DOI

10.5703/1288284313353

Abstract

The influence of speed limits on roadway safety is an extremely important social issue and is subject to an extensive debate in the State of Indiana and nationwide. With roughly 900 fatalities and sixty thousand injuries annually in Indiana, traffic accidents place an incredible social and economic burden on the state. Still, speed limits posted on highways and other roads are routinely exceeded as individual drivers try to balance safety and mobility (speed) as well as risks of penalties from enforcement efforts. This research explores the relationship between speed limits and roadway safety. Specifically, the research focuses on the influence of the posted speed limit on the causation of accidents (the likelihood that unsafe speed would be listed as the primary cause of the accident), the severity of accidents, and speed-limit compliance with an emphasis on speed-limit changes from 65 mph to 70 mph on rural interstates and from 55 mph to 60 mph on select non-freeway multilane rural highways. Data were considered on individual accidents from the Indiana Electronic Vehicle Crash Record System in 2004 and 2006. Data from 2005 were excluded because Indiana raised some interstate and non-interstate speeds in July 1, 2005 – thus making 2005 a transition year as drivers adjusted to new speed limits. With these data, appropriate statistical models were estimated for the causation and severity of different types of accidents on all road classes. The results of the modeling show that higher speed limits did not have a statistically significant effect on the likelihood of unsafe-speed being listed as the primary cause of the accident on interstate highways. For some non-interstate highways, higher speed limits were found to significantly increase the likelihood of unsafe speed being listed as the primary cause of the accident, and for others, the higher speed limits were found to decrease this likelihood. With regard to accident severity, the results show that speed limits did not have a statistically significant effect on the severity of accidents on interstate highways. However, for some non-interstate highways, higher speed limits were found to be associated with higher accident severities – suggesting that future speed limit changes on non-interstates need to be carefully assessed on a case-by case basis. The findings in this project provide some information to the Indiana Department of Transportation on the potential consequences of future speed-limit policies in the state.

Report Number

FHWA/IN/JTRP-2007/06

Keywords

speed limits, speed-limit compliance, speed limits and safety, vehicle-accident severity, vehicle-accident causation, SPR-3030

SPR Number

3030

Project Number

C-36-59VV

File Number

8-5-48

Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, IN

Date of this Version

2007