THE DEVELOPMENT OF SAFETY PERFORMANCE FUNCTIONS FOR ROUNDABOUTS IN WISCONSIN
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Crash occurrences at roadways or intersections are associated with a large variety of factors. The safety performance function (SPF) is a statistical regression model that determines the relation between crash frequency and potential geometric features that lead the crashes at a location. As the funds for safety improvements are limited, it is important to identify the design decisions which will result in the maximum safety benefits. By improving SPFs, engineers can estimate the number of crashes and improve the roadway or intersection safety through applying appropriate safety countermeasures. The number of roundabouts has increased rapidly throughout the state of Wisconsin in the last several years. There is a strong need to develop an assessment model to evaluate the safety performance of Wisconsin roundabouts. In 2010 Highway Safety Manual (HSM) published by American Association of State Highway and Transportation Officials (AASHTO), SPFs of roundabouts have been developed. However, the development of the SPFs is based on the data collected from locations across the country, the real condition of local roundabouts, such as geometry design, driver behavior and weather condition, cannot be reflected in these models. Two types of safety performance functions of roundabouts in Wisconsin are developed in this thesis by using the negative binomial regression models. The first type is the intersection-level SPFs, which considers the crash frequency, traffic volume and geometric features of whole roundabouts. The second type is the single approach-level SPFs, which considers the entering-circulating locations at roundabouts with their crash frequency and geometric features. The results show that for the intersection-level SPFs, crash frequency has a strong relation with traffic volume (AADT). For the single approach-level SPFs, crash frequency are related to traffic volume, lane width, and entry angle. To evaluate how the Wisconsin roundabouts SPFs perform, a comparison with models developed by National Cooperative Highway Research Program (NCHRP) is provided. The results reveal that Wisconsin roundabouts SPFs tend to have less crash frequency. The potential reasons of these findings are discussed.