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FHWA Highway Safety Programs

Speed Concepts: Informational Guide

Referenced by the Resource : Speed Concepts: Informational Guide
fhwasa10001.pdf (1.42 MB)

September 2009

Cover image - Speed Concepts: Informational Guide


The speed at which drivers operate their vehicles directly affects two performance measures of the highway system—mobility and safety. Higher speeds provide for lower travel times, a measure of good mobility. However, the relationship of speed to safety is not as clear cut. It is difficult to separate speed from other characteristics including the type of highway facility. Still, it is generally agreed that the risk of injuries and fatalities increases with speed. Designers of highways use a designated design speed to establish design features; operators set speed limits deemed safe for the particular type of road; but drivers select their speed based on their individual perception of safety. Quite frequently, these speed measures are not compatible and their values relative to each other can vary. This guide discusses the various speed concepts to include designated design speed, operating speed, speed limit, and a new concept of inferred design speed. It explains how they are determined and how they relate to each other.

The purpose of this publication is to help engineers, planners, and elected officials to better understand design speed and its implications in achieving desired operating speeds and setting rational speed limits.

Joseph S. Toole
Associate Administrator,
Office of Safety,
Federal Highway Administration


This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. This report does not constitute a standard, specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.


Technical Documentation Page

1. Report No. FHWA


2. Government Accession No.

3. Recipient's Catalog No.

4. Title and Subtitle

Speed Concepts: Informational Guide

5. Report Date

December 2009

6. Performing Organization Code


7. Authors

Eric T. Donnell, Ph.D., P.E; Scott C. Hines, Kevin M. Mahoney, D. Eng., P.E., Richard J. Porter, Ph.D., Hugh McGee, Ph.D., P.E.

8. Performing Organization Report No.

9. Performing Organization Name and Address

Thomas D. Larson Pennsylvania Transportation Institute
The Pennsylvania State University
201 Transportation Research Building
University Park, PA 16802

Vanasse Hangen Brustlin, Inc.
8300 Boone Boulevard
Suite 700
Vienna, Virginia 22182

10. Work Unit No.


11. Contract or Grant No.


12. Sponsoring Agency Name and Address

Office of Safety
Federal Highway Administration
U.S. Department of Transportation
1200 New Jersey Avenue, S.E.
Washington, D.C. 20590

13. Type of Report and Period Covered


14. Sponsoring Agency Code


15. Supplementary Notes:

FHWA COTR: Edward Sheldahl, Office of Safety.
Document was prepared by the Pennsylvania State University under subcontract to Vanasse Hangen Brustlin. Inc.

16. Abstract

Traffic speed is an important yet complex topic in the transportation engineering community. Furthermore, speed is of considerable interest to enforcement agencies, safety advocates, property owners, users of the transportation system, and the public at-large because of its perceived effect on crash risk. Each of these stakeholders perceives speed measures differently; therefore, many issues related to speed are either misunderstood or remain unanswered.

The objectives of this guide is to:

  • Define common speed-related terminology so that the guide’s contents can be clearly conveyed.
  • Explain the differences between designated design speed, inferred design speed, operating speed, and posted speed limits.
  • Illustrate perceptions and research conclusions related to the effects of speed.
  • Document speed-based technical processes.
  • Summarize State and local government agency roles and actions related to traffic speed.
  • Highlight speed management and mitigation measures.
The informational guide uses a combination of speed profile plots, tabular summaries, illustrative examples, and narrative discussion to describe each of the objectives noted above.

17. Key Words

Traffic speed, design speed, inferred design speed, operating speed

18. Distribution Statement


19. Security Classif. (of this report)


20. Security Classif. (of this page)


21. No. of Pages:


22. Price

Form DOT F 1700.7 (8-72) - Reproduction of completed pages authorized

SI (Modern Metric) Conversion Factors