Enhancement of Roadside Design Features Safety Performance Models for the Highway Safety Manual

The AASHTO Highway Safety Manual (HSM) is a tool that allows safety practitioners to consider safety fully and quantitatively in project decisions. The first edition of the HSM (HSM1) provides a method for considering roadside conditions in analyses of two-lane facilities that is based on a qualitative and visual index. The user selects a factor, called a Roadside Hazard Rating (RHR), using a series of photographs and descriptions to represent the roadside on the existing or proposed facility undergoing analysis. This rating is used in a limited number of safety performance functions (SPFs) and/or crash modification factors (CMFs) to account for roadside features on two-lane rural roads in crash predictions. Roadside information is limited to side-slopes on undivided multi-lane roadways and is not incorporated into analyses for other multi-lane divided facility types addressed in the HSM1. Since publication of the HSM1 in 2010 there has been research to fill in the knowledge gaps in the associated SPFs and those used in the Roadside Design Guide (RDG) for roadside features. NCHRP Project 17-54, “Consideration of Roadside Features in the Highway Safety Manual” developed models to quantitatively consider the roadside in safety analyses. Upon completion of this work, the AASHTO Highway Safety Manual Steering Committee, TRB Committee on Safety Performance, and others reviewed the research results and proposed a draft chapter for the second edition of the HSM (HSM2) that is currently being developed. Because the NCHRP Project 17-54 models were of a different form than the existing HSM1 models, using exposure versus predictive models, the AASHTO HSM Steering Committee undertook a demonstration project to determine the ability to use the models in the HSM context. In a separate effort, results from NCHRP Project 17-82, “Proposed Guidance for Fixed Objects in the Roadside Design Guide” were reviewed. . This analysis was also considered by the AASHTO committee. The TRB Committee on Highway Safety Performance also provided input to AASHTO on these analyses and all the analyses were discussed with the panel for NCHRP Project 17-71, “Proposed AASHTO Highway Safety Manual, Second Edition.” In summary, the NCHRP Project 17-54 findings for SPF and CMFs could not be validated by small sample testing by several states in the AASHTO HSM Steering Committee. Of particular concern was the shape (reduced road departure crash rates) of the SPF at higher volumes and the associated effects of roadside fixed objects and their offsets to the travel way. Research is needed to collect additional data and validate proposed models and then determine whether improvements are needed for the roadside-related crash prediction tools proposed for future editions of the HSM. If new data are available or readily collectable, then the model improvements could be performed. The same validation and model development would also benefit the existing and future efforts to develop a performance-based RDG. The objectives of this research are to (1) validate and/or develop enhanced roadside safety performance functions including associated design element variables that are appropriate for inclusion in the HSM and (2) prepare appropriate draft text for AASHTO to consider for inclusion in appropriate HSM chapters. The research will include a review of available roadway and roadside design element inventory and associated crash data. Both the roadway departure SPFs and related CMFs for design elements will be provided for the roadway facility types prioritized by AASHTO members based on available or collected data.


  • English


  • Status: Proposed
  • Funding: $500000
  • Contract Numbers:

    Project 17-104

  • Sponsor Organizations:

    National Cooperative Highway Research Program

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

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Harrigan, Edward

  • Start Date: 20210527
  • Expected Completion Date: 0
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01773412
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 17-104
  • Files: TRB, RIP
  • Created Date: May 24 2021 3:14PM