Synthesis of Information Related to Highway Practices. Topic 54-10. State Customization of Highway Safety Manual Methods

The AASHTO Highway Safety Manual provides tools for predicting the safety performance of a roadway facility. These tools include safety performance functions (SPFs) which incorporate geometric and other conditions to predict the crashes expected on a facility. SPFs are developed using crash numbers, geometric conditions, traffic conditions, and other data from one or more states, resulting in less accurate safety performance measures when the SPFs are used for analyses in other states. SPFs can be customized for a specific condition or region using a calibration factor. The calibration factor is then multiplied by the HSM model results to yield crash predictions that better represent the observed crash number in that state. Jurisdictions can develop jurisdiction-specific SPFs using their own data, allowing analyses that more closely represent their individual experiences. Although the development of customized SPFs is generally considered more accurate for crash predictions, it involves a higher level of data needs, expertise, and cost.  As the state of the practice in data-driven safety analysis advances, states are challenged to calibrate or develop models that meet their needs. Specific challenges include the availability of sufficient data or funding to collect data. In addition, states increasingly have questions about whether and how to apply particular factors or models to facility types that are not exactly the same as the ones used to develop the models. Practitioners have questions about whether calibration factors or SPFs are transferable and could be used by other jurisdictions. An initial step to addressing these questions is to synthesize the work states have already done to calibrate the Highway Safety Manual SPFs or develop their own SPFs.  The objective of this synthesis is to document state DOT practice on calibration factors and development of jurisdiction-specific SPFs.   Information to be gathered includes (but is not limited to): (1) Calibration factors and SPFs that states have developed; (2) Have cities, counties, and MPOs within a state developed their own SPFs or calibrated existing SPFs?; (3) Are CFs developed in-house or by consultants, and what are the respective general levels of effort?; (4) What type of analysis tools, e.g., FHWA Calibrator,  are used for development of calibration factors?; (5) Factors, including barriers and challenges, and the process used to make a decision to calibrate existing SPFs or develop jurisdiction-specific SPFs; (6) What calibration factors are used and how often they are updated?; (7) Development of region-specific calibration factors or SPFs (i.e., mountain, piedmont, coastal) and how they are used; (8) Metrics and methods of validation on calibrated factors; (9) Adoption of calibration factors from other states and metrics and techniques used to assess transferability; and Relevant factors, such as crash reporting threshold, that would impact the applicability of calibration factors or SPFs to other jurisdictions.


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  • Status: Active
  • Funding: $55000
  • Contract Numbers:

    Project 20-05, Topic 54-10

  • Sponsor Organizations:

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Project Managers:

    Harrigan, Edward

  • Performing Organizations:

    Pennsylvania State University, University Park

    Thomas D. Larson Pennsylvania Transportation Institute
    Research Office Building
    University Park, PA  United States  16802-4710
  • Principal Investigators:

    Gayah, Vikash

  • Start Date: 20230424
  • Expected Completion Date: 20241024
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01845578
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 20-05, Topic 54-10
  • Files: TRB, RIP
  • Created Date: May 17 2022 6:12PM