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Publication Number: FHWA-RD-02-089
Date: July 2002

Safety Effectiveness of Intersection Left- and Right-Turn Lanes

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1. INTRODUCTION

This report presents the results of research on the safety effectiveness of geometric design improvements for at-grade intersections. The objective of the research was to perform a well-designed before-after evaluation of selected types of intersection design improvements. The research was performed as part of a pooled-fund study; a portion of the funding for the research was contributed by highway agencies in the District of Columbia and the states of Iowa, Illinois, Louisiana, Minnesota, Montana, Nebraska, New Jersey, North Carolina, Oregon, and Virginia.

Representatives of the participating highway agencies met on three occasions during the study to assist in guiding the research. In particular, the types of intersection improvements to be evaluated in this study were selected in consultation with the participating state highway agencies. Based on a review of safety literature concerning a broad range of intersection design improvements, presented in section 2 of this report, it was decided that the before-after evaluation should focus on intersection design improvements involving left- and right-turn lanes.

Geometric design, traffic control, traffic volume, and traffic accident data were gathered for a total of 280 improved sites under the jurisdiction of the participating states, as well as 300 similar intersections that were not improved during the study period. The types of improvement projects evaluated included installation of added left-turn lanes, installation of added right-turn lanes, installation of added left- and right-turn lanes as part of the same project, and extension of the length of existing left- or right-turn lanes. An observational before-after evaluation of these projects was performed using evaluation approaches recommended in a recent report by Griffin and Flowers(1) and a recent book by Hauer.(2) Three contrasting approaches to before-after evaluation were used—the yoked comparison or matched-pair approach, the comparison group approach, and the Empirical Bayes approach. The research not only evaluated the safety effectiveness of left- and right-turn lane improvements, but also compared the performance of these three alternative approaches in making such evaluations.

This report presents the research methodology and evaluation approaches used to evaluate left- and right-turn lane projects and presents the results of the evaluations. Section 2 of the report presents a review of the safety literature concerning intersection design improvements; the results of this review were considered in the decision to focus the study of left- and right-turn lane improvements. Section 3 describes the selection of the evaluation sites. The collection of data concerning those evaluation sites is described in section 4. Section 5 presents the evaluation plan for the study, including the three specific evaluation approaches that were used, while section 6 presents and interprets the evaluation results. The conclusions and recommendations of the study are presented in section 7.

Appendix A summarizes the results of safety studies concerning intersection improvements published in the literature. Appendix B presents the results of negative binomial regression modeling on relationships between intersection accident frequency and traffic volumes performed for use with the comparison group and Empirical Bayes approaches. Appendix C presents the detailed results of all before-after evaluations performed in the research; this appendix includes all evaluation results that are discussed in section 6 of the report, as well as other evaluations whose results were not statistically significant and were, therefore, not used. Finally, appendix D presents the definitions for geometric design and traffic control data items that were collected in the field concerning the study intersections.

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