Calibrating the Highway Safety Manual Crash Prediction Models for Idaho’s Highways Grant uri icon



  • AASHTO’s Highway Safety Manual (HSM) provides methods to integrate quantitative estimates of crash frequency and severity into planning, project alternatives analysis, and program development and evaluation, allowing safety to become a meaningful project performance measure. The HSM supports ITD progress toward federal and state safety goals to reduce fatalities and serious injuries. As ITD works toward its safety goals, the quantitative methods in the HSM can be used to evaluate which programs and project improvements are achieving desired results; as a result, agencies can reallocate funds toward those that are having the greatest benefit. In the HSM predictive methods, the total expected crash frequencies for a facility are estimated by combining safety performance functions (SPFs) and crash modification factors (CMFS). The SPFs are first used to calculate estimated crash frequency for a base condition. Next, the estimates are modified by applying CMFs to address non-base condition characteristics for specific segment and intersection locations. The predictive method can be used to estimate safety separately for intersections and segments. The SPF’s included in the HSM, however, were developed using crash data from several states other than Idaho. Because there are differences in driver population, highway geometric characteristics, crash reporting procedures, animal populations, weather conditions, etc., ITD needs to use calibrated SPFs when applying the HSM procedures in Idaho.

date/time interval

  • January 1, 2013 - January 1, 2014