United States Department of Transportation
i
Evaluating the Impacts of the 2017 Legislative Mandated Speed Limit Increases
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2022-06-30
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[PDF-4.21 MB]
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Edition:Final Report, 02/01/2019 – 6/30/2022
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Abstract:This study evaluated the impacts of speed limit increases that occurred following the enactment of Michigan Public Acts 445 and 447 of 2016. Between May and June of 2017, the maximum speed limits were increased from 70 to 75 mph on 614 miles of rural, limited access freeways. During the same period, the speed limits were increased from 55 mph to 65 mph on 943 miles of rural two-lane roads. In addition, the maximum speed limits for trucks were increased from 60 to 65 mph on all routes where the passenger car limit was at least 65 mph. To assess the impacts of these increases, speed data were obtained from multiple sources including roadside spot-speed studies, permanent traffic recorder stations, and probe vehicles. These data were supplemented by statewide crash data from the Michigan State Police. A series of statistical analyses were conducted to evaluate changes in various speed metrics, including mean and median speeds, various speed percentiles of interest, and the variability in speeds within and across locations. The results showed consistent increases in speeds, ranging from 1.1 mph to 3.2 mph on freeways, and 3.8 mph to 5.1 mph on non-freeways. Crash analyses showed increases in both the frequency and severity of crashes following the speed limit increases. These increases tended to be more pronounced on the freeway network. Economic analyses were conducted to compare the costs incurred in the form of infrastructure upgrades with the benefits of reduced travel times and dis-benefits in the form of increased crashes and fuel consumption. These results showed a positive benefit-to-cost ratio for non-freeways and a larger, negative benefit-to-cost ratio for freeways. These findings provide important insights to inform future policy decisions related to speed limits. The effects of the COVID-19 pandemic on travel behavior were also investigated by examining changes in speed and crash data. The reductions in travel did not show meaningful impacts on speeds at the locations where limits were increased; however, speeds were shown to increase at control sites. Traffic crashes were lower following the onset of the pandemic, though the rate of crashes resulting in fatal or severe injuries increased at the sites that retained lower speed limits, suggesting adverse impacts that may be associated with the higher speeds.
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