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Publication Number: FHWA RD-03-081
Date: June 2003

Updated Minimum Retroreflectivity Levels

Final Report

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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-03-081 presents the results of a study that updated the minimum levels of traffic sign retroreflectivity for regulatory, guide, and warning signs which had been generated in 1993. The research team identified the need to update the basic input parameters for headlight illumination patterns and intensity, the effects of larger vehicles in the and the associated changes in driver eye height and headlight positions, the new legibility requirements of the MUTCD, the needs of older drivers, and the performance features of new sign materials. A new analysis tool was developed that computed retroreflectivity needs considering the relative illumination provided by each headlight for traffic signs in various positions (right-side, left-side, and overhead) relative to the roadway. Detailed tables of minimum levels of traffic sign retroreflectivity were produced to allow analysis of the sensitivity of factors such as speed, driver accommodation levels, the features of available materials, sign legend, and other factors. The detailed tables were subsequently collapsed to address AASHTO concerns about the requirements being too complicated. The end result of these efforts was that the three tables from the 1993 research and the three tables from the recent research on overhead guide and street names signs were collapsed into a single table. This single table is described in 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-03-081

2. Government Accession No.

3. Recipient's Catalog No.

4. Title and Subtitle
Updated Minimum Retroreflectivity Levels for Traffic Signs

5. Report Date
July 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
3135 TAMU
College Station, TX 77843-3135
under contract to
Batelle
505 King Avenue
Columbus, OH 43201-2639

10. Work Unit No.(TRAIS)

11. Contract or Grant No.
DTFH61-01-C-00182/
WO BA82B004

12. Sponsoring Agency Name and Address
Federal Highway Administration
Office of Safety
400 7th Street, SW
Washington, DC 20590

13. Type of Report and Period Covered

14. Sponsoring Agency Code

15. Supplementary Notes
Research conducted under subcontract to Battelle as part of a contract with the
U.S. Department of Transportation, Federal Highway Administration. Project COTR: Kenneth S. Opiela
TFHRC Safety R&D (HRDS-5)
Federal Highway Administration (HSA-10)

16. Abstract
The development of minimum retroreflectivity (MR) levels for traffic signs has been going on for more than two decades, but it was significantly accelerated in 1984 when the Center for Auto Safety petitioned the Federal Highway Administration (FHWA) to establish retroreflectivity standards. During the past decade, several sets of recommended MR levels for traffic signs have been proposed. However, these preliminary recommendations have been based on a headlamp-beam pattern that represents vehicle design from the mid- to late-1980s. Vehicle headlamps have changed significantly since then. Other significant changes also have prompted the need to update the recommended MR levels for traffic signs before FHWA initiates the rule-making process.

This report includes an updated set of recommended MR levels for traffic signs based on recent developments in vehicle headlamps, vehicle types/sizes, nighttime driver needs, and newer sheeting materials. The updated MR levels are also based on more robust computer modeling of retroreflective sheeting performance.

17. Key Words
Retroreflectivity, Traffic Control Devices, Traffic Signs

18.  Distribution Statement
No restrictions. This document is available to the public through the:
National Technical Information Service (NTIS)
5285 Port Royal Road
Springfield, VA 22161

19. Security Classif. (of this report)
Unclassified

20. Security Classif. (of this page)
Unclassified

21. No. of Pages
107

22. Price

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


SI* (Modern Metric) Conversion Factors


TABLE OF CONTENTS

Report Summary

  1. Introduction Identifying the Needs for An Update
  2. Fundamental Concepts Retroreflectivity Metric
  3. Updated Factors
  4. Updated MR Levels
  5. Assumptions and Limitations

LIST OF APPENDICES

Appendix A: AASHTO Policy Resolution

Appendix B: RAW Data From Mercier et.al.

Appendix C: Subjective Results From National MR Workshops

LIST OF FIGURES

Figure 1. Minimum Luminance Data for Warning Signs

Figures 2 a and b. Minimum Luminance Requirements

Figure 3. Scatterplot of Data from Mercier et al. and TTI

Figure 4. Comparison of Data for Older Drivers Only

Figure 5. Effect of Contrast Ratio on Legibility

Figure 6. Cumulative Percentage of Driver Population as a Function of Driver Age for Trips at Different Times of Day

Figure 7. Rotational Sensitivity of four types of Retroreflective Sheeting Materials

Figures 8 a and b. Observation Angle Profiles as a Function of Weathering

Figures 9-45. Comparison of Human Subjects' Performance Against Demand Luminance Predicted by CARTS Using 15 Different Signs

Figure 46. Results for the CURVE Sign (i.e., Bold Warning Sign)

Figure 47. Results for the DIVIDED HIGHWAY ENDS Sign (i.e., Fine Warning Sign)

Figure 48. Results for STOP Sign (i.e., White-on-Red Iconic Sign)

LIST OF TABLES

Table 1. Research Recommendations for Updated MR Levels

Table 2. Threshold Luminance Values by Accommodation Level (cd/m2)

Table 3. Replacement Luminance Values

Table 4. Minimum Internal Contrast Criteria

Table 5. Headlamp Descriptions

Table 6. Vehicle Dimensions for MR Calculations

Table 7. Initial MR Levels for Large Guide Signs

Table 8. Initial MR Levels for Small Guide Signs

Table 9. Assumed Characteristics and Criteria for Street Name Signs

Table 10. Initial MR Levels for Street Name Signs

Table 11. Inventory of Standard Size Warning Signs

Table 12. Criteria for Text-Based Warning Signs

Table 13. Initial Retroreflectivity Levels for Text Warning Signs

Table 14. Initial Retroreflectivity Levels for Symbol Warning Signs

Table 15. Criteria for SPEED LIMIT Signs

Table 16. Initial Retroreflectivity Levels for SPEED LIMIT Signs

Table 17. Criteria for KEEP RIGHT Signs

Table 18. Criteria for ONE WAY Signs

Table 19. Criteria for NO RIGHT TURN Signs

Table 20. Initial MR Levels for KEEP RIGHT Signs

Table 21. Initial MR Levels for ONE WAY Signs

Table 22. Initial MR Levels for NO RIGHT TURN Signs

Table 23. Criteria for STOP Signs

Table 24. Initial MR Levels for STOP Signs

Table 25. Criteria for DO NOT ENTER Signs

Table 26. Initial Retroreflectivity Levels for DO NOT ENTER Signs

Table 27. Updated MR Levels for Traffic Signs

Table 28. Summary of Variability Across Sign Faces

Table 29. Rotational Sensitivity of Unweathered Materials

Table 30. Rotational Sensitivity of Weathered Materials

Table 31. Description of NTPEP Panels Measured

Table 32. Description of Signs Used in Visibility Study

Table 33. Description of Signs Used in Nighttime Evaluations

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