Variability of the calibration factors of the HSM safety performance functions with traffic, region and terrain

Crash prediction is a crucial step for each part of the road safety management process. The HSM first edition provided safety performance functions (SPF) in order to predict crash frequencies on different types of road infrastructures. However, crash frequencies of similar roadway segments or intersections can vary widely from one jurisdiction to another. Hence, a calibration process to local conditions is necessary before the application of HSM SPFs. HSM (2010) provides guidelines for the calibration process. However, the recent NCHRP 20-07(332) (2014) emphazises the need of considering the variability of the local calibration factor with other specified variables (severity, traffic, segment lengths, regions, terrain). Also, in that report, the matter of the assessment of calibration results is addressed. However, there is still uncertainty on which and how many variables should be considered for the calibration with regards to their influence on the overall jurisdiction factor. In this paper, the influence of traffic volume ranges, terrain types and regions on the results of the calibration study is analyzed. The NCHRP 20-07 (332) recommendations about the calibration process and the assessments of its results were applied. For the purpose of the study, a sample of roads belonging to the Italian two-lane undivided highways network was employed, considering a five-years study period. Calibration factors at the nationwide scale and for different sub-groups of the sample based on traffic volumes, terrain types and regions (macro-regions and administrative regions) were computed. The influence of those variables was assessed through the analysis of the obtained results.