Development of a strategy to prioritize candidate locations for speed limit increases on a high-speed, undivided highway network

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Timothy J. Gates
Raha Hamzeie
Peter T. Savolainen
Timothy P. Barrette

The 1995 repeal of the National Maximum Speed Limit (NMSL) in the United States returned all speed limit policy authority back to the individual states. In the time since the 1995 repeal, a wide variety of speed limit policies have been enacted and modified, with nearly all states eventually choosing to raise maximum speed limits on limited access freeways above the NMSL of 55 mph. Rural non-freeway roadways have also experienced speed limit increases above 55 mph in many states. As of late-2015, speed limits greater than 55 mph were used on rural undivided roadways in 21 states. Several other states have also considered increasing speed limits on undivided roadways beyond 55 mph. This includes the state of Michigan, where recently proposed legislation would increase speed limits on state-maintained rural undivided highways from 55 mph to 65 mph. In contrast to prior initiatives of this nature, many of the recent speed limit increases were done on a selective basis (rather than system-wide) in consideration of factors such as prevailing speeds, recent crash history, roadway geometry, and costs required for infrastructure upgrades. This is likely due in large part to the substantial infrastructure costs associated with geometric modifications along certain segments that will ultimately be necessary to achieve compliance with state and/or federal design speed requirements. However, a survey of states that recently increased speed limits on rural non-freeway roadways did not provide specific details as to how such roadways were selected or prioritized for selection. Furthermore, given how recently these increases were implemented, none of these states had been able to determine whether the speed limit changes had a measureable effect on traffic safety. This paper presents the results from a study aimed at developing a prioritization strategy to identify prospective candidate locations for potential speed limit increases in Michigan. The development of such a prioritization strategy was motivated by the estimated high costs associated with geometric modifications, specifically horizontal or vertical realignment, that would be required to achieve system-wide design speed compliance during rehabilitation or reconstruction, in addition to expected impacts on traffic safety. Consequently, it was determined that a prioritization strategy was needed to identify roadway segments that possess a low safety risk and where costly geometric improvements can be avoided.

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