A Recommended Speed Zoning Practice

Speed Limits

January, 1996 Draft Version

It should be noted that this report from the 4M-25 Committee on Speed Zoning, a part of the Institute of Transportation Engineers, is a draft version and is not yet an official ITE recommendation. Permission to post this on this web site was granted by the committee.

Introduction

The purpose of speed zoning as stated in the Uniform Vehicle Code is to establish a speed limit which is "reasonable and safe for a given section of roadway." There are at least two difficulties when interpreting this statement. The first is a question of "reasonable to whom?", and the second is the implication that there is truly a cause and effect relationship between speed limits and safety.

It is clear from the controversy surrounding the use of speed zones that there are differences of opinion as to what is a reasonable speed among drivers, residents, legislators and enforcement officers. Thus, compliance with the vehicle code in determining the appropriate speed limit to post in a speed zone requires a definition of the term "reasonable speed."

The use of speed zones to increase safety depends on the assumption that a direct relationship exists between a change in the speed in the speed limit and a change in driver behavior which results in increased safety. Changing speed limits alone seldom changes speed characteristics of the traffic stream, indicating that this assumption is likely invalid. In fact the net result of current practice which results in decreasing the speed limit is that a higher percentage of the drivers are now in violation of the speed limit, while their speeds have not changed. It is improbable that this results in increased safety. It may only serve as an ineffective substitute for other traffic engineering measures that could result in increased safety.

However, speed zoning as a traffic engineering tool should not be ignored. It is widely used, and many states, counties and cities have developed policies and guidelines for the implementation of speed zones. Speed zoning, when properly applied and enforced, contributes to highway safety.

Inconsistencies in Speed Zoning

In a survey conducted by this committee, the most frequently cited reasons for establishing speed zones is to increase safety and to inform the motorist of the reasonable speed for this segment of the road. However, there are serious inconsistencies in the practice of speed zoning which make it difficult to justify speed zoning as either a safety measure or a means of communicating the reasonable speed to the motorist. These inconsistencies are described as follows:

Location of Speed Zones. Even though traffic engineers and the public both perceive speed zoning to be a safety tool, such zones are frequently established primarily in response to citizen demands rather than where an accident or accident potential problem exists. It is not surprising, then, that studies of accident reductions resulting from speed zoning are inconclusive. If speed zones are to be an effective safety device, the profession needs consistent guidelines, based on an engineering analysis, to determine where and when to establish these zones.
Speed Limits. Most traffic engineers support the use of the 85th percentile speed as a basis for determining the appropriate speed limit. However, the majority of posted speed limits in speed zones nationwide are much lower than the 85th percentile speed and in many cases lower than the average speed. The limits are often set to reflect either legislatively created limits or simply to accommodate the demands of the public. The profession also needs to be consistent in applying guidelines for posting speed limits in speed zones. Uniformity in the application of traffic control devices is one of the goals of the traffic engineering profession.
Enforcement tolerance. This final inconsistency refers to the practice of enforcing the speed limit in speed zones. Where speed limits are artificially low, the enforcement tolerance must be high. Since enforcement action against a large proportion of a traffic stream is not possible, the enforcement tolerance must be increased when the speed limit is set below the 85th percentile speed. While a large tolerance may be necessary for zones where the speed limit is artificially low, it is not appropriate to use this same tolerance where the speed limit is set at or near the 85th percentile speed. Since all speed zones have identical signing, the motorist cannot distinguish between the two types of speed zones.

Rationale for Consistent Speed Zone Guidelines

Several studies have demonstrated that drivers who travel either slower or faster than the 85th percentile speed of the traffic stream have a higher accident involvement rate than those drivers whose speed is close to the 85th percentile speed. Posting the speed limit at the 85th percentile speed informs the motorist of the speed which is expected to minimize their risk of an accident. Thus, the overriding basis (from a safety perspective) for speed zoning should be that the creation of the zone, and the speed limit posted, reflects the maximum speed considered to be safe and reasonable (i.e., the 85th percentile speed).

A second rationale for consistency in speed zoning practice is the desire for equitable treatment of motorists. When speed limits are set artificially low, and enforcement action cannot be directed at all the violators, the enforcement officer has too much discretion in selecting the motorists to be penalized. The cost of being selected can include both a fine and an increase in the cost of insurance. This type of enforcement ultimately leads to poor public relations for both the traffic engineering agency and the enforcement agency.

A third rationale is the need for consistency between the speed limit and other traffic control devices. Signal timing and sight distance requirements, for example, must be based on the prevailing speed of traffic. If these values are based an a speed limit that does not reflect the prevailing speed of traffic, safety may be compromised.

If speed zones are to fulfill their intended function as a traffic control device used to enhance highway safety and operations, these inconsistencies must he eliminated. As currently practiced, speed zoning violates one of the basic traffic engineering premises stated in the national Manual on Uniform Traffic Control Devices: "..uniformity means treating similar situations in the same way. The use of a standard device does not, in itself, constitute uniformity. A standard device used where it is not appropriate is as objectionable as a nonstandard device..."

The most desirable method of eliminating these inconsistencies would be to require all speed zones to be based on an engineering study and to enforce all speed limits with equal rigor. Unfortunately, traffic engineers, enforcement agencies and the courts seem to be moving in the opposite direction, with more speed zones being established based on other criteria, more speed limits being set incorrectly, and greater tolerance in the enforcement of these speed limits.

Guidelines

The following guidelines will provide a consistent basis for the application of engineering principles to speed zoning.

Speed zones shall only be established on the basis of an engineering study. Each speed zone should be periodically restudied to determine that the established speed limit is appropriate. The suggested maximum interval is five years. In addition, an engineering study should be conducted whenever there is a change in the roadway that would affect the prevailing speed. Such changes would include elimination of parking, added lanes, signal coordination, changes in roadside development, etc.
The engineering study include an analysis of the current speed distribution of free- flowing vehicles. The speed limit within a speed zone shall be set at the nearest 5 MPH increment to the 85th percentile speed or the upper limit of the 10 MPH pace. No speed zone shall be established in a location where the 85th percentile speed is within +/- 3 MPH of the statutory speed limit. The existing speed limit within a speed zone shall not be changed if the 85th percentile speed is within +/- 3 MPH of the posted speed limit.
The engineering study may include other factors such as:
1. Geometric features including: vertical and horizontal alignment, and sight distance;
2. Roadside development;
3. Road and shoulder surface characteristics;
4. Pedestrian and bicycle activity;
5. Speed limits on adjoining highway segments:
6. Accident experience or potential.
However, in no case should the speed limit be set below the 67th percentile speed of free flowing vehicles.
Speed zones should not be used to warn motorists of hazardous conditions. If a hazardous condition exists within the road segment under study, this condition should be corrected or an appropriate warning sign in conjunction with an advisory speed plate should be posted.
Enforcement of speed limits within speed zones should be uniform. Efforts should be made to coordinate the implementation of speed zones and the enforcement policies with the governing enforcement agency.

Legal Issues

In addition to the application of the Speed Zoning Guidelines, some changes in laws or ordinances would be required to eliminate inconsistencies in speed zoning. This would include the codification of the requirement for engineering study, justification of speed zones as well as a requirement for a periodic restudy. It should also include some distinction in terms of enforcement. For example, on roadways where these guidelines are not followed, enforcement should only be based on violation of the Basic Speed Law. [NMA's Model Speed Zoning Law meets these requirements.]

Definitions

As used in this document, the following terms are defined as:

Speed zone - A section of street or highway where a speed limit different than the statutory speed limit has been established.
Speed limit - The maximum (or minimum) speed permitted on a section of street or highway. This limit may be statutory or it may be established within a speed zone on the basis of an engineering study.
Basic Speed Law - no person shall operate a motor vehicle at a speed greater than is reasonable and proper for the prevailing conditions.
85th (67th) percentile speed - The speed at or below which 85 (67) percent of the sample of free flowing vehicles are traveling. This speed should be determined by conducting a spot speed study following the procedure contained in the Manual of Transportation Engineering Studies. [Note: The 85th percentile speed would be the maximum speed limit, and the 67th percentile speed would be the minimum speed.]
Pace - The 10 MPH band of travel speeds containing the largest number of observed vehicles.
Advisory speed - The speed at which a specific feature along the street or highway may be traversed.
Tolerance - the numerical difference between the speed limit and the minimum speed at which enforcement action is taken.

Bibliography

Speed Zoning In America: Some Preliminary Research Results. U.S. Department of Transportation, Federal Highway Administration, November 1989.
Assessment of Current Speed Zoning Criteria. Analysis Group, Inc., January 1989.
Synthesis of Speed Zoning Practices. U.S. Department of Transportation, Federal Highway Administration, July 1985.
Manual of Transportation Engineering Studies, 1st Edition, 1994.

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The NMA's Position

Speed limits should be based on sound traffic-engineering principles that consider responsible motorists' actual travel speeds. Typically, this should result in speed limits set at the 85th percentile speed of free-flowing traffic (the speed under which 85 percent of traffic is traveling). These limits should be periodically adjusted to reflect changes in actual traffic speeds.