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 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.