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Effect of speed limit increase on crash rate on rural two-lane highways in Louisiana

This study investigated the impact of a speed limit increase on the crash rate on rural two-lane roads in Louisiana. The Louisiana crash database for 1999-2004 was used to compare rates of different crash severities and types before and after a speed limit change on rural roads during the observation period. The comparison was made among homogeneous data groups established using a classification procedure that sought to control as many of the other factors contributing to the high crash rate on rural two-lane roads as possible. The natural trend in crash rates was observed by first dividing the road sections in the data into both those that had experienced a speed limit change in the last five years and those that had not, and then observing the crash trend among those that had not had any speed limit change. The speed limit change group was divided into before and after speed change sections, and the after speed change crash rate values were adjusted for any significant trend in the corresponding no speed limit change cases. These final before and after crash rate values adjusted for the trend were compared statistically to test the null hypothesis that crash rate does not increase with speed limit increase. Based on the results, the null hypothesis that an increase in speed limit had no impact on crash rate was rejected for 6 out of the 39 cases at the 5% level of significance. The cases that were found to be significantly affected by an increase in speed limit included run off road, rear-end, and single-vehicle crashes involving no impact with another object or impact with a fixed object, animal, cyclist, or pedestrian.