Developing Safety Performance Functions for Rural Two-Lane Highways that Incorporate Speed Measures

The Highway Safety Manual (HSM) currently provides Safety Performance Functions (SPFs) for several roadway types and intersection types that highway agencies can calibrate to local conditions for use in predicting expected safety outcomes for given roadway designs and safety features. These SPFs, and others developed in-house by highway agencies, are a function of AADT, since the number of vehicles on a facility is directly related to the likelihood of crashes. However, speed is also an important predictor of roadway safety. The severity of the crash is particularly sensitive to vehicle speeds since the crash energy increases by the square of the vehicle velocity. Speed also may have an impact on the probability of crash occurrence, although this is less well-understood. Despite the importance of speed on safety, speed measures are generally not included in most SPFs. Highway agencies have little guidance for understanding the impact of speed policies on safety outcomes. The measures used to assess safety in most crash data are usually determined subjectively after the crash and do not provide clear information about the safety effects of vehicle operating speeds for a road section or other location type by time, weather, etc. Rural two-lane roads are a priority for understanding the speed-safety relationship because more than half of all fatal crashes occur on rural roads. Nearly 40 percent of those are deemed to be speeding-related (i.e., racing, exceeding speed limits, or too fast for conditions). It is not well-understood which speed measures (such as average speed, 85th percentile speed, speed variance, speed limit violation rate, etc.) are the best predictors of crash likelihood and severity. The objectives of this research are to identify the most relevant speed measures for predicting safety and evaluate the potential of including speed measures in SPFs for rural two-lane facilities. The research products will help to improve the performance and accuracy of the HSM predictive methods and help agencies in deciding about appropriate speed limits.


  • English


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

    Project 17-92

  • Sponsor Organizations:

    American Association of State Highway and Transportation Officials (AASHTO)

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

    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
  • Project Managers:

    Harrigan, Edward

  • Start Date: 20180619
  • Expected Completion Date: 0
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01672553
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 17-92
  • Files: TRB, RiP
  • Created Date: Jun 18 2018 3:03PM