Effects of Raising and Lowering Speed Limits

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(Abstract and Finding)

Report No. FHWA-RD-92-084                                      October 1992

U.S. Department of Transportation      Research, Development, and Technology
Federal Highway Administration         Turner-Fairbank Highway Research Center
                                       6300 Georgetown Pike
                                       McLean, Virginia 22101-2296

The objectives of this research was to determine the effects of raising and lowering posted speed limits on driver behavior and accidents for non-limited access rural and urban highways. Speed and accident data were collected in 22 States at 100 sites before and after speed limits were altered. Before and after data were also collected simultaneously at comparison sites where speed limits were not changed to control for the time trends. Repeated measurements were made at 14 sites to examine short - and long-term effects of speed limit changes.

The results of the study indicated that lowering posted speed limits by as much as 20 mi/h (32 km/h), or raising speed limits by as much as 15 mi/h (24 km/h) had little effect on motorist' speed. The majority of motorist did not drive 5 mi/h (8 km/h) above the posted speed limits when speed limits were raised, nor did they reduce their speed by 5 or 10 mi/h (8 or 16 km/h) when speed limits are lowered. Data collected at the study sites indicated that the majority of speed limits are posed below the average speed of traffic. Lowering speed limits below the 50th percentile does not reduce accidents, but does significantly increase driver violations of the speed limit. Conversely, raising the posted speed limits did not increase speeds or accidents.


This study was conducted to examine driver behavior and accident effects of raising and lowering posted speed limits on nonlimited access rural and urban highways. While much research in recent years has focused on the effects of the 55 and 65 mi/h (89 and 105 km/h) speed limits on limited access facilities, the major emphasis of this research is on streets and highways that were posted between 20 and 55 mi/h (32 and 89 km/h)

A maximum speed limit is posted or set by statute on a highway to inform motorists of the highest speed considered to be safe and reasonable under favorable road, traffic, and weather conditions.

A review of early vehicles speed legislation in the United States suggests that regulations were established to improve public safety. The rational for government regulation of speed is based on the fact that unreasonable speed may cause damage and injury. Speed laws also provide a basis for punishing the unreasonable behavior of an individual driver.

Every State has a basic speed statute requiring drivers to operate their vehicles at a speed that is reasonable and prudent under existing conditions. This law recognizes that the maximum safe speed varies due to traffic, roadway, weather, light and other conditions, and places the responsibility of selecting a safe and reasonable speed on the driver.

The majority of motorists select a speed to reach their destination in the shortest time possible and to avoid endangering themselves, others, and their property. In selecting their speed, motorist consider roadway, traffic, weather, and other conditions. The collective judgment of the majority of motorists represents the level of reasonable travel and acceptable risk. Prior research has shown that the upper region of acceptable risk is in the vicinity of the 85th percentile speed.

Most traffic engineers believe that speed limits should be posted to reflect the maximum speed considered to be safe and reasonable by the majority of drivers using the roadway under favorable conditions. Procedures used to set speed limits have evolved through years of experience and research. Most States and localities set safe and reasonable maximum speed limits based on the results of an engineering and traffic investigation. While all States and most jurisdictions use the 85th percentile speed as a major factor n selecting the appropriate speed limit for a given street or highway, other factors such as roadside development, accident experience, and design speed are often subjectively considered.

The lack of consensus on how to establish safe and reasonable speed limits has led to nonuniform limits. While newspapers and scientific articles dating to the early 1900's discuss the problem and need for uniform limits, engineers such as Bearwald, in 1964, criticized traffic engineers for using nonuniform limits in both rural and urban areas and called for the establishment of speed zones of a factual and scientific basis as opposed to opinion and political expediency. Bearwald's suggestion apparently received little attention. For example, Harkey recently examined speed limits in rural and urban areas in four States and found that speed limits were set from 6 to 14 mi/h (10 to 23 km.h) below the 85th percentile speed.

One primary reason for setting speed limits lower than speed considered safe and reasonable by the majority of motorists is based on the belief that lower speed limits reduced seeds and accidents. Also it has been frequently suggested that most motorists drive 5 to 10 mi/h (8 to 16 km/h) over the posted speed limit, so lower limits should be established to account for this condition.

Conversely, it is believed that raising the speed limit increases speeds and accidents. For example, following a severe accident, one of the most frequent requests made to highway jurisdictions is to lower the speed limit. These requests are founded on public knowledge that accident severity increases with increasing vehicle speed because in a collision, the amount of kinetic energy dissipated is proportional to the square of the velocity. Simply stated, when a vehicle is involved in a crash the higher the vehicle speed, the greater the chance of being seriously injured or killed. However, as noted by a number of researchers, the potential for being involved in an accident is highest when traveling at speed much lower or much higher than the majority of motorists.

Arbitrary, unrealistic and nonuniform speed limits have created a socially acceptable disregard for speed limits. Unrealistic limits increase accident risks for persons who attempt to comply with limit by driving slower or faster than the majority of road users, Unreasonably low limits significantly decrease driver compliance and give road users such as person not familiar with the road and pedestrians, a false indication of actual traffic speeds.

Unrealistically high speed limits increase accident risk for drivers who are inexperienced or who disregard the basic speed law. Unrealistic limits also place enforcement officials and judges in the position of subjectively selecting and punishing violators. This practice can result in punishing average drivers, as well as high-risk violators.

For years, traffic engineering texts have supported the conclusion that motorists ignore unreasonable speed limits. Both formal research and informal operational observations conducted for many years indicate that there is very little change in the mean or 85th percentile speed as the result of raising or lowering the posted limit. Very few accident studies have been conducted to determine the safety effects or altering posted speed limits.

Highway administrators, enforcement officials, the judiciary system, and the public need factual information concerning the effects of speed limits to address pertinent issues. For example, do lower posted speed limits reduce vehicle speeds and accidents? If the speed limit is raised, will speeds and accidents increase? Do most motorists driver 5 to 10 mi/h (8 to 16 km/h) above the posted speed limit. What are the effects or lowering and raising speed limits on driver compliance? Answers to these questions and related issues are addressed in this report.

Summary of Findings

The pertinent findings of this study, conducted to examine the effects of lowing and raising posted speed limits on nonlimited access rural and urban highways, are listed below:


[Figure 10]

The primary conclusion of this research is that the majority of motorist on the nonlimited access rural and urban highways examined in this study did not decrease or increase their speed as a result of either lowering or raising the posted speed limit by 4, 10, or 15 mi/h (8, 16, or 24 km/h). In other words, this nationwide study confirms the results of numerous other observational studies which found that the majority or motorist do not alter their speed to conform to speed limits they perceive as unreasonable for prevailing conditions.

The data clearly show that lowering posted speed limits did not reduce vehicle speeds or accidents. Also, lowering speed limits well below the 86th percentile speed did not increase speeds and accidents. Conversely, raising the posted speed limits did not increase speeds and accidents. The majority of motorist did not drive 5 to 10 mi/h (8 to 16 km/h) above the posted speed limit when speed limits were raised, nor did they reduce their speed by 5 or 10 mi/h (8 to 16 km/h) when speed limits were lowered.

Because there were few changes in the speed distribution, it is not surprising that the overall effects of speed limit changes on accidents were minor. It is interesting to note that compliance decreased when speed limits were lowered and accidents tended to increase. Conversely, when compliance improved after speed limits are raised, accidents tended to decrease.

Based on the sites examined in 22 States, it is apparent that the majority of highway agencies set speed limits below the average speed of traffic as opposed to setting limits in the upper region of the minimum accident risk band or about 85th percentile speed. This practice means that more than one-half of the motorist are in technical violation of the speed limits laws.

Although there are variations from State to State, on average, speed limits were posed 5 and 16 mi/h (8 and 26 km/h) below the 85th percentile speed. As all States use the 85th percentile as a major criterion for establishing safe and reasonable speed limits, it is surprising that the new speed limits posted on the experimental sections examined in this study deviated so far from the 85th percentile speed. There are several plausible reasons. Once commonly cited reason for posting unreasonably low speed limits is public and political pressure. While individuals and politicians clearly influence some speed limit decision, there are other factors involved.

Although the 85th percentile speed is used as the major guideline in setting speed limits, other factors such as land use, pedestrian activity, accident history, etc., are often subjectively considered in the decision making process. Together, these factors can account for sped limits that are set 10 mi/h (16 km/h) below the 85th percentile speed. In addition, the 85th percentile speed is often estimated based on a minimum of 200 vehicles or 2 h sample. This process does not take into account the wide hourly fluctuations in the 85th percentile speed over a 24-h period. Furthermore, the vehicle selection process use of radar which is detected by motorist contribute to a bias sample, i.e., usually lower then the average 24-h 85th percentile speed.

Although the study sites could not be randomly selected, they represent a wide range of rural and urban conditions, traffic volume, and regional situations. As large changes in the posted speed limit did not create a meaningful increase or decrease in the motorists' speeds at the study sites, it is plausible that this effect would also be found on other nonlimited rural and urban access highways.

The data collected during this study indicate that there are no benefits, either from a safety or operational point of view, from establishing speed limits less than the 85th percentile speed. This does not mean that all speed limits should be raised. Traffic and engineer investigations should be conducted to obtain an accurate measure of the speed distribution. Greater emphasis should be placed on using the 85th percentile speed in setting safe and reasonable speed limits. These studies should be repeated as land use and traffic characteristics change.

The information provided in this report will be useful to highway agencies, enforcement officials, and other involved in establishing uniform safe and reasonable speed limits on the nation's highways. The graphics, such as figure 10 on p.15 [above], can be used to illustrate the effects of speed limit changes on vehicle speeds. As shown below, figure 41 (which shows the changes in accidents, as well as the 95th percentile confidence limits of the changes) can be used to illustrate the effects of lowering and raising speed limits in accidents. This figure should only be used by persons who have read the accident analysis section in this report and have a basic understanding of the analysis results.

[Figure 41]

Performing Organization Name and Address:
Martin R. Parker & Associates, Inc.
38549 Laurenwood Drive
Wayne, Michigan 48184-1073

Sponsoring Agency Name and Address:
Office of Safety and Traffic Operations R&D
Federal Highway Administration
6300 Georgetown Pike
McLean, Virginia 22101-2296

Contracting Officer's Technical Representative (COTR): Howard H. Bissell, HSR-30 and Davey L. Warren, HSR-10.
Contract or Grant Number: DTFH61-85-C-00136.
Type of report and dates covered: Final, October 1985 - June 1992

The entire report is 84 pages long. You can try calling the National Technical Information Service (general info 703-487-4770, sales 703-487-4650) and asking for this report. Even with the report number they won't be able to find it. It's being buried since it says things that certain organizations (both governmental and private) don't want to be made public. However, NMA is selling this report for $15 plus $4 shipping and handaling. They can be contacted at 608/849-6000; nma@motorists.com ; or 6678 Pertzborn Road, Dane, Wisconsin 53529.

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