Assessment of Operational and Safety Impacts of Speed Limit Increases in Michigan

The state of Michigan increased the passenger car speed limits on 614 miles of limited-access roadways from 70 mph to 75 mph following enactment of Michigan Public Acts 445 and 447 of 2016. This same legislation also increased the speed limits on 943 miles of trunk line non-freeways from 55 mph to 65 mph. Speed limits for trucks were also increased to 65 mph on all state trunk lines where passenger car speed limits were 65 mph or higher. This study investigates the impacts of these speed limit increases on travel speeds and traffic safety on both freeways and non-freeways. To assess the impacts of these increases on vehicle speeds, data were compared between those sites where the speed limits were increased and similar control sites that retained the lower speed limits. Speed data were collected from various sources and compared. The 5-mph increase in speed limits on the freeway network was associated with an increase of 1.1 mph to 2.8 mph in free-flow speeds. Aggregate speeds increased on average by 1.4 mph to 3.2 mph. The standard deviation in speeds was also shown to increase by significant amount. Turning to the non-freeway network, spot-speed data were collected from free-flow vehicles using LIDAR and high-definition video cameras. The 10-mph speed limit increases corresponded with increases of 2.8 to 4.8 mph among various speed metrics for passenger cars and heavy vehicles. Across both types of roadways, speeds at the control sites remained relatively consistent. For the speed limit increase locations, the magnitude of the increases in speeds was found to be the highest among the highest-speed drivers, while the lowest speed drivers tended to increase their speeds by lesser amounts. The magnitude of the changes in speeds also varied based on roadway characteristics, including traffic volume, presence of horizontal curves, and roadway cross-sectional characteristics. Safety impacts were examined using empirical Bayes (EB) evaluations. Safety performance functions (i.e., crash prediction models) were also developed for total crashes, for non-animal related crashes, and for various injury severity levels, among other subsets. On the limited access freeway network, total crashes increased by 9 percent. For fatal and incapacitating injuries, the increases ranged from 25 to 33 percent. Increases were also experienced among non-incapacitating injury and property-damage-only crashes. Additional regression analysis to relate crash frequency with speed metrics on the freeway network showed consistent increases in crashes across all severity levels after controlling for mean speed, variability in speeds and other site-specific variables. On the non-freeway system, total crashes and serious injury crashes increased by 11 percent and 1.7 to 4.7 percent, respectively. Ultimately, the effects of speed limit increases were largely consistent with results from the extant research literature. The results have also shown that speed selection varies significantly both within and across locations. Changes in the characteristics of the roadway driving environment also affect speeds differently and, thus, careful consideration should be given in considering any subsequent speed limit increases. This is particularly true since the sites where speed limits were increased tended to be the safest when considering historical crash data. As a result, speed limit increases on other segments may be expected to experience larger increases in crashes as compared to these lower risk sites. In addition to crash history, other factors, such as the variability in speeds, roadway context, and geometric characteristics should be considered in determining speed limits.