The National Academies

NCHRP 17-79 [Active]

Safety Effects of Raising Speed Limits to 75 mph and Higher

  Project Data
Funds: $500,000
Staff Responsibility: David M. Jared
Research Agency: Texas A&M Transportation Institute
Principal Investigator: Dr. Karen Dixon
Effective Date: 9/19/2016
Completion Date: 12/31/2020

Over the past decade, there has been a trend toward higher speed limits, especially on rural interstates/freeways. Twelve states have increased speed limits to 75 mph, some as high as 85 mph on freeways and 75 mph on other roads. A number of states have increased speed limits up to 65 to 70 mph on 2-lane rural roads. In some cases these higher posted speed limits exceed design speeds. The impacts of these recent speed limit increases on safety have not been thoroughly studied. There are concerns about increased crash severity resulting from higher speeds. For example, roadside hardware is tested at lower speeds under AASHTO MASH standards, and it is not well understood how various devices may perform on roadways with higher speed limits. Impact performance of a highway feature cannot be measured by a series of crash tests only; even the most carefully researched device has performance limits dictated by physical laws, vehicle stability, and crashworthiness.
The objective of this research is to provide guidance to assist highway agencies in estimating the safety impacts of increasing speed limits to 75 mph or more. This should include information to support the consideration of where, when, and by how much speed limits may or may not be increased on existing facilities and newly constructed roads. The guidance should also identify mitigation strategies that may be implemented to offset potential impacts.
The NCHRP is seeking the insights of proposers on how best to achieve the research objective. 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 objective. The work proposed must be divided 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 research plan should include, as a minimum, the following components as a suggested overall framework of the proposed research:
1.  An analysis of existing research on level of speed limit changes in the states; the mileage and types of highways with raised speed limits; practices, procedures, and considerations used to determine if and where speed limits are or have been raised; and internal studies on safety and other impacts.
2.  Collection and analysis of data on the effects of speed limit increases. These should include, but not limited to:
  • Before and after crash rates, speed, traffic flow, volume and capacity;
  • Engineering, enforcement and educational mitigation strategies that were implemented in concert with the increase in speed;
  • Operating speeds, speed differentials, and other appropriate measures such as incremental operating speed increases over time, and spillover effects on adjacent roads or sections;
  • Crash frequency and severity levels, trends and contributing factors;
  • Performance of roadway and roadside elements, such as roadside hardware, and barrier systems;
  • Effects of geometric design, roadside elements, and placement of traffic control devices, such as signing, pavement markings, merging, rumble strips, lighting; and
  • Assessment how engineering, enforcement, operations, maintenance, educational programs, mitigation strategies and other activities affect safety results.
3.  Guidance and implementation strategies highway agencies should consider when determining whether to increase speed limits to 75 mph or greater. The guidance may also include procedures and tools such as crash modification factors or models, or evidence from prior research that would support these decisions including mitigation strategies that may be implemented to offset potential impacts.
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 existing data that should be completed within the first 14 months of the effective date of contract start. The results of Phase 1, including an outline, annotated table of contents or framework of the draft guidance will be presented in Interim Report 1, and an in-person meeting will be held with the project panel to discuss Interim Report 1.
Subsequent phases will involve executing 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 including the guidance; (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; and (3) a stand-alone technical memorandum titled “Implementation of Research Findings and Products”.
Status: Research in progress; contractor's draft final report pending.

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