Report No. FHWA/RD-85/096
Speed zoning is the establishment of reasonable and safe speed limits based on an engineering study. Speed zoning incorrectly used on streets and highways can lead to driver non-compliance with speed limits.
This study reviewed the principles and practices used to set speed limits. It is based mainly on a survey of traffic officials conducted by the American Association of State Highway and Transportation Officials (AASHTO) subcommittee on Traffic Engineering. All States and 44 city and county agencies responded to the survey.
Some of the problems found with current speed zoning practices include:
- Lack of understanding and support for current speed zoning criteria.
- Difficulty of using other factors such as road characteristics and accident experience in conjunction with prevailing speed.
- Public pressure based on concern about past accidents.
- Concealing radar and obtaining speed samples on low volume roads.
Traffic officials generally agree speed limits should reflect the speed of most drivers. All States and most of the local agencies use the 85th percentile speed of free flowing traffic as the basic factor. However, it is fairly common to reduce the speed limit based on a subjective consideration of other factors.
The main factors used in setting speed limits are shown in Table 1. The most commonly reported lower level of the speed limit is 5 mph below the 85th percentile speed with 10 mph below being the extreme.
Based on the best available evidence, the speed limit should be set at the speed driven by 85 to 90 percent of the free-moving vehicles rounded up to the next 5 mph increment. This method results in speed limits that are not only acceptable to a large majority of the motorists, but also fall within the speed range where accident risk is lowest. Allowing a 5 mph tolerance, enforcement would be targeted at drivers who are clearly at risk.
Table 1. Main Factors Used to Set Speed Limits
|Factor||Percent of States||Percent of Locals|
|85th percentile speed|
|Adjacent speed limits|
|10 mph pace|
|Average test run speed|
No other factors need to be considered since they are reflected in the drivers’ speed choice. If there are unusual hazards not readily apparent to drivers, then a warning sign could be installed giving the nature of the hazard and, if necessary, supplemented with a realistic advisory speed.
This technical summary is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The summary provides a synopsis of the study’s final report. The summary does not establish policies or regulations, nor does it imply FHWA endorsement of the study’s conclusions or recommendations. The United States Government assumes no liability for the contents or their use.
This summary is distributed directly to Regions and Divisions. The final report for this study will be available in October 1987. Copies will be available only form the National Technical Information Service, 5285 Port Royal Road, Springfield, Virginia 22161
Report Number/ Title: FHWA/RD-85/096.
“Synthesis of Speed Zoning Practices”
Martin R. Parker & Associates
44236 Suffolk Court
Canton, Michigan 48187-2129