The web Browser you are currently using is unsupported, and some features of this site may not work as intended. Please update to a modern browser such as Chrome, Firefox or Edge to experience all features Michigan.gov has to offer.
Many people ask what is the 85th percentile speed, and why is it used to set speed limits? State law dictates that MDOT and the Michigan State Police (MSP) jointly set speed limits that are based on the 85th percentile speed, which is the speed at or below which 85 percent of drivers are currently driving a given section of road. For example, if 85 percent of drivers on a section of road are driving 55 mph or less, the 85th percentile speed would be 55.
Michigan uses this methodology because it is the national standard for setting speed limits, recognizing that the great majority of drivers instinctively drive at a speed that is safe and comfortable based on the road design and other factors. This also results in fewer conflicts between drivers, which lead to unsafe actions such as tailgating and improper passing.
Routes that qualified for speed limit increases to 65 mph or 75 mph had new speed limit signs posted. MDOT maintenance funding (state funds) paid for the initial changes; however, future costs will likely be paid through a combination of state and federal construction funds.
Additionally, initial changes included posting new or modified speed limit signs on affected routes, moving no-passing zone signs where applicable, installing new advisory speed limit signs where needed, and changing pavement markings to match modified no-passing zones and turning/passing lanes. Long-term, increased speed limits may prompt road design changes and modifications of other features, such as guardrails. MDOT and MSP focused on selecting routes that were safe and appropriate based on current conditions, which will minimize the number of needed improvements.
Michigan Freeway/Non-Freeway Speed Limit Increases
Public Act 445 of 2016, tasked MDOT and MSP with increasing speed limits on a minimum of 600 miles of limited access freeway to 75 mph, and 900 miles of trunkline highway to 65 mph based on the results of engineering and safety studies that utilize the 85th percentile speeds (the speed at or below which 85 percent of traffic is moving).
Increased Speed Limit Legislation
Public Act 445 - Increased speed limits on freeways and non-freeways, hospital zones, truck and bus speed limits.
Public Act 446 - School zone speed limits.
Public Act 447 - Determination of minimum and maximum speed limits, and local speed limits.
Public Act 448 - Michigan Vehicle Code violations and traffic control devices.