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Speed Management

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Speed Management Safety

CHAPTER 8. SUMMARY

Following are some of the key points included in this Informational Guide:

CHAPTER 7. MANAGING TRAFFIC SPEED

Traffic speeds involve a complex set of interactions between engineering, legal and driver performance factors. Currently, knowledge of speed behavior is limited. Although the research literature contains a variety of speed prediction models for rural two-lane highways and low-speed urban streets, accurate speed prediction models for other road and street types is limited. As such, the ability to accurately predict speeds on all road and street types does not exist.

CHAPTER 6. GOVERNMENT ROLES RELATED TO TRAFFIC SPEEDS

Government agencies, mostly at the state, county and local levels, own and operate approximately four million miles of roads and streets in the U.S. Through road ownership and construction, statutes, traffic regulation and enforcement, government agencies create the physical and legal driving environment. Not all drivers respond in the same way to the same driving environment and these differences extend to speed selection. An understanding of speed issues requires some basic insights of agency functions and human behavior in a regulated environment.

CHAPTER 4. ENGINEERING AND TECHNICAL CONCEPTS

A number of technical and engineering techniques are closely associated with traffic speeds. The designed driving environment consists primarily of infrastructure and traffic control devices. The geometric design process is used to define the location and dimensions of road and street infrastructure which consists of the horizontal and vertical alignment, cross section features, intersection type and all the associated details. Traffic control devices are used to regulate, warn and guide drivers through the use of signs, traffic signals, pavement markings and other devices.

CHAPTER 3. CONSEQUENCES OF SPEED

Traffic speeds are relevant and of interest to nearly everyone. Our preferences and judgments of appropriate speed are strongly influenced by setting and perspective. The speed we prefer as a driver or passenger traveling long-distance along a rural Interstate highway is much different than what we prefer as a pedestrian crossing an urban street. Context is everything.

CHAPTER 2. TERMINOLOGY AND NOTATION

The following terms and abbreviations are commonly used in speed literature and discussions. Other references may use or define these terms somewhat differently than as defined below and used in this publication.

15th percentile speed – the speed at or below which 15 percent of vehicles travel. Also, see Speed distribution.

85th percentile speed – the speed at or below which 85 percent of vehicles travel. Also, see Speed distribution.

CHAPTER 1. INTRODUCTION AND PURPOSE

Traffic speed is a subject of considerable interest and importance to transportation and traffic enforcement agencies, safety advocates, motorists, non-motorist street and highway users (e.g., pedestrians and bicyclists), residential and commercial property occupants and the public at-large. Speed is a controversial and complex subject. Even after significant research, many questions remain unanswered about speed relationships.