INTRODUCTION

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.[1] 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.[2] 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.[3-6]

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.[7] 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.[8] 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. [6]

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.[3-6]

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.[9] 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.[10] 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.

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