Safety performance functions for crash severity on undivided rural roads

tThe objective of this paper is to explore the effect of the road features of two-lane rural road networkson crash severity. One of the main goals is to calibrate Safety Performance Functions (SPFs) that canpredict the frequency per year of injuries and fatalities on homogeneous road segments. It was foundthat on more than 2000 km of study-road network that annual average daily traffic, lane width, curvaturechange rate, length, and vertical grade are important variables in explaining the severity of crashes. Acrash database covering a 5-year period was examined to achieve the goals (1295 injurious crashes thatincluded 2089 injuries and 235 fatalities). A total of 1000 km were used to calibrate SPFs and the remaining1000 km reflecting the traffic, geometric, functional features of the preceding one were used to validatetheir effectiveness. A negative binomial regression model was used. Reflecting the crash configurationsof the dataset and maximizing the validation outcomes, four main sets of SPFs were developed as follows:(a) one equation to predict only injury frequency per year for the subset where only non-fatal injuriesoccurred, (b) two different equations to predict injury frequency and fatality frequency per year per sub-set where at least one fa tality occurred together with one injury, and (c) only one equation to predict thetotal frequency per year of total casualties correlating accurate percentages to obtain the final expectedfrequency of injuries and fatalities per year on homogeneous road segments. Residual analysis confirmsthe effectiveness of the SPFs.