Lowering Speed Limits

The following information was provided by the Traffic Improvement Association (TIA).


Speed limits should be set so that the majority of drivers observe them voluntarily and law enforcement can be directed to the few speed violators. An unrealistically low speed limit can actually lead to crashes.

Here's why: 

  • First, many studies conducted over several decades in all parts of the country have shown that a driver's speed is influenced more by the appearance of the roadway and the prevailing traffic conditions than it is by the number on speed limit signs.
  • Second, some drivers will obey the lower posted speed while others will feel it's unreasonable and simply ignore it. This disrupts the uniform traffic flow and increases accident potential between the faster and the slower vehicles.
  • Third, when traffic is traveling at different speeds, the number of breaks in traffic to permit vehicles to enter the roadway is reduced. Pedestrians also have greater difficulty in judging the speed of approaching vehicles.

Traffic Engineering Surveys

Speed limits should always be based on traffic engineering surveys, which include an analysis of roadway conditions, crash history, and the prevailing speed of prudent drivers.

Michigan's basic speed law states that drivers must observe speed limits and must also drive at a speed not greater than, nor less than, is reasonable and proper having regard for all existing conditions. The law continues: ... and no person shall drive any vehicle upon a highway at a speed greater than will permit him to bring it to a stop within the assured clear distance ahead.

Roadway & Traffic Conditions

Any speed limit is reasonable only for the roadway and traffic conditions for which it was set. Limits based on prevailing speeds of free flowing vehicles obtained during good weather will be reasonably high for extreme weather conditions. Under Michigan law, the maximum speed limit for any passenger vehicle is 70 miles per hour. All other speed limits are called prima facie limits - speeds that are safe and prudent under normal conditions.

These limits include 25 miles per hour in business and residential districts. These speeds aren't always posted, but motorists are required to know them.

Key Factors

The key factor in any speed zone is the 85th Percentile speed. This is related to accident risk on the theory that the majority of motorists traveling upon a city street are competent drivers and possess the ability to determine and judge the speed at which they operate safely. A majority of motorists are reasonable and prudent persons who do not want to become involved in an accident and desire to reach their destination in the shortest time possible.