Hu, Wen; McCartt, Anne T.
Insurance Institute for Highway Safety
In January 2009, the speed limit for all vehicles was raised from 75 to 80 mph on two sections of rural interstate highway I-15 in Utah. The current study evaluated the effects of the speed limit increase on vehicle speeds.
Vehicle speeds were measured at sites within and near the 80 mph speed zones and at more distant control sites in May 2008, May 2009, and May 2010. Log-linear regression models for passenger vehicles and for large trucks estimated percentage changes in vehicle speeds associated with the speed limit increase. Logistic regression models estimated the effects of the speed limit increase on the probability of passenger vehicles exceeding 80 mph. Models of the probability of large trucks exceeding 80 mph were not estimated due to very small proportions of trucks exceeding 80 mph in 2009 and 2010.
Relative to baseline speeds measured before the speed limit change, mean passenger vehicle and large truck speeds and the proportion of vehicles exceeding 80 mph declined substantially for all site groups four months after the speed limit increase took effect, with larger declines at sites with the 80 mph limit. One year later, speeds had increased somewhat at sites within or near the 80 mph speed zones, but not at control sites, and all speeds still were lower than at baseline. Based on patterns in speeds at the control sites, mean passenger vehicle speeds and the probability of passenger vehicles exceeding 80 mph generally were lower than expected in 2009 and higher than expected in 2010 at sites within the 80 mph speed zones and at nearby sites, although not all the differences were statistically significant. Notably, the odds of passenger vehicles exceeding 80 mph at sites within the 80 mph speed limit zones were estimated to be 31 percent higher in 2010 than would have been expected without the speed limit increase, a marginally significant effect. At sites near the 80 mph speed zones, there was a non-significant increase of 10.3 percent in the odds of passenger vehicles exceeding 80 mph in 2010 compared with the odds that would have been expected without the speed limit increase. Large truck speeds at sites within the 80 mph speed zones were significantly lower by an estimated 7.3 percent in 2009 and an estimated 3.7 percent in 2010, on average, than would have been expected without the speed limit increase. At sites near the 80 mph speed zones, large truck speeds were higher by an estimated 0.6 percent in 2009 and 1.6 percent in 2010, relative to expected without the speed limit increase. Only the increase in 2010 was significant.
Contrary to prior studies on the effects of speed limit increases in other states, observed travel speeds on affected roads in Utah decreased relative to the speeds observed before the speed limit increase in both the first and second years of the speed limit increase. The widespread reduction in speeds may reflect the effect of the unusually deep and persistent recession in the United States, along with volatile gas prices. Although speeds did not recover to their baseline levels, passenger vehicle speeds within or near the 80 mph speed zones were increasing faster than at the control sites from the first to the second year of the speed limit change, Thus, 16 months after the change, there was evidence that sections with the higher limit were encouraging faster travel than would have been expected, had the speed limit not been raised. There was no evidence that increasing the speed limit was associated with increased large truck speeds, which could reflect a greater sensitivity of these vehicles to the depths of the U.S. recession and complex and unknown ways these vehicles may have been affected.