FHwA Speed Limit Synthesis
Technical Summary
Synthesis of Speed Zoning Practice
Report Number FHWA/RD-85/096, July 1985
US Department of Transportation
Federal Highway Administration
FHwA contact: Davey L. Warren, HSR-30, FTS 285-2426
Research, Development and Technology
Turner-Fairbank Highway Research Center
6300 Georgetown Pike
McLean, VA 22101-2296
Limit Should Reflect Traffic Speed
Introduction
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 this survey.
Results
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 based on other factors.
The main factors used in setting speed limits are shown below. The
most commonly reported lower level of the speed limit is 5 mph below
the 85th percentile with 10 mph below being the extreme.
Main Factors Used to Set Speed Limits
Factor Percent of states/locals
-----------------------------------------
85th percentile speed 100/86
Roadside development 85/77
Accident experience 79/81
10 mph pace 67/34
Roadway geometrics 67/57
Average test run speed 52/34
Pedestrian volumes 40/50
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 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.
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.
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