Effects of vehicle power on passenger vehicle speeds

May 2016

Objectives: During the past two decades, there have been large increases in mean horsepower and the mean horsepower-to-vehicle-weight ratio for all types of new passenger vehicles. This study examined the relationship between travel speeds and vehicle power, defined as horsepower per 100 pounds of vehicle weight. Methods: Speed cameras measured travel speeds and photographed license plates and the faces of drivers of passenger vehicles traveling on roadways in northern Virginia during daytime off-peak hours in spring 2013. The driver licensing agencies in the District of Columbia, Maryland, and Virginia provided vehicle information numbers (VINs) by matching license plate numbers with vehicle registration records and provided the age, gender, and zip code of the registered owner(s). VINs were decoded to obtain the curb weight and horsepower of vehicles. The study focused on 26,659 observed vehicles for which information on horsepower was available and the observed age and gender of drivers matched vehicle registration records. Log-linear regression estimated the effects of vehicle power on mean travel speeds, and logistic regression estimated the effects of vehicle power on the likelihood of a vehicle traveling over the speed limit and more than 10 mph over the limit. Results: After controlling for driver characteristics, speed limit, vehicle type, and traffic volume, a 1-unit increase in vehicle power was associated with a 0.7% increase in mean speed, a 2.7% increase in the likelihood of a vehicle exceeding the speed limit by any amount, and an 11.6% increase in the likelihood of a vehicle exceeding the limit by 10 mph. All of these increases were highly significant. To illustrate the findings, a 3-unit increase in vehicle power, which is equivalent to the difference between the 10th and 90th percentile vehicle power for the study vehicles, is associated with a 38% increase in the likelihood that a vehicle exceeds the speed limit by more than 10 mph. Conclusions: Speeding persists as a major factor in crashes in the United States. There are indications that travel speeds have increased in recent years. The current findings suggest that the trend toward substantially more powerful vehicles is contributing to higher speeds. Given the strong association between travel speed and crash risk and crash severity, this is cause for concern.