Observations on the Use of Crash Modification Factor-Corrected Crash Prediction Models to Identify Sites with Promise

The Highway Safety Manual (HSM) provides guidance on the application of crash prediction models for network screening, evaluation of alternative designs, and evaluation of implemented safety improvements. A variety of models are available. They may be classified as network screening level (or simple models), project level [or crash modification factor (CMF)–corrected models], and evaluation-level models. The use of crash prediction models to identify sites with promise of safety improvement is often based on assessing the difference between the expected number of crashes (N_e) at a site and the predicted number of crashes (N_p) for similar sites within the population. A large difference between N_e and N_p may denote a safety problem and be used to identify and rank sites with promise. The HSM indicates that CMF-corrected models may be used for network screening purposes. However, issues arise in the analysis regarding the definition of similar sites. When CMF-corrected models are used, the definition of similar sites changes with changes in the CMF adjustments particular to each site. Use of (N_e − N_p) for identifying and ranking sites with promise does not work well when the analysis uses a CMF-corrected estimate of N_p. A large combined CMF will increase N_p and thereby decrease the value (N_e − N_p). But a large combined CMF may point to a site with promise. This paper examines this phenomenon in detail, reviews a case study, and suggests that simple models may be preferable for identifying sites with promise.