Calibration of the Highway Safety Manual for Missouri

The new Highway Safety Manual (HSM) contains predictive models that need to be calibrated to local conditions. This calibration process requires detailed data types, such as crash frequencies, traffic volumes, geometrics, and land-use. The HSM does not document in detail techniques for gathering such data, since data systems vary significantly across states. The calibration process also requires certain decisions, such as the correct sampling approach, determination of the minimum segment length, the treatment of left-turn phasing, and the inclusion or exclusion of speed-change lane crashes. This report describes the challenges, practical solutions, and results from a statewide HSM calibration in Missouri, including lessons learned from other states such as Kansas, Illinois, and New Hampshire. The models calibrated included eight segment and eight intersection site types, as well as three freeway segment types that will be part of the next edition of the HSM. The applied random sampling technique ensured geographic representativeness across the state. A variety of data processing techniques were utilized, including CAD, which was used to obtain geometric data. Some of the challenges encountered during calibration included data availability, obtaining a sufficient sample size for certain site types, maintaining a balance between segment homogeneity and minimum segment length, and excluding inconsistent crash data. The calibration results indicated that the HSM predicted Missouri crashes reasonably well, with the exception of a few site types for which it may be desirable for Missouri to develop its own SPFs