United States Department of Transportation
i
Safety Comparisons Between Interchange Types
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2023-04-01
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[PDF-6.27 MB]
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Edition:Final Report
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Abstract:Documentation of potential safety performance impacts is required for Interchange Justification Reports (IJRs) to validate the need for new or modified interchanges.(2) Typically, IJRs are written early in the project planning and design process, with details generally consistent with conceptual design. As such, the design details required for using the Highway Safety Manual (HSM) Part C predictive models may not be known, only the general form of the interchanges.(1) Without these details, using the HSM models to develop an accurate prediction of crash frequency and severity of the design is difficult. In addition, aggregating site-by-site predictions may not fully capture the safety performance impacts when considering the project location as a whole. The Federal Highway Administration sought to develop planning-level models and tools to predict crash frequency and severity for an existing or proposed interchange to further explore and address an IJR application. This predictive model estimates crash frequency and severity for service interchange configurations comprising more than 75 percent of those typically considered in IJRs. The safety performance function includes freeway and crossroad annual average daily traffic (AADT) per lane, ramp AADT, and interchange configuration. The base estimate is adjusted for freeway lanes, crossroad lanes, area type, interchange skew, a nearby adjacent interchange gore, managed lanes, the number of crossroad left-turn lanes at terminals, and variation in ramp volumes.
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