Jackson County Road Commission Speed Limits Setting Realistic Speed Limits Speed limits can and should provide motorists, police and traffic courts with information on reasonable and safe speeds that will facilitate the safe, orderly flow of traffic under normal conditions. The public should be encouraged to bring to the attention of public officials what they perceive to be speeding problems. But speed limits should not be set based on casual observations or uniformed opinion. Public agencies have responsibility to establish speed limits based upon thorough traffic engineering surveys. What factors are considered when establishing a speed limit? Traffic engineers and police officers examine many traffic and road conditions to determine a reasonable speed limit. These inlcude number and type of crashes, speed of vehicles and number of cars, pedestrians, and bicycles. Also considered are physical conditions of the road such as sidewalks, hills, curves, lanes, driveways, intersections, roadway surface and traffic controls.

How Speed Limits Are Established Basic Speed Law Under the Michigan Vehicle Code, the Basic Speed Law states "a person driving a vehicle on a highway shall drive at a careful and prudent speed not greater than nor less than is reasonable and proper". The maximum speed limit on all highways is 55 mph. All other speed limits are called prima facie speed limits - those safe and prudent under normal conditions. The prima facie speed limit in residential and business districts is 25 mph. These speeds may not always be posted, but motorists are required to know them. 85^th Percentile Speed Speed limits between 25 and 55 mph are established on the basis of traffic engineering surveys. A traffic engineering survey analyzes the road conditions, crash history and prevailing speeds along the road. The speed limit is then set at the speed at or below which 85^th percent of the drivers drive, otherwise known as the 85^th percentile speed. Studies have shown that posting the speed limit at the 85^th percentile speed results in more uniform traffic flow and fewer accidents. Drivers tend to be less impatient, pass less often and tailgate less, thereby reducing crashes. In addition, posting an appropriate speed limit simplifies law enforcement since blatant speeders are easily identified and law enforcement officers are not trying to enforce unrealistic and arbitrary speed limits. Under Michigan law, speed limits on county roads are determined by unanimous agreement between the county road commission, the township board and the director of the State Police. Lower Speed Limits The Jackson County Road Commission receives many requests to post a lower speed in order to slow down traffic on a road. Many studies have demonstrated that drivers tend to operate their vehicles at speeds that are reasonable and proper, regardless of the posted speed limit. Since it is the driving environment that mainly influences speed, posting signs at higher or lower speeds does not significantly change the 85^th percentile speed. In addition, if an artificially low speed limit is posted, some drivers will obey the lower speed limit while many others will simply ignore it. This difference in speed disrupts traffic flow and can lead to an increase in crashes. Finally, when traffic is traveling at different speeds, the number of gaps in traffic to allow safe crossing is reduced and pedestrians and other drivers have a more difficult time in judging the speed of approaching vehicles. Gravel Roads Due to the constantly changing conditions of these roads, the Road Commission does not post speed limits on all gravel roads. Gravel roads that do not have posted speed limits, therefore, fall under Michigan’s Basic Speed Law which requires a driver to drive in a safe and reasonable manner with a maximum speed of 55 mph. This position has been supported by the Michigan Department of State Police. Home Page | Information | Commission | Departments Contact Us | Permits For more information: Jackson County Road Commission 2400 N. Elm Road Jackson, MI 49201 US mailto:jcrc@jcrc-roads.org 517-788-4230 1-800-718-3537 Fax: 517-788-4237 © Copyright 2007 Jackson County Road Commission. All Rights Reserved.