Surrogate Safety Measures from Traffic Simulation Models

Safety is emerging as an area of increased attention and awareness within transportation engineering. Historically, the safety of new and innovative traffic treatments has been difficult to assess, primarily because of a lack of good predictive models of crash potential and a lack of consensus on what constitutes a safe or unsafe facility. An FHWA-sponsored research project investigated the potential to derive surrogate measures of safety from existing traffic simulation models. These surrogate measures could then be used to support evaluations of various traffic engineering alternatives, including facilities that have not yet been built and strategies that have not yet been used. Each surrogate measure is collected on the basis of the occurrence of a conflict event, which is an interaction between two vehicles in which one vehicle must take evasive action to avoid a collision. The surrogate measures that are proposed as the best are time to collision, postencroachment time, deceleration rate, maximum speed, and speed differential. Time to collision, postencroachment time, and deceleration rate can be used to measure the severity of the conflict. Maximum speed and the speed differential can be used to measure the severity of the potential collision (by use of additional information about the mass of the vehicles involved to assess momentum). After the simulation model is executed for a number of iterations, a postprocessing tool would be used to compute the statistics for the various measures and perform comparisons between design alternatives.