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

85th percentile speed
Roadside development
Accident experience
10 mph pace
Roadway geometrics
Average test run speed
Pedestrian volumes

Percent of states/locals

100 / 86
  85 / 77
  79 / 81
  67 / 34
  67 / 57
  52 / 34
  40 / 50

Conclusion

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.

Chart

FHWA: Synthesis of Speed Zoning Practice
Report # FHWA/RD-85/096