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The Michigan State Police (MSP) is responsible for making joint investigations and recommendations concerning: 1) reasonable and safe speed limits; 2) parking restrictions; and 3) traffic preference at intersections of state trunkline highways. This duty falls on the Traffic Services Section. Traffic services personnel located at each district headquarters conduct joint investigations with county road commission personnel for county roads or the Michigan Department of Transportation (MDOT) for state trunkline highways in each of the three previously listed areas.
The basis for all speed controls is predicated upon the nationally accepted principle that the majority of drivers are cautious, prudent, and drive at speeds that are reasonable and proper regardless of the posted speed limit. This "reasonable and proper" theme is part of the basic speed law as set forth in the Michigan Vehicle Code (Section 257.627). Basically, this law states that motorists must always drive at a speed which allows them to stop safely. This Basic Speed Law governs the speed of all drivers regardless of any speed controls.
There are several types of speed controls used in Michigan. The first is Advisory speed controls, which are recommended safe driving speeds to alert drivers of the maximum recommended speed through a curve or for other special roadway conditions. Advisory speeds are not enforceable in Michigan courts except under the Basic Speed Law provisions and are posted only in combination with the appropriate warning sign. The second is Regulatory speed controls which are enforceable and are categorized as either statutory or modified.
Statutory speed limits are set either as maximum/minimum speed limits or as prima facie restrictions. Prima facie is Latin for "on the face of it" and is the speed limit under most circumstances. These speed limits are set legislatively and apply throughout the state.
Modified speed controls are utilized in areas requiring speed limits between the statutory freeway speed limits, 55 mph maximum speed limits on state and county roadways, and the 25 mph prima facie speed limits in business and residential areas. These modified speed limits are established by administrative action based upon a traffic engineering study. The limits can be set only by agencies having the legal authority and jurisdiction over the respective roadway (MDOT and county road commissions working with MSP). These modified speed limits are referred to as absolute speed limits and are not to be exceeded regardless of conditions.
The establishment or review of speed zones originates for a variety of reasons, such as road construction, changes in land use, crashes, poor compliance with an established speed limit, or citizen request. The survey team, an engineer from MDOT or county road commission (depending on the roadway in question), and a State Police officer gather all the data needed to analyze the roadway conditions, such as speed studies, traffic crash statistics, driving environment, and other pertinent information.
Driver behavior is an extension of societal attitudes. Most drivers respond to traffic regulations in a safe and reasonable manner as demonstrated by their consistently favorable driving records. Traffic laws which reflect the behavior of the majority of motorists are usually respected and obeyed. In order for any traffic law to be enforceable, voluntary compliance must be practiced by the vast majority of drivers so violators can be easily identified. Realistic speed limits reflect this fact and recognize that unreasonable restrictions encourage widespread violations and disrespect for the entire traffic control system. Arbitrary laws unnecessarily restrict drivers, encourage violations, and lack public support. Studies show that the driving environment, not the posted speed limit, is the main influence on motorist speeds.
Speed studies are taken during times that represent normal free-flow traffic. Since modified speed limits are the maximum allowable speeds, roadway conditions must be close to ideal. 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. Return to reference
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.
Crash locations are identified by use of spot maps or computer programs. The action of each vehicle involved in a crash is also identified, and 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. Return to reference
The design, physical condition, and use of a roadway has 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 survey team makes a personal inspection of the roadway to verify the accuracy of their data. They drive the roadway to determine if there are any hazards not readily apparent to the motoring public. If hazardous conditions are found to exist, an attempt is made to correct them. If they cannot be corrected, consideration is given to posting an advisory speed control sign; or if several conditions are present, then the speed limit may be reduced.
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. Return to reference
Once all the data has been collected and reviewed by the traffic survey team, the facts are analyzed and a recommendation is made. When the survey members agree that a modified speed zone should be established, their proposal is communicated to the party that requested the change and local units of government. While local concurrence is desirable, it is not required by law. If the traffic survey team agrees that a modified speed limit is not justified or if they 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 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.
The primary function of a roadway is to provide for the safe and efficient movement of vehicles operating on that roadway system. Generally, outside of cities and villages, stopping, standing, and parking is prohibited on the main traveled portion of the roadway by the Michigan Vehicle Code. However, the demands of the land use adjacent to the roadway may require that parking be allowed within the right of way.
Overly restrictive prohibitions of parking that are not based on a genuine need cause widespread non-compliance and are unenforceable. Overly permissive parking situations lead to decreased capacities, traffic crashes, and a dysfunctional transportation system. Parking restrictions must reflect a narrow band between these two extremes. In all cases, however, the rights and safety of the driver take precedence over the demands of adjacent land owners.
The basis for taking action in parking situations is stated in Section 257.675 (4) of the Michigan Vehicle Code. The section states:
"The state transportation commission with respect to state trunkline highways and the board of county road commissioners with respect to county roads, acting jointly with the director of the department of state police, may place signs prohibiting or restricting the stopping, standing, or parking of vehicles on a highway where in the opinion of the officials as determined by an engineering study the stopping, standing, or parking is dangerous to those using the highway or where the stopping, standing, or parking of vehicles would unduly interfere with the free movement of traffic on the highway or street . . . ."
Recommendations for parking restrictions may be made on highways when: 1) there have been crashes involving cars parked or entering or leaving parked positions; 2) cars are continually stopped on the highway because of roadside interests; 3) parked cars will not permit the continuous free movement of traffic on the main traveled portion of the highway; 4) parked cars are creating vision obstructions or other hazardous situations for traffic utilizing the highway; or 5) parked cars obstruct, hinder, or interfere with the general maintenance of the highway.
The parking investigation is much like a speed investigation. The survey team (consisting of an engineer from MDOT or county road commission and an MSP officer) makes a visual inspection of the area under study noting conditions that exist, such as roadway width and condition, shoulder width and condition, adjoining land use, vehicle traffic volumes, speed limit, and the availability of off-street parking. This data is then analyzed for its effects on the capacity of the roadway and safety to drivers and pedestrians. The survey party then makes a recommendation regarding any parking restrictions. These restrictions become effective when the signs are posted and a Traffic Control Order has been signed by both parties and filed with the county clerk.
Traffic preference refers to the assignment of vehicular right of way to one or more directions of traffic at an intersection. The ultimate goal of any intersection is to safely and efficiently accommodate the required traffic volume on each roadway segment.
Where traffic volume is relatively low, sight distance is long, or approach speeds are relatively high, intersection traffic controls are not normally necessary. However, when either of these conditions is not present, traffic control devices may be necessary to regulate, warn, or guide intersection traffic.
The types of traffic control devices commonly used at the right of way at intersections are the traffic signal and merge, yield, and stop signs. The use of these controls will interrupt the traffic flow and increase delays in traffic, thus affecting the capacity and level of service for that roadway. For these reasons, traffic control devices are used only where conditions warrant their installation. Alternatives such as improved sight distance by removing vision obstructions (parked cars, tree limbs, brushes, etc.), changing the roadway alignment, or improving the grade separation are considered before restricting traffic flow.
Generally the roadway with the greater traffic volume will be awarded the traffic preference at an intersection. Three- and four-way controls will be considered when traffic volumes on each intersecting roadway are approximately equal in number. Occasionally, more efficient traffic flow may result by requiring traffic on one or more of the state highway legs to stop or yield to non-trunkline traffic.
Right-angle crashes are an important consideration in establishing right of way. A certain number of these types of crashes will occur at an intersection even with a traffic control device present. However, when right-angle crashes become disproportionately high in relation to total crashes, causation factors are determined. If warranted, traffic controls at these intersections may reduce the number and severity of right-angle crashes; however, the number of rear-end collisions and other types of crashes tends to increase.
This survey is done in a similar manner as the speed and parking investigations, with the exception that MSP is required to jointly conduct investigations only with MDOT at intersections with state trunkline highways. The survey team visits the survey location and notes the conditions present, including traffic volumes for each roadway and intersection crash experience, paying particular attention to right-angle crashes and visibility and capacities for each roadway. The information is analyzed and a recommendation is made based on sound engineering principles.
