Use of Monte Carlo Simulation for a Sensitivity Analysis of Highway Safety Manual Calibration Factors

The Highway Safety Manual (HSM) is a comprehensive traffic safety tool that allows administrators, planners, engineers, and other agency representatives to make informed decisions about programs for highway safety improvement. Part C of the HSM contains predictive models for quantifying the safety of roadways and intersections; these models require significant data inputs. Because the models were developed with data from specific states, the HSM recommends the use of a calibration factor to account for local conditions when a model is applied elsewhere. The calibration process uses a set of sites and criteria for calculating the calibration factor. The accuracy and the precision of these models depend on the calibration process that develops the factor. This study evaluated the HSM calibration criteria's guidance for two-lane, two-way undivided urban arterial roadways and examined a method for improving the accuracy and precision of calibration factors. A sensitivity analysis used calibration sets of various sizes in a Monte Carlo simulation to resample sites. Results indicated that the current HSM criterion of 30 to 50 sites was insufficient for evaluating locations with many sites and that the minimum threshold of observed crashes per year should be reconsidered. A traffic volume analysis revealed a strong correlation between the traffic volume distribution of all sites in a jurisdiction in comparison with selected sites. This relationship was directly linked to the derivation of high-quality calibration factors. The ratio formula for the calibration factor in Part C, Appendix A, of the HSM was validated through another site allocation strategy. The findings provide prompts for future research into enhancing the HSM calibration process.