Non-Prima Facie Speed Limits

When the prima facie limit is considered too high on a county road, the State Police, in conjunction with the road commission and local township board, conduct a speed study to determine the "reasonable and proper" speed for the road.


Road agencies around the country have established standardized methods for conducting speed studies. These methods include: 

  • Accident rates
  • The character of the street (whether there are sidewalks, the number of driveways, sight obstructions, etc.)
  • Engineering and traffic studies that examine such things as current traffic speed
  • Pedestrian activities
  • Potential hazards that might not easily be detected by drivers
  • Traffic volume

In most cases, speed limits are based on the speed that the majority of the existing traffic is traveling. This is called the "85th percentile rule." This rule, which is used nationwide, dictates that whatever speed 85 percent of the traffic is traveling is the appropriate speed for the location under study.

The theory is that most drivers are responsible and will accurately judge on their own the proper speed for the conditions of the road.

Drivers slow down where there are curves and hills and/or other factors that might affect vehicle control or sight distance, and will go faster where the road is straight and level with no sight obstructions.

85th Percentile

The 85th percentile rule is based on the following: 

It is generally agreed that with no traffic controls, the driver would adopt a reasonable speed for the prevailing conditions. Further, it is sometimes assumed that a certain percentage (usually 15 percent) of drivers will normally exceed a safe and reasonable speed.

To get an enforceable speed limit set or changed on a county road, it is necessary that the state police conduct a speed study and that the state police and the Road Department concur on the speed limit. Unless the state police concur with the proposed speed limit, it is not legally enforceable.

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