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How Speed Limits are Determined on County Roads

The purpose of a speed limit is to provide a safe roadway environment for motorists and pedestrians. This speed limit must be acceptable to motorists and enforceable by police.

Working With the State Police

To establish speed limits for county roads, the Michigan Vehicle Code requires that county road commissions work with the Michigan Department of State Police. A survey team, consisting of a county traffic engineer and a State Police Officer, conduct field studies and recommend speed limits on county roads. The team uses three important factors to help determine the appropriate speed limit; the 85^th percentile speed, traffic crash data, and the driving environment.

The 85^th Percentile Speed

The primary basis for establishing a proper, realistic speed limit is the nationally recognized method of using the 85th percentile speed. This is the speed at or below which 85% of the traffic moves. Studies have shown that changing the posted speed limit does not significantly change the 85th percentile speed. The driving environment, including other traffic on the roadway, roadway conditions, pedestrian traffic, etc., is a significant factor which influences the prevailing speed.

Use of the 85th percentile speed acknowledges that 15% of the drivers are traveling above a speed that is reasonable and proper. It is to this 15% that enforcement action is directed. Studies have shown that these are the drivers who cause many of the crashes and have the worst driving records.

Traffic Crash Data

Contrary to popular belief, lower speed limits do not necessarily improve safety. The more uniform the speeds of vehicles in a traffic stream, the less chance there is for conflict or crashes. Posting speed limits lower or higher than what the majority of drivers are traveling produces two distinct groups of drivers: 1) those attempting to observe the speed limit and 2) those driving at speeds they feel are reasonable and prudent. These differences can result in increased crashes due to tailgating, improper passing, and reckless driving.

The survey team looks for any pattern of crashes that emerges that is speed-related. The traffic crash pattern is then taken into consideration when determining the speed limit. Regardless of the roadway involved, there is a statistical number of crashes that can be expected to occur no matter how safe the roadway. The survey team determines if the number of crashes is unusually high by analyzing the crash rate based on the number of crashes and amount of traffic for the section of roadway under study. This crash rate is compared to the crash rate of similar type roadways for the purpose of relative safety.

Studies have been conducted over the years to relate crashes to speed. Findings show that a driver's risk of being involved in a crash is lowest at 5 to 10 mph above the average speed of traffic. This is coincidentally usually the 85th percentile speed. Based on those studies, the lowest risk of being involved in a crash occurs at approximately the 85th percentile speed.

Driving Environment

The design, physical condition, and use of a roadway have a profound effect on vehicle speeds because motorists vary their speeds depending on the driving environment. The traffic survey team considers significant items in the driving environment, such as traffic volumes, roadside development, roadway and shoulder widths, number of lanes, driveways, intersections, hills, curves, sidewalks, and any other factors that apply. These factors are all reflected in the 85th percentile speed.

The number of changes in the speed limit along a given route should be kept to a minimum. With this in mind, the length of the speed zone should be at least one-half mile. Survey team members base their recommendations on the conditions that exist at the time of their evaluation and should not attempt to consider such things as future growth or conditions not presently existing.

Recommendation and Summary

Once all the data has been collected and reviewed by the traffic survey team the State Police write a report of the findings and determine the proper speed limit for the section of road being evaluated. If the traffic survey team cannot agree on a recommendation, the survey is concluded with no change in the existing speed limit.

The modified speed limits become effective when the Traffic Control Order has been signed by both agencies, a copy of the order has been filed with the county clerk, and the signs have been erected.

Realistic speed limits provide for a uniform and orderly movement of traffic. It is important to encourage smooth traffic flow, not only for safety, but also for the convenience and economy of every motorist. Speed limits are based upon driving speeds--yours, your neighbors, and a percentage of everyone traveling on a roadway.

Frequently Asked Questions

Most drivers drive 5 to 10 mph over the speed limit. Why not establish the speed limit with this in mind?

While some drivers drive faster than the speed limit, this is not true of everyone. Experience shows that 85% of the drivers adhere to properly established speed limits that they feel are reasonable, comfortable and safe for conditions at the time. This is what we call the 85th percentile speed. This is the speed at, or below which, 85% of the traffic moves. Police officers can then target their speed enforcement efforts at the remaining 15% of drivers who are not in compliance with the speed limit.

Isn't a lower speed limit always safer?

No, lower speed limits do not necessarily improve safety. The more uniform the speeds of vehicles in a traffic stream, the less chance there is for conflict and crashes. Posting speed limits lower or higher than what the majority of drivers are traveling produces two distinct groups of drivers - those attempting to observe the limit and those driving at what they feel is reasonable and prudent. These differences in speeds may result in increased crashes due to tailgating, improper passing, reckless driving and weaving from lane to lane. Inappropriate established speed limits also foster disregard for other speed limits, traffic signs and signals and contribute to driver frustration.

Why not simply post a lower speed limit and have the police enforce it?

Posting speed limits lower than the 85th percentile speed does not result in voluntary motorist compliance with the posted speed limit unless there is strict, continuous and visible enforcement. Increased enforcement is effective only at the immediate time and in the area where the police officer is present. The availability of police officers is limited and their services must be shared with other police responsibilities. Since these lower speed limits cannot be properly enforced, they will be consistently violated and will breed disregard for speed limits in general.

I am only one person among thousands of other drivers. What can I do to reduce the speeding problem in my community?

Speed limits are based upon studies of driving speeds - yours, your neighbors, and a percentage of everyone traveling on a roadway. You have shown that you are concerned about speed limits just by taking the time to read this article. Please obey the speed limit, not only on your street but also on all streets and highways of our state.

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