United States Department of Transportation
i
Improved Guidelines for Estimating the Highway Safety Manual Calibration Factors
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2016-01-01
[PDF-1.44 MB]
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Abstract:Crash prediction models can be used to predict the number of crashes and evaluate roadway safety. Part C of the first edition of the Highway Safety Manual (HSM) provides safety performance functions (SPFs). The HSM addendum that includes freeway and ramp chapters consist of severity distribution functions (SDFs) to estimate the crash severity as a function of geometric and traffic characteristics. In order to account for the differences in factors that were not considered or cannot be considered in the development of SPFs and SDFs, it is essential to calibrate them when they are applied to a new jurisdiction. The HSM recommends a one‐size‐fits-all sample size for calibration procedures that require crash data collected from randomly selected sites. However, the recommended sample size is not fully supported by documented studies, and several agencies have initiated SPF calibration efforts. In addition, there are no clear guidelines on when an agency should update their calibration factors (C-factors) and how they should make a decision on the need of region-specific calibration factors. The objectives of this research are to (1) review and document issues with the existing calibrating method in the HSM, (2) identify factors that influence the selection of the sample size for the SPFs calibration (or recalibration), (3) determine how frequently or when an agency should update their calibration factors, (4) determine whether or not having region-specific C-factors are justified and when they are needed, and (5) identify factors that influence the selection of the sample size for the SDFs calibration (or recalibration). The study objectives were accomplished using simulated and observed data. The guidelines included a discussion on (1) the sample size that is required to calibrate SPFs; (2) when the models should be recalibrated; (3) when the region-specific C-factors are recommended; and (4) the sample size that is required to calibrate SDFs.
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