Calibration of highway safety manual predictive models for Kansas freeway segments

The Safety Performance Functions (SPFs) in the Highway Safety Manual (HSM) are statistical formulas developed based on limited data gathered from selected few states. Therefore, the HSM recommends calibrating the HSM-default SPFs to local conditions or developing local SPFs to enhance the accuracy of crash prediction. This paper demonstrates the HSM calibration procedure for freeway segments and the quality assessment of the calibration process using Kansas freeway geometric and crash data. A minimum sample size of 446 freeway segments was calculated corresponding to 95% confidence level and 5% error; consequently, data for 521 freeway segments were collected and utilized in this freeway calibration. Results indicated that the HSM methodology overpredicts fatal and injury crashes and underpredicts property damage only crashes for freeway segments in Kansas. Results of quality assessment of the calibration process showed that estimated calibration factors were satisfactory for all freeway types considered in this study.