Evaluation of the traffic safety benefits of a lower speed limit and restriction of trucks to use of right lane only on I-10 over the Atchafalaya Basin.

To improve traffic operation and safety, several states have implemented truck lane restriction and differential speed limit policies on freeways. In response to an 11-vehicle crash in September 2003, the Louisiana State Department of Transportation and Development (LADOTD) introduced such operational policies on the 18-mile elevated section of Interstate 10 over the Atchafalaya Basin, where trucks were restricted to the right lane and their speed limit was reduced to 55 mph. The speed limit for all other types of vehicles was also reduced to 60 mph. The primary objectives of this research study were to examine the traffic characteristics and truck compliance behavior to the newly implemented policies, as well as evaluate the overall safety impact of such policies. Additionally, two opinion surveys were conducted to determine the truck drivers’ perception and opinions with respect to these policies. Traffic data were collected and analyzed for four different sites along the freeway corridor using multiple linear regression, pairwise comparison and two sample t tests. The basic statistical analysis showed that the speed in the left lane was much higher than it was in the right lane as a result of the imposed differential speed limit. The results also showed more trucks in the right lane than in the left lane, with a compliance rate in the range of 60% to 80% most of the time. Further statistical analysis showed that the truck speeds were generally lower than the rest of the vehicles because of their reduced speed limit. For mixed traffic composition, truck speeds were significantly higher than 55 mph, but lower than 60 mph on the right lanes. The truck speeds, however, exceeded 60 mph in the left lane. Thus, trucks generally violated both lane restriction and speed limit. However, it should be noted that truckers were informed through the local trucking association that they were permitted to overtake in the left lane. Thus, their presence in the left lane and their increased speed may be at least partially due to the overtaking maneuver. The results from both surveys were consistent and showed that the truckers were not in favor of the restrictions and did not perceive that significant safety benefits would be gained from such restrictions. In fact, several responses indicated that it would be safer to have uniform speed limits and freedom to select a travel lane. The crash data analysis clearly showed a reduction in the number of crashes, and particularly the number of truck crashes but in addition to the imposed restriction policies there were other improvements made such as shot abrasion and raised pavement markers, whose safety impact could not be independently evaluated.