MODELING SAFETY EFFECTS OF GEOMETRIC DESIGN CONSISTENCY ON TWO-LANE RURAL ROADWAYS USING MIXED EFFECTS NEGATIVE BINOMIAL REGRESSION

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MODELING SAFETY EFFECTS OF GEOMETRIC DESIGN CONSISTENCY ON TWO-LANE RURAL ROADWAYS USING MIXED EFFECTS NEGATIVE BINOMIAL REGRESSION

ABSTRACT

 

Previous research has examined the relationship between roadway safety and design consistency using measures such as the difference between design and operating speeds and the difference in operating speeds on successive elements. While such measures have proven effective in identifying inconsistencies in the roadway, they do not directly identify the conditions associated with safety performance. The purpose of this research was to directly quantify the effects of geometric design consistency on roadway safety using measures that can be linked to specific geometric elements. To do so, five years of crash data and roughly 5,000 miles of alignment data from the state of Washington were utilized to model crash experience on 2.5 mile segments.

Using mixed effects negative binomial modeling, three safety performance functions (SPFs) were developed. The first contained typical roadway parameters that were suggested for use by several contemporary safety management tools, while the second contained various geometric design consistency measures developed from the dataset. The final SPF contained both typical and design consistency parameters. After Empirical Bayes adjustments were applied using the conditional overdispersion parameters from the mixed effects negative binomial models, sites with potential (SWiPs) for safety improvements were ranked for each model using the scaled differences in frequencies between the predicted and adjusted number of crashes.

A comparison was then made based on differences in SWiP rankings between the typical parameter model and the final model containing additional design consistency parameters. Ultimately, 40 unique segments were identified by each SPF out of the top

220 segments ranked; this constitutes a 19 percent change in the top 10 percent of segments identified as SWiPs. Additionally, there was marked variation in the order in which SWiPs were ranked. This disparity may lend credence to the incorporation of geometric design consistency parameters in the development of predictive safety models. Ultimately, by directly modeling the inconsistencies in geometric roadway design, practitioners may be able to better identify and categorize unsafe roadways both in the design stage and post-construction. However, it is important to note that the use of design consistency parameters does not ameliorate the modeling process solely based on a difference in SWiP identification; rather, it should encourage further avenues of research into the use of such measures in predictive safety modeling. Although this investigation is only preliminary, the results may help to burgeon the ever-expanding body of literature regarding the relationship between geometric design consistency and roadway safety.

TABLE OF CONTENTS

LIST OF FIGURES………………………………………………………………………………………………….. vi

LIST OF TABLES…………………………………………………………………………………………………… vii

ACKNOWLEDGEMENTS………………………………………………………………………………………… viii

Chapter 1. INTRODUCTION……………………………………………………………………………………… 1

1.1.  General Background…………………………………………………………………………………… 1

1.2.  Purpose of Research…………………………………………………………………………………… 3

Chapter 2. BACKGROUND AND LITERATURE REVIEW…………………………………………………. 5

2.1.  Design Consistency……………………………………………………………………………………. 5

2.2.  Speed Differences………………………………………………………………………………………. 6

2.3.  Alignment Indices…………………………………………………………………………………….. 17

2.4.  Vehicle Stability………………………………………………………………………………………. 21

2.5.  Driver Workload………………………………………………………………………………………. 24

2.6.  Perceived Radius……………………………………………………………………………………… 28

2.7.  Summary…………………………………………………………………………………………………. 33

Chapter 3. METHODOLOGY…………………………………………………………………………………… 34

3.1.  Development of Safety Performance Functions……………………………………………. 34

3.2.  Empirical Bayes Adjustments……………………………………………………………………. 38

3.3.  Identification of Sites with Potential for Safety Improvements………………………. 40

Chapter 4. DATA ACQUISITION AND PREPARATION………………………………………………….. 42

4.1.  Database Acquisition………………………………………………………………………………… 42

4.2.  Preparation of Fixed-Length Segments……………………………………………………….. 43

Chapter 5. RESULTS…………………………………………………………………………………………….. 47

5.1. Safety Performance Functions…………………………………………………………………….. 47

5.2. Empirical Bayes Adjustments…………………………………………………………………….. 57

5.3.  Ranking of Sites with Potential………………………………………………………………….. 58

Chapter 6. CONCLUSIONS……………………………………………………………………………………… 64

6.1. Summary…………………………………………………………………………………………………. 64

6.2. Conclusions……………………………………………………………………………………………… 66

6.3.  Future Work…………………………………………………………………………………………….. 68

REFERENCES………………………………………………………………………………………………………. 71

APPENDIX…………………………………………………………………………………………………………… 75

Chapter 1. INTRODUCTION

1.1. General Background

One of the foremost aspirations of transportation professionals, regardless of realm of expertise, is to maintain the highest levels of safety throughout the roadway network. Over the past decade, there has been a marked decrease in the number of fatal automobile crashes, even with a steadily increasing number of vehicle-miles-traveled (VMT) by motorists. It remains to be seen whether this trend comes by dint of the recent economic downturn or through the efforts of programs like AASHTO’s Towards Zero Deaths and the methodologies established in the Highway Safety Manual (HSM). However, one fact remains evident. Current levels of safety, both those perceived by the roadway user and analytically derived through crash statistics, should leave transportation professionals far from complacent. It is imperative that innovative and more proficient methods for evaluating roadway safety are continuously being developed through

research efforts at all levels of the profession.

Although novel in terms of the overall history of transportation safety, the currently-established method for evaluating roadway safety utilizes statistical regression modeling to estimate crash frequency. These safety performance functions (SPFs) utilize historical crash data to estimate the predicted number of crashes for a roadway segment based on a set of baseline conditions. The disparity between the actual number of crashes experienced on a segment and the number predicted by the SPF may be an indication of a roadway that would benefit from investments in safety improvements. Though the parameters included in these models vary significantly, they typically include measures of exposure, such as Annual Average Daily Traffic (AADT) and roadway segment length. The HSM provides recommendations for several other general roadway parameters to help predict crashes; however, the means and procedures utilized for the development of safety performance functions are far from being perfected. Therefore, considerable research has been directed towards developing more proficient and efficacious means to help estimate levels of safety.

One such method, which has warranted significant study over the past decade, utilizes inconsistencies in roadway design to help identify potentially unsafe sections of roadway. Since these inconsistencies may take various different forms, the recent literature is rather diffuse; the full breadth of these current evaluation practices is evaluated in the subsequent section. It is important to note, however, that some of these methods developed for assessing design consistency, such as measuring the disparity between 85th percentile speeds on successive elements, may require extensive financial and development efforts on the behalf of practitioners. Although the development of speed profile equations have allowed for the estimation of 85th percentile speeds, these equations require field validation to ensure circumstantial applicability. Furthermore, such measures of consistency may only become efficacious in the evaluation of existing roadway systems. If, for example, a practitioner was attempting to evaluate the potential safety performance of several design alternatives, they would have to place their faith in the pertinence of speed profile equations to estimate 85th percentile speeds; the practitioner has no method for verifying the applicability of the selected speed models to their potential designs.

 

1.2. Purpose of Research

Therefore, it is the objective of this study to develop a methodology for assessing potential levels of roadway safety that do not require surrogate measures of design consistency. This is achieved through the direct use of geometric design inconsistencies, such as changes to intra-segmental horizontal curve radii and the number of changes in vertical grade within a segment. By incorporating these parameters into safety performance functions, levels of design consistency can be evaluated in a more direct manner.

One advantage of using actual geometric alignment parameters to measure consistency stems from their general accessibility; most public agencies possess records of the geometric layout of their roadway network. Although the precision and diligence by which these files are maintained vary from agency to agency, geometric alignment parameters are much more readily available for utilization in safety performance functions (SPFs) than the values for 85th percentile speed or driver workload for each particular segment of roadway under the agency’s control. By directly modeling inconsistencies in the geometric alignment, practitioners will also be afforded the ability to estimate the safety performance of existing roadways, as well as the performance between several alternatives in the design stage. The geometric data required to utilize the safety performance functions should be available to safety professionals conducting safety analysis on a single roadway or an entire network of roadways. Therefore, the incorporation of changes to geometric elements into current safety evaluation methods may serve practical applications with a trivial amount of effort. Before this analysis is performed, however, it is imperative to first gain an understanding of the current state of practice of design consistency in the field of roadway safety.

MODELING SAFETY EFFECTS OF GEOMETRIC DESIGN CONSISTENCY ON TWO-LANE RURAL ROADWAYS USING MIXED EFFECTS NEGATIVE BINOMIAL REGRESSION

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