................................................................................. V
LIST OF TABLES ....................................................................................................... IX
LIST OF FIGURES ..................................................................................................... XI
CHAPTER 1 INTRODUCTION .................................................................................. 1
1.1 BACKGROUND.................................................................................................... 1
1.2 PROBLEM STATEMENT AND INTENTION OF THIS THESIS ...................................... 3
1.3 RESEARCH AIM AND OBJECTIVES ....................................................................... 6
1.4 CLARIFICATIONS OF THE TERMS “TRAFFIC CONGESTION”, “ROAD ACCIDENTS”
AND “SAFETY” USED IN THIS THESIS .................................................................. 7
1.5 OUTLINE OF THE THESIS ..................................................................................... 9
CHAPTER 2 LITERATURE REVIEW OF FACTORS AFFECTING ROAD
ACCIDENTS ................................................................................................................ 11
2.1 INTRODUCTION ................................................................................................ 11
2.2 AN OVERVIEW OF FACTORS AFFECTING ROAD ACCIDENTS ............................... 11
2.3 TRAFFIC CHARACTERISTICS ............................................................................. 14
2.3.1 Speed ........................................................................................................... 16
2.3.2 Traffic density ............................................................................................. 19
2.3.3 Traffic flow .................................................................................................. 20
2.3.4 Traffic congestion ....................................................................................... 22
2.4 ROAD GEOMETRY AND INFRASTRUCTURE ........................................................ 26
2.5 DEMOGRAPHIC, DRIVING BEHAVIOUR AND LAND USE ...................................... 28
2.6 ENVIRONMENT ................................................................................................. 32
2.7 CONCLUSION ................................................................................................... 33
CHAPTER 3 ECONOMETRIC METHODS USED IN ACCIDENT
MODELLING ............................................................................................................... 36
3.1 INTRODUCTION ................................................................................................ 36
Table of Contents vi
3.2 ROAD SEGMENT LEVEL ANALYSIS .................................................................... 36
3.3 AREA-WIDE ANALYSIS ..................................................................................... 38
3.4 ECONOMETRIC MODELS ................................................................................... 40
3.4.1 Accident frequency models ......................................................................... 41
3.4.2 Accident severity models ............................................................................. 49
3.4.3 Practical applications of accident prediction models ................................ 53
3.5 CONCLUSION ................................................................................................... 58
CHAPTER 4 METHODOLOGY ............................................................................... 63
4.1 INTRODUCTION ................................................................................................ 63
4.2 RESEARCH DESIGN ........................................................................................... 63
4.3 CONGESTION MEASUREMENT ........................................................................... 64
4.3.1 Review of congestion measurements ........................................................... 65
4.3.2 Congestion measurements used in this thesis ............................................. 67
4.3.2.1 Congestion index ................................................................................ 67
4.3.2.2 Total delay per length ......................................................................... 68
4.4 MAPPING ACCIDENTS TO THE CORRECT ROAD SEGMENTS ................................ 68
4.5 ACCIDENT FREQUENCY MODELS ...................................................................... 70
4.5.1 Classical count outcome models ................................................................. 70
4.5.2 Spatial models using a full Bayesian hierarchical approach ..................... 73
4.6 ACCIDENT SEVERITY MODELS .......................................................................... 79
4.6.1 Ordered response models ........................................................................... 79
4.6.2 Nominal response models ........................................................................... 82
4.7 SITE RANKING .................................................................................................. 84
4.8 SUMMARY ........................................................................................................ 87
CHAPTER 5 DATA DESCRIPTION ........................................................................ 89
5.1 INTRODUCTION ................................................................................................ 89
5.2 CONGESTION MEASUREMENTS ......................................................................... 90
5.3 DATA FOR ANALYSING ACCIDENT FREQUENCY ................................................ 98
5.4 DATA FOR ANALYSING ACCIDENT SEVERITY .................................................. 106
5.5 DATA VALIDATION ......................................................................................... 109
5.6 SUMMARY ...................................................................................................... 113
CHAPTER 6 RESULTS FROM ACCIDENT FREQUENCY MODELS ............ 114
Table of Contents vii
6.1 INTRODUCTION .............................................................................................. 114
6.2 A SPATIAL ANALYSIS OF THE M25 ................................................................. 114
6.2.1 Congestion ................................................................................................ 122
6.2.2 Other contributing factors ........................................................................ 128
6.3 A SPATIO-TEMPORAL ANALYSIS OF THE M25 AND SURROUND ....................... 130
6.3.1 Classical count outcome models ............................................................... 132
6.3.2 Spatial models using a full Bayesian hierarchical approach ................... 136
6.4 SUMMARY ...................................................................................................... 155
CHAPTER 7 RESULTS FROM ACCIDENT SEVERITY MODELS ................. 157
7.1 INTRODUCTION .............................................................................................. 157
7.2 ORDERED RESPONSE MODELS ........................................................................ 157
7.2.1 Findings for traffic congestion ................................................................. 166
7.2.2 Findings for other contributing factors .................................................... 166
7.3 NOMINAL RESPONSE MODELS ........................................................................ 170
7.3.1 Findings for traffic congestion ................................................................. 173
7.3.2 Findings for other contributing factors .................................................... 176
7.4 SUMMARY ...................................................................................................... 179
CHAPTER 8 SITE RANKING ................................................................................. 181
8.1 INTRODUCTION .............................................................................................. 181
8.2 RESULTS FROM THE TWO-STAGE MODEL ........................................................ 181
8.3 SITE RANKING RESULTS ................................................................................. 186
8.4 SUMMARY ...................................................................................................... 193
CHAPTER 9 DISCUSSION AND POLICY IMPLICATIONS............................. 194
9.1 INTRODUCTION .............................................................................................. 194
9.2 TRAFFIC CONGESTION AND ROAD ACCIDENT FREQUENCY .............................. 194
9.3 TRAFFIC CONGESTION AND ROAD ACCIDENT SEVERITY .................................. 197
9.4 TRAFFIC CONGESTION AND ROAD SAFETY ...................................................... 198
9.5 POLICY IMPLICATIONS ................................................................................... 199
9.6 SUMMARY ...................................................................................................... 204
CHAPTER 10 CONCLUSIONS AND FURTHER RESEARCH ........................ 206
10.1 SUMMARY AND CONCLUSIONS ....................................................................... 206
Table of Contents viii
10.2 CONTRIBUTION TO KNOWLEDGE .................................................................... 209
10.3 LIMITATIONS AND FURTHER RESEARCH ......................................................... 211
10.3.1 Data ...................................................................................................... 211
10.3.2 Econometric models .............................................................................. 213
10.3.3 Extensions of the study area ................................................................. 215
10.3.4 Policy formulation and testing .............................................................. 215
REFERENCES ............................................................................................................ 217
APPENDIX .................................................................................................................. 238