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

Speed limit policies can be broadly classified into two categories. Uniform speed limit policies establish the same maximum limit for all vehicles, while differential speed limit policies set a lower limit for heavy trucks in comparison to cars and light duty trucks. While differential speed limits are common on limited access freeways in the United States, Montana is currently the only state that maintains a differential speed limit policy on two-lane rural highways, with a current daytime statutory limit of 70 mph for cars and light duty trucks and 60 mph for trucks with greater than one-ton payload capacity. However, in April 2013, speed limits were changed to a uniform 65 mph for all vehicles along 55 miles of MT-16 and MT-200 between Glendive and Fairview in eastern Montana. This change was made in response to observations of aggressive passing behavior by motorists queued behind heavy trucks with little opportunity to pass. Consequently, it was necessary to assess the impacts associated with these speed limit changes to determine if further application of the uniform 65 mph speed limit is warranted.