The National Academies

NCHRP 17-23 [Completed]

Safety Impacts and Other Implications of Raised Speed Limits on High-Speed Roads

  Project Data
Funds: $350,000
Research Agency: Charles River Associates
Principal Investigator: Jon Bottom and Kara Kockelman
Effective Date: 7/23/2002
Completion Date: 4/1/2006

Background: In December 1995, Congress repealed the National Maximum Speed Limit and thus returned to states the right to set speed limits without the threat of losing federal highway funds. As a result of the repeal, many states have ongoing programs for analyzing speed limits on high-speed highways and raising speed limits when appropriate. After 1995, some states initially reported increases in fatalities, and others experienced decreases. Previous studies on this topic examined only short-term effects or failed to control for a host of factors that could affect the results. Higher speed limits could also have other implications, such as the reallocation of enforcement resources to higher-risk areas, or impacts on the environment or the economy due to changes in traffic flows. Research that accounts for all these factors is needed to determine the long-term safety impacts and other implications of raising speed limits.

Objective: The objectives of this research were to study the effects of raised speed limits on high-speed roads and, based on the results, to recommend models and procedures that will assist highway agencies to determine when and where speed limits should be changed. High-speed roads are defined as those with speed limits raised from 55 mph or greater, including freeways and nonfreeways, in both rural and urban settings. The primary emphasis of the study should be on the safety impacts, but attention should also be paid to the implications of raised speed limits on operations, socioeconomics, energy, and the environment.

Tasks: Accomplishment of the project objective required the following tasks: (1.) Analyze, describe, and critique pertinent domestic and international research findings, on the basis of applicability, conclusiveness, and usefulness for the analytical needs of the project. Include both completed research and research currently underway. (2.) Conduct a survey to collect data on the experiences of state DOTs that have raised speed limits. This survey should be designed to collect information including. (3.) Based on the results of Tasks 1 and 2, update the work plan for data collection and statistical analysis to assess the effects of changed speed limits. (4.) Deliver an interim report on Tasks 1, 2, and 3. The contractor will be expected to meet with the project panel approximately 1 month later to review and approve the proposed data collection and analysis plan. (5.) Collect data and conduct the analysis in accordance with the methodology approved by the project panel. (6.) Based on the analysis, develop conclusions on the effects of raised speed limits from 55 mph or greater on freeways and nonfreeways, in both rural and urban settings. Describe models that will assist highway agencies in estimating the effects of raised speed limits on high-speed roads and identify procedures to determine where speed limits should be changed. (7.) Prepare a final report that documents the entire research effort.

The final report describes the analyses performed and results obtained by a study of safety and other impacts of speed limit changes on high-speed roads. The information will be valuable for researchers studying the complex relationship between safety and vehicle speeds, and also serves to highlight limitations in the data needed to make sound regulatory decisions. The study offers a number of recommendations for future improvements in data collection that could yield a more rigorous analysis of the impacts of speed limit changes in the future. While the present study offers an incremental step in the analysis of speed limit impacts, it is not expected that the results would be sufficient to impact regulatory decisions.

Status: The project is completed.

Product availability: The final report is now available as NCHRP Web-Only Document 90. An expanded summary of the report has been published as NCHRP Research Results Digest 303.

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