OLR Report 2013-R-0074 examines whether raising speed limits results in an increase in the number or rate of motor vehicle accidents. Much of the information in this report is from a 2006 National Cooperative Highway Research Program (NCHRP) report. NCHRP researches problems affecting highways.
Numerous studies of the relationship between increased speed limits and accident rates have been conducted since 1987, when states were allowed to increase maximum speed limits from 55 miles per hour (mph) to 65 mph. According to the NCHRP, the studies did not identify a clear relationship.
Consequently, NCHRP conducted its own study to help guide state highway officials and policy makers in setting speed limits. It examined these earlier studies, surveyed state transportation and police departments, and collected and analyzed relevant data.
The NCHRP study found that higher speed limits were associated with an increased likelihood of deaths and incapacitating injuries. It found that increasing a speed limit from 55 to 65 mph on an "average" section of high speed road resulted in about a 3% increase in the total number of crashes and a 24% increase in the likelihood that a vehicle occupant would be fatally injured. This increased crash rate would yield a 28% increase in the number of fatalities following the speed limit increase.
The study also found a similar, but lesser, impact when speed limits were raised from 65 to 75 mph. In those cases, the total number of crashes increased by 0.64%, increasing the probability of a fatality by 12%, with an overall increase of 13% in total fatalities. Although the analysis did not explain why a smaller increase occurred at the higher speeds, the study suggested that people may drive more cautiously when driving faster, or that roads deemed appropriate for a 75 mph limit are safer.
A subsequent study published in the American Journal of Public Health found about a 3.2% increase in road fatalities attributable to the raised speed limit on all roads in the U.S. The highest increases were on rural and urban interstates. The researchers attributed 12,545 deaths and 36,583 injuries in fatal crashes over a 10-year period to increased speed limits nationwide.
For more information, read the full report.
