How are speed limits determined?

The Michigan Department of Transportation (MDOT) and the Michigan State Police (MSP) are jointly responsible for establishing speed limits along all state trunklines. The objective of any speed limit is that it be reasonable, enforceable, and that it assures a maximum limit.

MDOT and MSP conduct a speed study and analyze the data with the primary focus being on an arbitrary measurement called the '85th percentile' speed (that speed at or below which 85 percent of the vehicles are traveling).

The 85th percentile speed is considered by most agencies nationally to be the appropriate speed limit. It recognizes that most drivers voluntarily adjust their speed to the total roadway/roadside environment (width, alignment, surface condition, roadside development, pedestrian activity, weather, light conditions, etc.) For more on speed limits, visit the Office of Highway Safety Planning (OHSP).