Safety impacts of different speed limits on cars and trucks

The objectives of this study were to determine whether differential or uniform speed limits are more beneficial to transportation safety and traffic operations on interstate highways. The approach to achieving this objective was to examine speed and accident data from states employing both types of limits. Speed data were collected in 12 states at rural and urban locations representing all speed limits currently established on the interstate highway system for cars/trucks, ie 55/55 mi/h (89/89 km/h), 65/55 mi/h (105/89 km/h), 65/60 mi/h (105/97 km/h), and 65/65 mi/h (105/105 km/h). Accident data were obtained from nine states which were geographically distributed across the country and representative of all rural interstate speed limits currently established. For the speed data collected, a number of measures of effectiveness (MOEs) were examined including mean speed, speed variance, compliance, and speed distribution measures. For the accident data collected, types of crashes were examined (eg rear-end) along with vehicle involvement (eg car-into-truck) and crash severity. This final report summarizes the effects of uniform and differential speed limits on transportation safety and traffic operations as determined by the examination of speed and accident data.