Raising the speed limit from 75 to 80 mph on Utah rural interstates: effects on vehicle speeds and speed variance

2017

Introduction: In November 2010 and October 2013, Utah increased speed limits on sections of rural interstates from 75 to 80 mph. Effects on vehicle speeds and speed variance were examined. Methods: Speeds were measured in May 2010 and May 2014 within the new 80 mph zones, and at a nearby spillover site and at more distant control sites where speed limits remained 75 mph. Log-linear regression models estimated percentage changes in speed variance and mean speeds for passenger vehicles and large trucks associated with the speed limit increase. Logistic regression models estimated effects on the probability of passenger vehicles exceeding 80, 85, or 90 mph and large trucks exceeding 80 mph. Results: Within the 80 mph zones and at the spillover location in 2014, mean passenger vehicle speeds were significantly higher (4.1% and 3.5%, respectively), as were the probabilities that passenger vehicles exceeded 80 mph (122.3% and 88.5%, respectively), than would have been expected without the speed limit increase. Probabilities that passenger vehicles exceeded 85 and 90 mph were non-significantly higher than expected within the 80 mph zones. For large trucks, the mean speed and probability of exceeding 80 mph were higher than expected within the 80 mph zones. Only the increase in mean speed was significant. Raising the speed limit was associated with non-significant increases in speed variance. Conclusions: The study adds to the wealth of evidence that increasing speed limits leads to higher travel speeds and an increased probability of exceeding the new speed limit. Results moreover contradict the claim that increasing speed limits reduces speed variance. Practical applications: Although the estimated increases in mean vehicle speeds may appear modest, prior research suggests such increases would be associated with substantial increases in fatal or injury crashes. This should be considered by lawmakers considering increasing speed limits.