About the research
The purpose of this research was to assist in determining the potential impacts of implementing a proposed 65 mph speed limit on non-freeways in Michigan. Consideration was given to a broad range of performance measures, including operating speeds, traffic crashes and crash severity, infrastructure costs, fuel consumption, and travel times. Specifically, a prioritization strategy was developed to identify candidate MDOT non-freeway road segments possessing lower safety risks and potential infrastructure costs associated with raising the speed limit from 55 to 65 mph. Ultimately, approximately 747 miles of undivided and 26 miles of divided 55 mph non-freeways were identified as lower risk candidates, representing approximately one-eighth of the MDOT systemwide mileage posted at 55 mph. An economic analysis of the anticipated costs and benefits associated with the proposed speed limit increase was performed for these lower risk candidate segments, in addition to a systemwide estimate. As the travel time savings were expected to outweigh the fuel consumption costs, it was necessary to determine if these net operational benefits outweighed the expected infrastructure upgrade costs and increased crash costs. For roadways possessing horizontal and/or vertical alignments that are not compliant with a 65 mph speed limit, an unfavorable benefit/cost ratio would likely result due to the excessive infrastructure costs incurred during 3R (resurfacing, restoration, rehabilitation) or 4R (reconstruction) projects. Crashes were expected to increase for all implementation scenarios, with a particular increase in the risk of fatal and incapacitating injuries. Due to the substantially large infrastructure costs, application of the 65 mph speed limit is specifically not recommended for non-freeway segments requiring horizontal or vertical realignment to achieve design speed compliance. Even for segments where compliance with the increased design speed is maintained, careful consideration must be given to the potential safety impacts –particularly to fatal and injury crashes – that may result after increasing the speed limit.