Statistical versus simulation models in safety

Rational planning involves selecting from possible courses of action with regard to their predicted consequences. In road safety engineering, this requires the ability to predict the frequency and severity of crashes that are expected to result from design or operational changes. Statistical modeling is the dominant approach used in the first edition of the Highway Safety Manual to address this issue, but over time statistical models are expected to be replaced by models that explicitly describe mechanisms underlying crash occurrence. This development could be accelerated if the already substantial investment in statistical safety models could be used to calibrate or validate mechanism-based simulation models. This paper briefly reviews two efforts in which statistical models of median-crossing crash (MCC) frequency were developed. An objective of both statistical studies was to summarize how the expected frequency of MCCs varies with the average daily traffic and median width of a divided highway. After the nature of the simulation model is described, the summarized relationships are used as criteria for assessing its performance.