Geometric Categories as Intersection Safety Evaluation Tools

A number of operational and geometric factors impact the safety of at-grade intersections. Examples include the type of traffic control, the existence of left-turn lanes, and adequate sight distance. Determining whether an intersection of particular geometric design has unusual crash patterns could help identify those locations that need additional safety evaluation. During the last two years, the Wisconsin Department of Transportation and the University of Wisconsin-Madison created an intersection crash database segmented by area type (i.e., rural and urban), traffic control, traffic volumes, and general geometric categories. The overall database included crash report and entering volume information for more than 1,700 locations and 34,500 crashes. In addition, a series of 18 intersection categories were defined with respect to the number of intersection approach legs, the number of major road lanes, and the existence of a left-turn lane and/or a roadway median. More than 1,200 of the intersections were geometrically categorized, and the crash patterns at these summarized and evaluated. A portion of the results from the activities above is presented in this paper. More specifically, annual average crash frequencies and average crash rates are presented for intersections with varying operational and geometric characteristics. This integration of crash data and geometrics is not typically available for the general statewide safety management of intersections. These safety evaluation measures will be used to identify intersection locations of interest in Wisconsin, and are expected to be an invaluable resource for improving roadway safety in the state and possibly the region.