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

Hu, Wen
Journal of Safety Research
2017

Objective: Effective May 2008, Utah allowed the speed limit for all vehicles to exceed 75 mph on rural interstate highways, following an engineering and safety assessment. Speed limits were increased from 75 to 80 mph on several roadway segments in January 2009, November 2010, and October 2013. A prior study found that passenger vehicle speeds measured 16 months after the 2009 speed limit increase were lower than before the increase, but higher than would have been expected without the increase. The current study examined the effects of the speed limit increases in 2010 and 2013 on vehicle speeds and speed variance.
Methods: Vehicle speeds were measured in May 2010 and May 2014 at sites within and near the new 80 mph speed zones and at more distant control sites where speed limits remained 75 mph. Log-linear regression models for passenger vehicles and for large trucks estimated percentage change in mean 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, 85, or 90 mph and on the probability of large trucks exceeding 80 mph. The effect of the speed limit increase on speed variance also was estimated by a log-linear regression model.
Results: At sites within the 80 mph speed zones and at a nearby spillover location where the speed limit remained 75 mph, mean passenger vehicle speeds were 3.1 mph (4.1%) and 2.6 mph (3.5%) higher in 2014, respectively, than would have been expected without the speed limit increase. The probability that passenger vehicles exceeded 80 mph in 2014 was 122.3% higher than expected without the speed limit increase at sites within the 80 mph speed zones, and 88.5% higher than expected at the nearby spillover site. All increases were significant. Although not statistically significant, the probabilities that passenger vehicle speeds exceeded 85 and 90 mph were higher than expected without the speed limit increase within the 80 mph speed zones, and lower than expected at the nearby spillover site. The mean speed of large trucks and the probability that large trucks exceeded 80 mph were higher than expected within and near the 80 mph speed zones. Only the increases in mean speed were significant. The speed limit increase was associated with non-significant increases in speed variance within and near the 80 mph speed zones.
Conclusions: The current study adds to the wealth of evidence that increasing the speed limit leads to higher travel speeds and an increased probability that drivers exceed the new speed limit. Moreover, the results contradicted the claim that increasing the speed limit reduces speed variance as speed variance increased, though not significantly, after the speed limit increase. Although the estimated increases in mean vehicle speeds may appear modest, prior research suggests such increases would be associated with substantial increases in crashes, especially fatal or injury crashes. For example, an increase in mean speed from 75 to 78 mph on a freeway is associated with a 17% increase in fatal crashes, based on a model that quantified the relationship between changes in mean speed and changes in the number of crashes. This evidence should be considered by lawmakers and transportation agencies considering increasing speed limits on interstates and freeways.