Safety and Operational Impacts of Differential Speed Limits on Two-Lane Rural Highways in Montana

As of July 2016, Montana was the only state to maintain a differential speed limit on two-lane two-way rural highways, utilizing a daytime statutory speed limit of 70 mph for cars and light trucks and 60 mph for trucks exceeding a one-ton payload capacity. Although differential speed limits are common on freeways, the use of differential limits on two-lane roadways presents unique safety and operational issues due to passing limitations and subsequent queuing, and prior research on such issues is scarce. Consequently, research was performed to evaluate the safety and operational impacts associated with the aforementioned differential speed limit on rural two-lane highways in Montana, particularly when compared to a uniform 65 mph speed limit. A series of field studies were performed on two-lane rural highways in Montana, which predominately possessed the 70 mph/60 mph differential speed limit, and in neighboring states where uniform 65 mph speed limits prevailed. The locations with 65 mph speed limits generally displayed less variability in travel speeds, shorter platoon lengths, less high-risk passing behavior, and fewer crashes. Surveys were performed to determine the speed limit policy preferences among motorists and members of the trucking industry in Montana. Although motorist support for the uniform 65 mph speed limit was mixed, the trucking industry strongly supported the uniform 65 mph limit over the current differential limit. Overall, the collective findings support transitioning to a uniform 65 mph speed limit on two-lane rural highways in Montana. Selective implementation of this new speed limit is advised initially, and candidate highways should possess relatively high traffic volumes, relatively high truck percentages, and limited passing opportunities.