Montana Department of Transportation

Malcolm “Mack” Long, Director


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Safety Impact of Differential Speed Limits on Rural 2-Lane Highways in Montana

speed limit sign along roadway

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. This research study determined conditions under which differential speed limits or, alternatively, uniform speed limits should be utilized on two-lane rural highways.