MODELING INTERSECTION CRASH COUNTS AND TRAFFIC VOLUME

This research explored the feasibility of modeling crash counts at intersections with use of available exposure measures. The basic purpose of "exposure" is to serve as a size factor to allow comparison of crash counts among populations of different sizes. In the context of highway crash studies, at first glance, vehicle miles of travel (VMT) appears to be a natural exposure measure. However, VMT is closely related to traffic density and this raises doubts if it can serve the intended purpose of an exposure measure. Data from four-leg signalized intersections in Washtenaw County, Michigan, and the states of California and Minnesota were used in this study. Traffic volumes on the approaches are the routinely available exposure measure. It was noted that in these data sets the same values of traffic volume were often "carried over" several intersections. Using such values of traffic volumes as measures of exposure results in correlations between errors of the independent variables, which violates the requirements of standard statistical procedures. It was found that the relationships between crash counts and traffic volumes could not be adequately represented by the standard loglinear model that is also the basis for more sophisticated models. Therefore, nonparametric regression in the form of kernal smoothing was used. This allowed a realistic representation of complex relationships. The relationships found differed among the three data sets, with California showing dramatic deviations from the loglinear model. The relationship between crash counts and traffic volumes on the approaches to the intersection and those within the intersection were found to be very different. This makes it unlikely that realistic models for all intersection-related crashes can be developed. Turning counts are plausible candidates for exposure measures for turn-related intersection crashes. However, since turning counts are not routinely available, the possibility of using proportions of crashes involving turns was explored. The results were negative, but because of the small case number, no definite conclusions could be drawn.