In recent years the development and use of accident prediction models forroadway safety analyses have received substantial attention. These models, also known as Safety Performance Functions (SPFs), relate the expected crash frequency of roadway elements (intersections, road segments, on-ramps) to traffic volumes and other geometric and operational characteristics. A commonly practiced approach for applying SPFs is to assume that crash types occur in fixed proportions, e.g., rear-end crashes make up 20% of crashes, angle-crashes 35%, etc., and then apply these fixed proportions to crash totals to estimate crash frequencies by type. As demonstrated in this paper, this practice makes questionable assumptions and results in considerable error in estimating crash proportions. Using rudimentary safety performance functions based solely on major and minor road Average Annual Daily Traffic (AADT), the homogeneity-in-proportions assumption is shown to break down across AADT. The models suggest that crash proportions vary as a function of both major and minor road AADT, thus showing the homogeneity-in-proportions assumption to be invalid. For example, with minor road AADT of 400 vehicles per day, the proportion of intersecting direction crashes decreases from about 50% with 2000 major road AADT to about 15% with 82,000 AADT. Same direction crashes increase from about 15% to 55% over this same comparison. The homogeneity-in-proportions assumption should be abandoned and crash type models should be used to predict crash type proportions.SPFs using additional geometric variables would only exacerbate the problem quantified here. Comparing models for different crash types using additional geometric variables remains the subject of future research.
Abstract