Before discussing the gathering and analysis of the speed study data, there are some facts about driver behavior which are the basis of all traffic laws, including modified speed limits.
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.
Posting unrealistically low speed limits may create a false sense of security. Actually, studies show that the driving environment, not the posted speed limit, is the main influence on motorists’ speeds.
Speed Studies
Speed studies are taken during light to medium traffic conditions on a weekday. Rush hours and adverse weather conditions are avoided because they do not represent normal, free flow traffic. Areas such as intersections, railroad tracks, or other factors that will influence speed are avoided. Since modified speed limits are the maximum allowable speeds, the conditions under which speed studies are taken 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. For example, if 85 of each 100 motor vehicles were recorded at 45 mph or under, then 45 mph is the 85th percentile speed.
Historically, before and after traffic engineering studies have shown that changing the posted speed limit does not significantly affect the 85th percentile speed. The driving environment, which includes other traffic on the road and roadway conditions, is the primary factor which influences the prevailing speed.
The driving environment is reflected by the 85th percentile speed. The majority of drivers, consciously or unconsciously, consider the factors in the driving environment and travel at a speed that is safe and comfortable regardless of the posted speed limit.
The speed data are collected by recording the speeds of free flowing motor vehicles using a radar or other speed measuring device. A representative sample of vehicular speeds is recorded and these speeds would include local residents who drive through the zone.
Use of the 85th percentile speed acknowledges that 15% of the drivers are traveling above a speed that is reasonable and proper. This is the 15% of motorists at which enforcement action is directed. Studies have shown that this is the group of motorists that cause many of the crashes and have the worst driving records.
There are other parameters used to evaluate speed data, such as the average, median and pace speeds. However, the 85th percentile speed is the most critical criterion in establishing realistic speed limits.
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 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 speed limit and those driving at a speed they feel is reasonable and prudent. These differences in speeds can result in increased crashes due to tailgating, improper passing, reckless driving, and weaving from lane to lane. However, the number of traffic crashes along any highway is related to numerous factors.
Regardless of the roadway involved, there is a statistical number of crashes that can be expected to occur no matter how safe a roadway is made. The traffic survey team determines if the number of crashes is unusually high by analyzing the crash rate for the section of roadway under study. A crash rate is based on the number of crashes and amount of traffic for a given segment of roadway. The traffic crash pattern is then taken into consideration when determining the speed limit.
Investigations of crashes reveal that in the majority of cases there was a clear violation of a traffic law or rule of good driving. A review of crash experience is an important component of any analysis of speed limits. Proper analysis and evaluation of these factors require the experience and expertise of the traffic survey team.
Studies have been conducted over the years to relate crashes to speed. Based on these 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 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 which are all reflected in the 85th percentile speed. For example these may include: traffic volumes, roadside development, roadway and shoulder widths, condition of the roadway, and the number of lanes, intersections, driveways, hills, curves, sidewalks, schools, parks, and any other factors recorded by the team.
The traffic 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. Sometimes consideration is given to reduce a speed limit due to a certain condition. If a hazardous condition is found, an attempt should be made to correct it. If it cannot be corrected, consideration should begiven to posting an advisory speed control sign or, if several conditions are present, then the speed limit may be reduced. Of particular concern are hills and curves where vision is restricted. On long stretches of roadway, one or two hills or curves should not dictate the speed for the entire roadway. Motorists are warned of the reduced sight distance through the use of warning signs with advisory speed controls.
The number of changes in the speed limit along a given route should be minimized. With this in mind, the length of the speed zone should be a least one-half mile. Survey team members base their recommendation on the conditions that exist at the time of their evaluation and should not attempt to consider such things as future growth, anticipated enforcement, or concerns for something that hasn’t happened.
Realistic speed limits provide for a uniform and orderly movement of traffic. There is a need for uniformity on all roadways especially where they carry large volumes of traffic through various roadside conditions or numerous adjoining communities.
Recommendation
Once all the data have 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 requestor as well as any local units of government. If requested, a public presentation of their findings may be conducted. 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 traffic survey team then submits a written report of their findings and recommendations to their respective agencies. When a modified speed limit is recommended, a Traffic Control Order is submitted to the respective road agency and the Director of the Michigan Department of State Police for their approval and signatures.
Posting Speed Limit Signs
The modified speed limit becomes 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 installed. The Michigan Manual of Uniform Traffic Control Devices sets forth standards for installing speed limit signs and specifies the size, shape, color and location of the signs.
Signs should be installed at the start of a zone, beyond major intersections and at approximately one-half mile intervals. The speed limit is established in increments of 5 mph, as close as possible to the 85th percentile speed. “REDUCED SPEED AHEAD” signs may be posted to advise motorists of speed limit reductions. These signs are not normally required in urban areas where speeds are relatively low. Generally the entire speed survey process from request to signs being posted takes anywhere from 3 to 9 months, depending on the complexity of the situation.
