Understanding the Relationship between Operating, Posted, and Design Speeds and Safety in the Setting of Speed Limits

Speed management has long been a concern of transportation agencies, dating back to research from the 1960s which showed that vehicles traveling excessively below or above the speed limit were overrepresented in crashes on rural highways and interstates. Inappropriate speed limits are more likely to cause larger speed variability and a larger mean difference from the posted speed. It is desirable to achieve balance for safety between the various types of speed classifications: operating, posted, statutory, and design speed. Historically, research has shown that increases in speed differential tend to have more adverse safety impacts than higher mean speeds. Some evaluation studies have shown increases in traffic crashes and/or fatalities in some states where the statutory speed limit had been increased. However, other studies found either marginal or no changes in traffic safety, while others found safety improvements after speed limit increases. The repeal of the National Maximum Speed Law in 1995 led to a series of additional studies and statutory changes. Ideally, the design speed will exceed or match the posted speed and operating speed, but that is not always the case. While the design speed for an existing roadway is based on geometric features, posted speed limits have historically been established based on state statute or an engineering study. Engineering studies often consider the 85th percentile speed during free-flow conditions when determining the posted speed. In some cases, the 85th percentile speed is higher than the design speed. In these situations, the engineer is faced with the dilemma of determining a posted speed above the design speed without the ability to fully quantify the safety risk. NCHRP Report 504 identified general relationships between design speed, operating speed, and posted speed limit; however, it is unclear how these various speeds relate to safety performance. The objectives of this research are to (1) identify and describe factors that influence operating speed and (2) provide guidance to make informed decisions related to establishing speed limits on roadways. The guidance should address the following, at a minimum: 1. An approach for determining the relationship between operating, design, posted and statutory speeds and how the differences among them impact safety performance. This may also include quantitative and/or qualitative models to predict the safety performance associated with the differences between operating, posted, statutory and design speeds. Identification and classification of nationwide data including, but not limited to, geometric design, access density, signal density, traffic volume characteristics, and enforcement practices that may impact operating speed. 3. An analysis of the 85th percentile speed and other statistical measures and factors as appropriate methods for setting speed limits. 4. Implications of setting a speed limit lower than those recommended using the factors identified above. The focus of the research should be on roadway functional classifications of minor arterials and higher as defined by American Association of State Highway and Transportation Officials (AASHTO). The National Cooperative Highway Research Program (NCHRP) is seeking the insights of proposers on how best to achieve the research objectives. Proposers are expected to describe research plans that can realistically be accomplished within the constraints of available funds and contract time. Proposals must represent the proposers’ current thinking described in sufficient detail to demonstrate their understanding of the issues and the soundness of their approach in meeting the research objectives. The work proposed must be divided into tasks and/or phases. Proposers must describe the work proposed in each phase and task in detail. The research plan should build in appropriate checkpoints with the NCHRP project panel including, at a minimum (1) a kick-off teleconference meeting to be held within 1 month of the contract’s execution date; (2) 1 face-to-face interim deliverable review meeting at the end of Phase 1; and (3) any web-enabled teleconferences tied to panel review and/or NCHRP approval of any other interim deliverables deemed appropriate.  The work proposed must be divided into at least 2 phases and each phase must be divided into tasks. Proposers must describe the work proposed in each phase and task in detail, and identify specific deliverables for submission to NCHRP for review and approval.


  • English


  • Status: Active
  • Contract Numbers:

    Project 17-76

  • Sponsor Organizations:

    Federal Highway Administration

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

    American Association of State Highway & Transportation Officials

    444 North Capitol Street, NW, Suite 225
    Washington, DC  United States  20001

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Bush, Mark

  • Performing Organizations:

    Texas A&M Transportation Institute, College Station

    Texas A&M University System
    3135 TAMU
    College Station, TX  United States  77843-3135
  • Principal Investigators:

    Fitzpatrick, Kay

  • Start Date: 20161007
  • Expected Completion Date: 20190407
  • Actual Completion Date: 0
  • Source Data: RiP Project 40216

Subject/Index Terms

Filing Info

  • Accession Number: 01572384
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 17-76
  • Files: TRB, RiP
  • Created Date: Aug 7 2015 1:01AM