Updated Minimum Retroreflectivity Levels For Traffic Signs

Final Report

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Publication No: FHWA-RD-03-081

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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.tfhrc.gov/safety/safety.htm.

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

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
   
   • Demand Luminance
       Conflict Severity
       Contrast Ratio Issues
   • Supply Luminance
   • Summary
     
3. Updated Factors
   
   • Headlamps
   • Vehicle Type/Size
   • Retroreflective Sheeting Performance
   • Driver Accomodation Level
     
4. Updated MR Levels
   
   • Guide Signs
        Large Guide Signs
         Small Guide Signs
         Street-Name Signs
   • Warning Signs
   • Regulatory Signs
         Black-on White Signs
         White-on Red Signs
   • Consolidation
     
5. Assumptions and Limitations
   
   • Assumptions
         Demand Luminance
         Supply Luminance
   • Standards and Specifications
   • Measuring Retroreflectivity
         Measurement Error
         Measurement Variability
         Standardization
         Rotational Sensitivity
         Uniform Degradation
   • Future Work

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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