Methods for Maintaining Traffic Sign Retroreflectivity PUBLICATION NO. FHWA-HRT-08-026 NOVEMBER 2007 Research, Development, and Technology Turner-Fairbank Highway Research Center 6300 Georgetown Pike McLean, VA 22101-2296 FOREWORD Signs are considered essential to communicating regulatory, warning, and guidance information. It is critical that signs are able to fulfill this role during both daytime and nighttime periods. The ability of a sign to fulfill its role during nighttime periods is provided by a unique form of reflection known as “retroreflectivity.” The retroreflectivity of signs, however, degrades as the signs age in the field. A new standard in the Manual on Uniform Traffic Control Devices (MUTCD) requires that agencies maintain traffic signs to a minimum level of retroreflectivity. Various methods can be used within an agency’s sign management processes to meet and maintain a minimum retroreflectivity requirement for traffic signs. This report describes methods for maintaining traffic sign retroreflectivity that can be used by agencies to: • Systematically identify those signs that do not meet the minimum level of retroreflectivity. • Initiate activities that will upgrade signs that fall below the minimum required levels. • Monitor the retroreflectivity of in-place signs. • Create procedures that will assess the need to change practices and policies to enhance the nighttime visibility of signs. It is not appropriate to prescribe a single detailed method for all agencies to follow. The most cost effective and efficient method to maintain sign retroreflectivity will vary by agency, depending on the types of signs in service and the traffic and environmental conditions. Therefore, this report outlines several possible methods an agency can employ to maintain a minimum level of traffic sign retroreflectivity. Michael F. Trentacoste Director, Office of Safety Research and Development Notice 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 document 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. Form DOT F 1700.7 (8-72) 1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No. FHWA-HRT-08-026 4. Title and Subtitle 5. Report Date Methods for Maintaining Traffic Sign Retroreflectivity November 2007 6. Performing Organization Code 7. Author(s) 8. Performing Organization Report No. Paul J. Carlson and Matt S. Lupes 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) Texas Transportation Institute The Texas A&M University System 11. Contract or Grant No. College Station, TX 77843-3135 Project No. 474840 12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered Office of Safety Research and Development Final Report, March 2003–June 2005 Federal Highway Administration 14. Sponsoring Agency Code 6300 Georgetown Pike McLean, VA 22101-2296 15. Supplementary Notes Research performed in cooperation with the U.S. Department of Transportation and the Federal Highway Administration. Project Contracting Officer’s Technical Representative: Kenneth S. Opiela Turner-Fairbank Highway Research Center Safety Research and Development (HRDS-5) Federal Highway Administration (HSA-10) 16. Abstract In response to a Congressional directive, the FHWA has established minimum maintained traffic sign retroreflectivity levels that are incorporated into the Manual on Uniform Traffic Control Devices (MUTCD). One of the concerns expressed by agency personnel responsible for being in conformance with required minimums is the potential increase in tort exposure. The FHWA has developed retroreflectivity maintenance methods that, when implemented as intended, provide agencies with a flexible means of being in conformance with required minimum retroreflectivity levels and provide protection from potential tort claims. Other properly supported methods (i.e., through the completion of an engineering study) may be used to maintain signs at the required minimum retroreflectivity levels. Agencies can use the information in this report to help determine which retroreflectivity maintenance method or combination of methods best suits their needs. 17. Key Words 18. Distribution Statement RETROREFLECTIVITY, TRAFFIC CONTROL No restrictions. This document is available to DEVICES, TRAFFIC SIGNS, the public through: RETROREFLECTIVE SHEETING MATERIAL, National Technical Information Service SIGN MAINTENANCE, MINIMUM 5285 Port Royal Road RETROREFLECTIVITY, Springfield, VA 22161 RETROREFLECTOMET ER 19. Security Classif. 20. Security Classif. (of this page) 21. No. of Pages 22. Price (of this report) Unclassified 47 Unclassified Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 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 Celsius oC or (F-32)/1.8 ILLUMINATION fc foot-candles 10.76 lux lx fl foot-Lamberts 3.426 candela/m2 cd/m2 FORCE and PRESSURE or STRESS lbf poundforce 4.45 newtons N lbf/in2 poundforce per square inch 6.89 kilopascals kPa 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 "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 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 poundforce lbf kPa kilopascals 0.145 poundforce 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) ii TABLE OF CONTENTS CHAPTER 1. INTRODUCTION.................................................................................................1 BACKGROUND ...................................................................................................................... 1 RETROREFLECTIVE SHEETING MATERIALS AND STANDARDS .............................. 3 REPORT ORGANIZATION.................................................................................................... 4 CHAPTER 2. RETROREFLECTIVITY MAINTENANCE METHODS ...............................5 DEFINITIONS OF MAINTENANCE METHODS................................................................. 6 COMBINING MAINTENANCE METHODS......................................................................... 7 OBJECTIVES OF SIGN RETROREFLECTIVITY MAINTENANCE METHODS.............. 8 CHAPTER 3. ASSESSMENT METHODS ...............................................................................11 VISUAL NIGHTTIME INSPECTIONS ................................................................................ 11 Background....................................................................................................................... 11 Procedures......................................................................................................................... 12 Current Practices............................................................................................................... 13 Concerns ........................................................................................................................... 14 Linking Nighttime Visual Inspections to Minimum Retroreflectivity Levels.................. 14 Advantages and Disadvantages......................................................................................... 14 MEASURED SIGN RETROREFLECTIVITY...................................................................... 14 Background......................................................................................................................