Comparison of Simulated Freeway Safety Performance with Observed Crashes

This paper provides a link between simulated measures of safety performance and observed crash occurrence. Safety performance is expressed by using a crash potential index (CPI), established as a function of individual vehicle deceleration rates required to avoid a crash and of braking capabilities. Safety performance is compared with a sample of crashes observed on an instrumented segment of freeway. Three test results are reported: (a) comparing safety performance in 1-min increments for a period 5 min before the precise crash time, (b) comparing safety performance in 1-min increments over 5 min for matching crash and noncrash cases, and (c) comparing average safety performance with observed crash rates for a 1-h period at the same site. The results of this study confirm that crashes tend to occur when measures of safety performance at a given site are higher than normal and that this measure increases with the approaching time to crash. The results provide basic evidence that the CPI measure of safety performance tends to reflect or explain increased crash risk subject to real-time changes in traffic conditions.