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Publication Number: FHWA-RD-03-082
Date: December 2003

Minimum Retroreflectivity Levels for Overhead Guide Signs and Street-Name Signs

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U.S. Department of Transportation

Federal Highway Administration

Research, Development, and Technology

Office of Safety Research and Development

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, Virginia 22101


Foreword

Report FHWA-RD-03-082 presents the results of a study that investigated the nighttime visibility needs of drivers for viewing overhead guide signs and street name signs. This effort reviewed past research and current practices to design a series of field tests. Field experiments were conducted with drivers 55 years and older in which the headlight illumination was incrementally increased until they could correctly read the messages on the overhead signs. The results from these experiments provided the data needed to determine threshold levels of demand luminance necessary to meet driver needs. The researchers back calculated minimum sign retroreflectivity levels using information about the supply luminance associated with the various retroreflective sign materials and amounts of illumination provided. The research indicated that some combinations of sign materials were inadequate to meet driver needs given changes in the headlight design, legibility requirements, viewing position from larger vehicles, and other factors. Tables of recommended minimum retroreflectivity levels for overhead guide and street name signs were formulated from the data gathered in the research. These tables cover a subset of signs that had not been addressed in previous research.

It is important to note that this research was initially completed before the need for updates to the other previously developed tables of minimum levels of traffic sign retroreflectivity became apparent. Subsequently, another research effort was undertaken to determine the factors needing updating and to generate new tables. Following that updating effort, the contractor for this project reanalyzed the minimum levels for overhead guide and street name signs using updated inputs for vehicle dimensions, headlight characteristics, driver age, material performance, legibility, and other factors. The revised set of the tables for minimum retroreflectivity for overhead guide and street name signs is provided in Chapter 8 of this report.

Sufficient copies of this report have been produced to allow distribution to FHWA division offices, resources centers and each state highway agency. Copies can be requested from the FHWA Office of Safety or the Office of Safety R&D. In addition, this report is available on-line through the FHWA electronic library at http://www.fhwa.dot.gov/research/tfhrc/programs/safety/index.cfm.

Michael Trentacoste

Director, Office of Safety R&D

Notice:

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

The United States Government does not endorse products or manufacturers. Trade and manufacturers’ names appear in this report because they are considered essential to the object of the document.


Technical Report Documentation Page

1. Report No.
FHWA-RD-03-082

2. Government Accession No.

3. Recipient's Catalog No.

4. Title and Subtitle
MINIMUM RETROREFLECTIVITY LEVELS FOR OVERHEAD GUIDE SIGNS AND STREET-NAME SIGNS

5. Report Date
December 2003

6. Performing Organization Code

7. Author(s)
Paul J. Carlson and H. Gene Hawkins, Jr.

8. Performing Organization Report No.

9. Performing Organization Name and Address
Texas Transportation Institute
The Texas A&M University System
College Station, TX 77843-3135

10. Work Unit No. (TRAIS)

11. Contract or Grant No.
DTFH61-99-Q-00053

12. Sponsoring Agency Name and AddressOffice of Safety Research
and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296

13. Type of Report and Period Covered

Final Report February 2000-June 2001

14. Sponsoring Agency Code

15. Supplementary Notes
Contracting Officer's Technical Representative (COTR): Ken Opiela, Office of Safety Research and Development, HRDS-05

16. Abstract

In 1993, the Federal Highway Administration (FHWA) published research recommendations for minimum retroreflectivity (MR) levels for traffic signs. The recommendations included overhead signs, but not street-name signs. In revisions to the recommended MR levels in 1998, the overhead signs were removed because of unresolved headlamp issues. Since then, there have been changes in U.S. headlamp specifications that prompted FHWA to initiate research to address MR levels for overhead guide signs and, in order to fill a somewhat related void, street-name signs. The intent of the research was to develop recommendations compatible with the 1998 revised recommendations. This report describes the research activities and consequent findings related to the development of MR levels for overhead guide signs and street-name signs.

The research included a literature review of the pertinent studies and available photometric models. This review initiated the development of an analytical model to develop MR for overhead guide signs and street-name signs. Using the findings from the literature review and a state-of-the-practice survey, an initial set of MR levels was developed. After an analysis of the initial recommendations, a field investigation was initiated to determine the minimum luminance needed to read overhead guide signs and street-name signs. Special emphasis was devoted to accommodating older drivers. Once the minimum luminance values were determined, the analytical model was used to develop a set of recommendations. The sensitivity of key factors was studied to determine the most appropriate conditions under which to establish MR levels. Once these analyses were completed and the values of the key factors were established, the MR model was executed for the final runs.

The initial results are summarized in three tables: one for overhead guide signs, one for post-mounted street-name signs, and another for overhead (mast-arm-mounted or span-wire-mounted) street-name signs. However, these tables were superseded by additional research conducted after this project was terminated. This additional research is summarized and the final recommendations, which were consolidated into one table of MR levels for all white-on-green signs, are presented.

17. Key Words
Traffic control devices, overhead signs, street-name signs, retroreflectivity, visibility, luminance.

18. Distribution Statement
No restrictions. This document is available to the public through the National Technical Information Service, Springfield, VA 22161.

19. Security Classif. (of this report)
Unclassified

20. Security Classif. (of this page)
Unclassified

21. No. of Pages
118

22. Price

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



TABLE OF CONTENTS

CHAPTER 1. INTRODUCTION

CHAPTER 7. INITIAL RECOMMENDATIONS

CHAPTER 8. FOLLOWUP RESEARCH

APPENDIX A. SURVEY OF CURRENT PRACTICES

APPENDIX B. MODELING PROCESS

REFERENCES



LIST OF FIGURES

Figure 1. Questions Included in Transportation Agency E-Mail Survey

Figure 2. Weathering Degradation of Retroreflective Sheeting

Figure 3. Overhead Sign Retroreflectivity Values

Figure 4. Layout of Overhead Sign Panel and Legend

Figure 5. Supplied Legend Luminance Graphs

Figure 6. Ford Taurus Headlamp Output

Figure 7. Control Box

Figure 8. Aiming Laser

Figure 9. Laser Location

Figure 10. Use of Laser for Aiming

Figure 11. Luminance Readings

Figure 12. Chromaticity Color Shift (CIE, 1931)

Figure 13. Closeup Chromaticity Color Shift (CIE, 1931)

Figure 14. Color Temperature Shift

Figure 15. Test Course

Figure 16. Overhead Sign

Figure 17. Street-Name Sign

Figure 18. Overhead Sign Results

Figure 19. Street-Name Sign Results

Figure 20. Results for Overhead Signs

Figure 21. Results for Street-Name Signs

Figure 22. Isocandela Plots of CARTS50 (Top) and UMTRI25PC Headlamp

Figure 23. E-mail Survey Sent to State and Local Transportation Agencies



LIST OF TABLES

Table 1. Legibility Factors.

Table 2. Types of Retroreflective Sheeting

Table 3. Sign Dimension Conversions.

Table 4. Replacement Luminance Values.

Table 5. Recommended SIA Values for Green Background Areas of Overhead Guide Signs.

Table 6. MR for White Signs.

Table 7. MR Guidelines for Signs with Green Backgrounds.

Table 8. Headlamp and Driver's Eye Height

Table 9. List of Transportation Agencies That Responded.

Table 10. State Agency Responses to Overhead Sign Questions.

Table 11. Local Agency Responses to Overhead Sign Questions.

Table 12. State Agency Responses to Street-Name Sign Questions.

Table 13. Local Agency Responses to Street-Name Sign Questions.

Table 14. Average RA of New White Sheeting.

Table 15. Test Words.

Table 16. Supplied Legend Luminance Values (cd/m2)

Table 17. Subject Information.

Table 18. Threshold Luminance Values by Accommodation Level (cd/m2)

Table 19. Replacement Luminance Values.

Table 20. Initial MR Levels for Overhead Guide Signs (50-Percent Accommodation)

Table 21. Initial MR Levels for Post-Mounted Street-Name Signs (50-Percent Accommodation)

Table 22. Initial MR Levels for Overhead Street-Name Signs (50-Percent Accommodation)

Table 23. Comparison of Headlamp Profiles for Overhead Signs.

Table 24. Comparison of Headlamp Profiles For Right-Shoulder-Mounted Signs.

Table 25. Comparison of Headlamp Profiles for Left-Shoulder-Mounted Signs.

Table 26. Roadway Illuminance Measurements (in lux)

Table 27. Comparison of Specific Vehicles.

Table 28. Illuminance Data for Left-Shoulder-Mounted Signs.

Table 29. Minimum Overhead Retroreflectivity Levels (50-Percent Accommodation)

Table 30. Vehicle Dimensions.

Table 31. Vehicle Impacts on Overhead Guide Sign MR Levels.

Table 32. Average RA of New Unweathered Sheeting.

Table 33. Overhead Guide Signs.

Table 34. Post-Mounted Street-Name Signs.

Table 35. Overhead Street-Name Signs.

Table 36. Updated Vehicle Dimensions.

Table 37. Initial MR Levels for Overhead Guide Signs(cd/lx/m2)

Table 38. Assumed Characteristics and Criteria for Street-Name Signs.

Table 39. Initial MR Levels for Street-Name Signs (cd/lx/m2)

Table 40. Research Recommendations for Updated MR Levels.

Table 41. Overhead Sign Example.

Table 42. Example MR Calculations.

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