Methods and Practices for Setting Speed Limits: An Informational Report FHWA Safety Program Institute of Transportation Engineers FHWA-SA-12-004 http://safety.fhwa.dot.gov Disclaimer The contents of this handbook reflect the views of the authors, who are responsible for the facts and the accuracy of the data published herein. The contents do not necessarily reflect the official view or policies of the Federal Highway Administration (FHWA). This handbook does not constitute a standard, specification, or regulation. It is not intended for construction, bidding, or permit purposes. Notice The United States Government does not endorse products or manufacturers. Trade or manufacturers’ names appear herein solely because they are considered essential to the object of this handbook. Acknowledgments Gerald J. Forbes (F) served as the ITE Project Consultant and the overall technical editor for this report. He was responsible for the technical content and developing the case studies. This report was based on a draft report, Best Practices for Setting Rational Speed Limits, prepared by Vanasse Hangen Brustlin, Inc. for the Federal Highway Administration, Office of Safety in May, 2009. Westat and Raghavan Srinivasan assisted in the development of the FHWA draft report. ITE would like to acknowledge the following individuals who supplied information or provided comments in the development of this informational report: Tony S. Abbo (M) Lawrence T. Hagen (F) Martin R. Parker Jr. (M) Richard F. Beaubien (F) Wen Hu (M) Veronica Pelkey Leanna M. Belluz (M) Kenton R. Jones Eduardo A. Petil (M) Robert Bucholc Arash Khoshghalb (M) William B. Raffensperger (M) Harry A. Campbell (F) Chris King Lawrence E. Sefcik Tom Carmody Ryan C. Kump (M) Douglas A. Skowronek (M) Christopher J. Dack (F) Greg M. Laragan (F) Harry W. Thompson (F) Chris C. Day (M) Matthew P. Lawrie (M) Blair Turner Andrew W. Edgar (M) Mark A. Madden (M) Elia Twigg James W. Ellison (F) Sean P. Merrell (M) David C. Woodin (F) John E. Fisher (F) Rock E. Miller (F) Peter J. Yauch (F) Kay Fitzpatrick (F) Craig S. Neustaedter (F) Erik H. Zandvliet (M) Jenny L. Grote (F) Michael D. Nichols (M) Abdullah J. Habibzai Kwabena Ofosu (M) ITE would like to acknowledge the ITE Traffic Engineering Council technical committee members who reviewed and provided comments on this report: Gerald J. Forbes (F), Chair Melisa D. Finley (M) Rick J. Staigle (M) Geni B. Bahar (M) Paul Mackey (M) James E. Tobaben (F) Marcus A. Brewer (M) Richard J. Porter (M) John W. Van Winkle (M) Wen Cheng (M) Keith B. Rohling (M) John A. Davis (F) Kelly I. Schmid (M) ITE would also like to acknowledge the Federal Highway Administration staff that reviewed and provided comments on this report: Guan Xu (M), FHWA Project Manager James E. (Eric) Ferron (M) Craig Allred Michael S. Griffith (M) Note: Letters in parentheses indicate ITE member grade: M—Member, F—Fellow. SI* (Modern Metric) Conversion Factors Approximate Conversions to SI Units Symbol When You Know Multiply By To Find Symbol Length in inches 25.4 millimeters mm ft feet 0.305 meters m yd yards 0.914 meters m mi miles 1.61 kilometers km Area in2 square inches 645.2 square millimeters mm2 ft2 square feet 0.093 square meters m2 yd2 square yard 0.836 square meters m2 ac acres 0.405 hectares ha mi2 square miles 2.59 square kilometers km2 Volume fl oz fluid ounces 29.57 milliliters mL gal gallons 3.785 liters L ft3 cubic feet 0.028 cubic meters m3 yd3 cubic yards 0.765 cubic meters m3 NOTE: volumes greater than 1000 L shall be shown in m3 Mass oz ounces 28.35 grams g lb pounds 0.454 kilograms kg T short tons (2000 lb) 0.907 megagrams (or “metric ton”) Mg (or “t”) Temperature (exact degrees) oF Fahrenheit 5 (F-32)/9 or (F-32)/1.8 Celsius oC Illumination fc foot-candles 10.76 lux lx fl foot-Lamberts 3.426 candela/m2 cd/m2 Force and Pressure or Stress lbf pound force 4.45 Newtons N lbf/in2 pound force per square inch 6.89 kilopascals kPa * SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. (Revised March 2003) iv Approximate Conversions from SI Units Symbol When You Know Multiply By To Find Symbol Length mm millimeters 0.039 inches in m meters 3.28 feet ft m meters 1.09 yards yd km kilometers 0.621 miles mi Area mm2 square millimeters 0.0016 square inches in2 m2 square meters 10.764 square feet ft2 m2 square meters 1.195 square yards yd2 ha hectares 2.47 acres ac km2 square kilometers 0.386 square miles mi2 Volume mL milliliters 0.034 fluid ounces fl oz L liters 0.264 gallons gal m3 cubic meters 35.314 cubic feet ft3 m3 cubic meters 1.307 cubic yards yd3 Mass g grams 0.035 ounces oz kg kilograms 2.202 pounds lb Mg (or megagrams (or “metric 1.103 short tons (2000 lb) T “t”) ton”) Temperature (exact degrees) oC Celsius 1.8C+32 Fahrenheit oF Illumination lx lux 0.0929 foot-candles fc cd/m2 candela/m2 0.2919 foot-Lamberts fl Force and Pressure or Stress N Newtons 0.225 pound force lbf kPa kilopascals 0.145 pound force per square inch lbf/in2 * SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. (Revised March 2003) v TABLE OF CONTENTS BACKGROUND ........................................................................................................................... 1 PURPOSE AND SCOPE ................................................................................................................ 2 REPORT ORGANIZATION ............................................................................................................ 3 THE SAFETY OF SPEED ................................................................................................................. 4 SPEED LIMIT BASICS .................................................................................................................... 6 Types of Speed Limits ................................................................................................................................. 6 Statutory Speed Limits ........................................................................................................................... 6 Speed Zones .......................................................................................................................................... 8 SETTING SPEED LIMITS ................................................................................................................. 9 Methods of Setting Speed Limits ............................................................................................................ 10 Engineering Approach ....................................................................................................................... 11 Operating Speed Method ............................................................................................................ 12 Road Risk Method .......................................................................................................................... 15 Expert System—USLIMITS2 ................................................................................................................... 16 Overview of the Decision Rules and Data Requirements of USLIMITS2 .................................... 17 Optimal Speeds ................................................................................................................................... 21 Injury Minimization ............................................................................................................................... 22 Minimum Length of Speed Zones ........................................................................................................... 25 Special Situations ..................................................................................................................................... 27 Advisory Speeds .................................................................................................................................. 27 Nighttime Speed Limits ....................................................................................................................... 28 School Zone Speed Limits ................................................................................................................... 28 Work Zone Regulatory Speeds ........................................................................................................... 30 Truck Speed Limits ............................................................................................................................... 30 Minimum Speed Limits ........................................................................................................................ 30 Variable Speed Limits .......................................................................................................................... 30 Transition Zone Speed Limits ............................................................................................................... 31 Seasonal or Holiday Speed Limits ...................................................................................................... 31 Reevaluation ............................................................................................................................................ 32 SPEED LIMIT SIGN DESIGN AND PLACEMENT .......................................................................... 33 Speed Feedback Signs ...........................................................................................................................