The National Academies

NCHRP 17-76 [Active]

Guidance for the Setting of Speed Limits

  Project Data
Funds: $500,000
Staff Responsibility: Mark S. Bush
Research Agency: Texas A&M Transportation Institute
Principal Investigator: Dr. Kay Fitzpatrick
Effective Date: 10/7/2016
Completion Date: 4/7/2019


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 AASHTO.

The 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.

Phase 1 will result in the preparation of a detailed work plan based on a comprehensive literature review and analysis of available resource data and should be completed within at least the first 13 months from the effective date of contract start. The results of Phase 1 should include: an analysis of existing information; a research plan to address gaps to develop the guidance; and an outline or framework of the draft guidance and recommendations. The results will be presented in Interim Report 1. An in-person meeting will be held with the project panel to discuss Interim Report 1.

Subsequent phases will involve implementing the approved detailed work plan prepared in Phase 1 and preparing the final deliverables, including at a minimum (1) a final report that (a) documents the entire project, incorporating all other specified deliverable products of the research; (b) provides an executive summary that outlines the research results; and (c) includes the research team’s recommendation of research needs and priorities for additional research; (2) a PowerPoint presentation with speaker notes describing the project background, objective, research method, findings, and conclusions suitable for use in a webinar; (3) a brochure, video, informational graphics, or other communications product to convey the findings of this project to the general public; and (4) a stand-alone technical memorandum titled “Implementation of Research Findings and Products”

Status: Research in progress; phase 2 underway.

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