United States Department of Transportation
i
Freeway Segment Safety Performance Function (SPF) Development
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2021-06-01
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[PDF-7.20 MB]
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Edition:Final Report
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Abstract:This report documents the Ohio Department of Transportation’s effort to improve the reliability of safety decision-making through the development of planning-level and project design-level safety performance functions (SPFs). Planning-level SPFs will replace calibrated versions of national SPFs in Safety Analyst to support network screening. Project design-level SPFs will replace previously calibrated versions of the Highway Safety Manual (HSM) Chapter 18 SPFs through estimation of Ohio-specific SPFs, adjustment factors (AFs), and a one-directional approach to freeway safety analysis. This research evaluated the performance of the calibrated HSM predictive method against an Ohio-specific bidirectional predictive method, Ohio bi-directional SPFs for base conditions used in combination with HSM AFs, and a one-direction predictive method. The results indicated the one-direction predictive method provided reliable predictions for all crash types and severities when compared to the baseline of the calibrated version of the HSM predictive method. The one-direction method is easier to implement on complex alignments, provides a broad set of AFs, but does exclude some factors that may be relevant to practitioners. Additional factors may be considered along with the one-direction predictive method, as needed, as external crash modification factors to the predictive method.
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