The National Academies

NCHRP 17-79 [Final]

Safety Effects of Raising Speed Limits to 75 mph and Higher

  Project Data
Funds: $500,000
Research Agency: Texas A&M Transportation Institute
Principal Investigator: Dr. Karen Dixon
Effective Date: 9/19/2016
Completion Date: 2/28/2022

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.
STATUS: Research is complete; final report is available as NCHRP_Report_1006, Guide to Understanding Effects of Raising Speed Limits

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