Safety Assessment of Freeway Merging and Diverging Influence Areas Based on Conflict Analysis of Simulated Traffic

2012·2.6 MB·English

by  Atamo, Markus Alito

SAFETY ASSESSMENT OF FREEWAY MERGING AND DIVERGING
INFLUENCE AREAS BASED ON CONFLICT ANALYSIS
OF SIMULATED TRAFFIC

By
Markos Alito Atamo
B.Sc., Addis Ababa University, Ethiopia, 2002
M.Sc., Addis Ababa University, Ethiopia, 2005

A thesis submitted to the
University of Colorado Denver
in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
Civil Engineering
2012
©2012 by Markos Alito Atamo
All rights reserved

Atamo, Markos Alito (Ph.D., Civil Engineering)
Safety Assessment of Freeway Merging and Diverging Influence Areas Based
on Conflict Analysis of Simulated Traffic

Thesis directed by Professor Bruce N. Janson.

ABSTRACT
The safety of merging and diverging influence areas, intersections,
interchanges, and other traffic facilities is assessed by tracking and analyzing
police-reported motor vehicle crash records. Since the nature of road crashes
is random and infrequent, this process is slow to reveal the need for
remediation of the roadway design and traffic control strategy. Moreover, this
process is not applicable to assess new designs that have not yet been built,
or deployed in the real world.

This study summarizes a technique combining micro-simulation and
automated conflict analysis to assess the safety of traffic facilities without
waiting for a statistically above-normal number of crashes to occur. The
technique is also valuable in assessing the relative performance of one
design versus another. Traffic Conflict Technique (TCT) is among the most
common surrogate measures to study the safety of roadway facilities. The
software referred to as Surrogate Safety Assessment Model (SSAM) was
developed by FHWA and was used in this study. The SSAM software
application was designed to perform statistical analysis of vehicle trajectory
data. Trajectory data is the output from microscopic traffic simulation models.
Among the various traffic simulation modeling tools, VISSIM was chosen
because of its compatibility with SSAM, its versatility for analyzing networks of
big sizes and its ability to provide users with the capability to model any type
of geometric configurations.

Forty-two merging and forty-two diverging influence areas in Colorado, USA,
were modeled in VISSM and conflict analysis was performed using SSAM
under AM-peak traffic conditions. Five field validation tests were conducted.
Conflicts predicted by SSAM approach were compared with actual crash
records at merging and diverging influence areas in all statistical validation
tests. The results of the validation effort of safety assessment based on
conflict analysis of simulated traffic suggested that this technique is
recommended for safety assessment at merging and diverging locations.

This abstract accurately represents the content of the candidate’s
dissertation. I recommend its publication.

Signed
Bruce N. Janson
DEDICATION

I dedicate this thesis to my loving and devoted wife, Azeb, for her unfaltering
support and understanding while I was completing it. I also dedicate this to
my daughters Yohana, Heldana, and Yosabet and to my parents who gave
me an appreciation of learning and taught me the value of perseverance and
resolve.
ACKNOWLEDGEMENT

My thanks to each of the members of my doctoral committee for their support
during the completion of this thesis.

Very special thanks to my advisor, Professor Janson, for his wise guidance,
instruction and unlimited support. I am very grateful for having the opportunity
to learn from him.
TABLE OF CONTENTS

Figures xiii
Tables xv
Chapter
1. Introduction……………………………………………………….. 1
1.1 Background……………………………………………………….. 1
1.2 Traffic Safety Overview………………………………………….. 2
1.3 Statement of the Problem……………………………………….. 4
1.4 Research Objectives and Contributions to the
Transportation Industry………………………………………….. 5
1.5 Glossary of Terms- Quick Reference Guide………………….. 7
1.6 Study Flow Structure…………………………………………….. 11
2. Review of the Literature…………………………………………. 13
2.1 Traffic Conflict Technique……………………………………….. 13
2.2 Merging and Diverging Movements on Freeways……………. 15
2.3 Survey of Modeling Data………………………………………… 16
2.4 Interchanges and Ramps……………………………………….. 17
viii
2.5 Background of Safety on Freeway Ramps……………………. 22
2.6 Log-linear Regression Models………………………………….. 23
2.6.1 Poisson Regression Model……………………………………… 25
2.6.2 Negative Binomial Regression Model………………………….. 27
2.6.3 Goodness of Fit Measure………………………………………... 29
2.7 Traffic Crash Prediction Model…………………………………. 32
2.8 Empirical Bayes Technique……………………………………... 35
2.8.1 Empirical Bayes Safety Estimate……………………………….. 36
2.8.2 Bayesian Identification of Accident-Prone Locations………. 38
2.9 Simulation Modeling…………………………………………….. 42
2.9.1 Choice of a Simulation Tool……………………………………. 42
2.9.2 VISSIM Micro-Simulation Tool……………………….…………. 43
2.10 SSAM Conflict Analysis Tool……………………….…………… 44
2.11 SAS/STAT Software and the GENMOD Procedure………….. 49
3. Research Approach and Methodology…………………………. 51
3.1 Field Validation…………………………………………………… 51
3.2 Purpose……………………………………………………………. 51
3.3 Data Assembly and Geometric Definition……………………… 51
3.4 Simulation Modeling of Interchanges and
Modeling Assumptions…………………………………………… 55
ix
3.5 Identification and Removal of Outliers………………………… 59
3.6 Methodology……………………………………………………… 60
3.6.1 Field Validation Test 1: Merging and Diverging
Location Ranking by Total Incidents…….……………………... 60
3.6.1.1 Conflict Ranking………………………………………………….. 61
3.6.1.2 Crash Ranking……………………………………………………. 61
3.6.1.3 Ranking Comparison…………………………………………….. 62
3.6.2 Field Validation Test 2: Merging and Diverging
Location Ranking by Specific Incident Types……………….. 63
3.6.3 Field Validation Test 3: Crash and Conflict
Prediction Regression Model Paired Comparison…………… 64
3.6.4 Field Validation Test 4: Crash and Conflict
Prediction Regression Model Comparative
Analysis For Total Incidents…………………………………….. 65
3.6.5 Field Validation Test 4: Crash and Conflict
Prediction Regression Model Comparative
Analysis For Specific Incident Types………….…….……….. 69
3.7 Goodness of Fit Measure……………………………………….. 69
4. Test Results and Discussion…………………………………… 72

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