Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Methods for Maintaining 

Traffic Sign Retroreflectivity 

Publication No. FHWA-HRT-08-026  NOVEMB
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. 

FHWA-HRT-08-026 

 2. Government Accession No. 3. Recipient’s Catalog No. 

 5. Report Date 

November 2007 

 4. Title and Subtitle 

Methods for Maintaining Traffic Sign Retroreflectivity 

 6. Performing Organization Code 

 7. Author(s) 

Paul J. Carlson and Matt S. Lupes 

 8. Performing Organization Report No. 

 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

Texas Transportation Institute 

The Texas A&M University System 

College Station, TX 77843-3135 

11. Contract or Grant No. 

Project No. 474840 

13. Type of Report and Period Covered 

Final Report, March 2003–June 2005 

12. Sponsoring Agency Name and Address 

Office of Safety Research and Development 

Federal Highway Administration 

6300 Georgetown Pike 

McLean, VA 22101-2296 

14. Sponsoring Agency Code 

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 

RETROREFLECTIVITY, TRAFFIC CONTROL 

DEVICES, TRAFFIC SIGNS, 

RETROREFLECTIVE SHEETING MATERIAL, 

SIGN MAINTENANCE, MINIMUM 

RETROREFLECTIVITY, 

RETROREFLECTOME ER T

18. Distribution Statement 

No restrictions. This document is available to 

the public through: 

National Technical Information Service 

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 

47 

22. Price 

 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) o

F 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) o

C 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 th International System of Units. Appr e opriate 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....................................................................................................................... 15 

Procedures......................................................................................................................... 15 

Hand-Held Contact Instruments ....................................................................................... 16 

Differences in Point and Annular Instrument Measurements........................................... 17 

Current Practice ................................................................................................................ 17 

Concerns ........................................................................................................................... 17 

Linking Measurements to Minimum Retroreflectivity Levels......................................... 18 

Advantages and Disadvantages......................................................................................... 18 

CHAPTER 4. MANAGEMENT METHODS ...........................................................................19 

EXPECTED SIGN LIFE ........................................................................................................ 19 

Procedures......................................................................................................................... 19 

Current Practice ................................................................................................................ 22 

Concerns ........................................................................................................................... 22 

Linking Expected Sign Life to Minimum Retroreflectivity Levels.................................. 23 

Advantages and Disadvantages......................................................................................... 23 

BLANKET REPLACEMENT................................................................................................ 24 

Procedures......................................................................................................................... 24 

Current Practice ................................................................................................................ 24 

Concerns ........................................................................................................................... 25 

Linking Blanket Replacement to Minimum Retroreflectivity Levels .............................. 25 

Advantages & Disadvantages........................................................................................... 25 

CONTROL SIGNS ................................................................................................................. 25 

Procedures......................................................................................................................... 26 

Current Practice ................................................................................................................ 26 

Concerns ........................................................................................................................... 26 

Linking Control Signs to Minimum Retroreflectivity Levels........................................... 26 

Advantages and Disadvantages......................................................................................... 27 

iii 
iv 

APPENDIX A. ESTABLISHMENT OF MINIMUM MAINTAINED TRAFFIC SIGN 

RETROREFLECTIVITY LEVELS.................................................................................29

HISTORICAL BACKGROUND............................................................................................ 29 

TECHNICAL BACKGROUND ON RETROREFLECTIVITY LEVELS............................ 31 

RECOMMENDED MINIMUM RETROREFLECTIVITY LEVELS................................... 34 

KEY ASSUMPTIONS............................................................................................................ 36 

REFERENCES.............................................................................................................................37
LIST OF FIGURES

Figure 1. Annular and Point Aperture Instrument Angles........................................................... 16 

Figure 2. Upright Optical Schematics.......................................................................................... 17 

Figure 3. Examples of Wyoming DOT Signs.............................................................................. 21 

Figure 4. Example of Minnesota DOT Sign Sticker.................................................................... 22 

LIST OF TABLES 

Table 1. ASTM Retroreflective Sheeting Type Descriptions........................................................ 3 

Table 2. Outline of Methods for Maintaining Minimum Retroreflectivity of Traffic Signs. ........ 4 

Table 3. Retroreflectivity Maintanence Methods. ......................................................................... 7 

Table 4. Headlamp Aiming Procedure......................................................................................... 13 

Table 5. Typical Warranty Life. .................................................................................................. 23 

Table A1. Minimum Maintained Traffic Sign Retroreflectivity Levels...................................... 34 

v 

CHAPTER 1. INTRODUCTION 

BACKGROUND 

One of the Federal Highway Administration’s (FHWA) primary goals is to improve safety on the 

nation’s streets and highways.(1) Approximately 42,000 people have been killed on U.S. roads 

each year for the last eight years.(2)

 While only a quarter of travel occurs at night,(3) about onehalf of the traffic fatalities occur during nighttime hours.(2)

 This translates to a nighttime fatality 

rate that is approximately three times greater than that of daytime. There are many reasons for 

this disparity, and no one factor can be singled out. It is, however, reasonable to expect that 

critical traffic signs be visible to drivers at night to facilitate night driving. 

Maintaining traffic sign retroreflectivity is consistent with the FHWA’s goal of improving safety 

on the nation’s streets and highways. Safety and operational strategies are dependent on sign 

visibility that meets the needs of drivers. The FHWA expects that improvements to nighttime 

visibility of traffic signs will help drivers better navigate the roads at night and thus promote 

safety and mobility. Improvements in sign visibility will also support the FHWA’s efforts to be 

responsive to the needs of older drivers whose visual capabilities are declining. This is important 

because the number of older drivers is expected to increase significantly in the coming years. 

Currently, 26.2 million drivers are 65 or older, and by 2010 an estimated 33.7 million drivers 

will be 65 or older.(4)

 

The opening statements of the Manual on Uniform Traffic Control Devices (MUTCD) in Section 

1A.01 define the purpose of traffic control devices and the principles for their use to be the 

promotion of highway safety and efficiency by providing for the orderly movement of all road 

users.(5,6,7)

 Those devices notify road users of regulations, provide warning, and give guidance 

needed for the safe, uniform, and efficient operation of all elements of the traffic stream. (Note: 

The MUTCD is incorporated by reference in 23 CFR 655.601. It is available as prescribed in 49 

CFR Part 7 and on the FHWA’s Web site at http://mutcd.fhwa.dot.gov.) Requirements for 

nighttime sign visibility have been included in every version of the MUTCD since the first 

edition in 1935. The latest edition of the MUTCD, the 2003 edition, continues to address the 

visibility of signs. Three pertinent sections include: Section 2A.09 Maintaining Minimum 

Retroreflectivity, Section 2A.08 Retroreflectivity and Illumination, which states, “[r]egulatory, 

warning, and guide signs shall be retroreflective or illuminated to show the same shape and 

similar color by both day and night, unless specifically stated otherwise in the text discussion in 

this Manual of a particular sign or group of signs,” and Section 2A.22 Maintenance, which 

addresses maintenance of traffic signs. 

These MUTCD provisions have tasked each agency with actively managing its traffic signs 

according to the MUTCD guidance and ensuring that its traffic signs are performing as they are 

intended. It is generally believed that maintaining the daytime performance of traffic signs (i.e., 

placement, clarity of message, adequate sight lines, redundancy, and color) is more easily 

accomplished than maintaining the nighttime performance. Nighttime performance of traffic 

signs can be more difficult to maintain for a variety of reasons. One of the primary differences 

between daytime and nighttime sign performance is a material property called retroreflection. 

Retroreflection is a special type of reflection that redirects incident light (i.e., from headlights) 

1 
back toward the source. In the case of highway application, traffic signs are made with 

retroreflective sign sheeting material that redirects headlamp illumination back toward the 

vehicle, thereby making the sign visible at nighttime to the vehicle driver. The specific 

measurement of retroreflection that is of interest is the “coefficient of retroreflectance,” 

abbreviated as RA. The FHWA has adopted the SI units for retroreflection; thus RA is measured 

in units of candelas per lux per square meter (cd/lx/m2). When discussed in quantitative terms, 

the coefficient of retroreflection is commonly referred to as retroreflectivity. Throughout this 

document, the term retroreflectivity will be understood to mean the coefficient of 

retroreflectivity (RA), unless otherwise stated. 

The nighttime visibility of traffic signs that is provided through retroreflective sign sheeting 

materials is difficult to assess during daytime conditions using visual inspection methods. 

Furthermore, the retroreflective properties of all sign sheeting materials degrade over time 

making signs progressively less visible (i.e., less bright) at night. Environmental conditions, such 

as UV-radiation from the sun, moisture, and pollutants cause a substantial amount of the 

deterioration in retroreflective performance. However, loss of retroreflectivity can also occur due 

to vandalism, such as paint ball shots, gunshots, and spray paint. A good overall set of guidelines 

for the general maintenance of traffic signs is provided by the FHWA at 

http://www.fhwa.dot.gov/tfhrc/safety/pubs/90002/intro.htm. 

As signs degrade and become less retroreflective, their effectiveness in communicating 

regulatory, warning, and guidance messages to road users at nighttime diminishes to the point 

that they cannot be seen or read in time for the driver to react properly. Thus, to maintain 

nighttime effectiveness, signs must be replaced before they reach the end of their useful 

retroreflective life. Until recently, little information was available about the levels of 

retroreflectivity necessary to meet the needs of drivers and thereby define the useful life of 

signs.(8)

 Research has led to the development of recommended minimum maintained levels of 

traffic sign retroreflectivity for regulatory, warning, and guide signs for currently available 

materials, vehicle fleet characteristics, and capabilities of the driving population. 

The FHWA developed minimum maintained traffic sign retroreflectivity levels in response to a 

Congressional directive in the Department of Transportation and Related Agencies 

Appropriations Act, 1993 (Public Law 102-388; October 6, 1992). Section 406 of this Act 

directed the Secretary of Transportation to revise the MUTCD to include a standard for 

minimum levels of retroreflectivity that must be maintained for traffic signs and pavement 

markings, which apply to all roads open to public travel. As part of the FHWA’s plan to meet the 

Congressional directive described above, the FHWA has outlined methods that agencies can 

implement to maintain minimum traffic sign retroreflectivity levels in conformance with the 

MUTCD requirements. Furthermore, changes to Section 2A.22 of the MUTCD will clarify 

traffic sign maintenance criteria. As a result of rulemaking, agencies will need to implement sign 

maintenance methods that incorporate the consideration of minimum retroreflectivity levels to 

provide for nighttime visibility of signs. This document provides general information on methods 

for maintaining minimum traffic sign retroreflectivity levels. 

2 
RETROREFLECTIVE SHEETING MATERIALS AND STANDARDS 

Agencies updating their policies regarding retroreflective sheeting materials for traffic signs find 

that although there are more materials available today than ever before, there is very little 

guidance on the effective use of these materials for specific applications or on the adequacy of 

the materials for conditions within an agency’s jurisdiction. 

Many agencies look to the American Society for Testing and Materials (ASTM) specification 

D4956, Standard Specification for Retroreflective Sheeting for Traffic Control, for information 

concerning the most applicable use of certain kinds of materials.(9)

 D4956, however, only 

provides very general descriptions of the materials, which are grouped into “Types” based on a 

limited set of attributes. Table 1 provides the descriptions of the types of materials listed in the 

current version of ASTM Specification D4956-05. 

Table 1. ASTM Retroreflective Sheeting Type Descriptions. 

ASTM 

Type 

ASTM 

Description 

Typical 

Construction 

Suggested 

Use Typical Applications 

I Medium 

intensity enclosed lens none provided 

permanent highway signing, 

construction zone devices, and 

delineators 

II Medium highintensity enclosed lens none provided 

permanent highway signing, 

construction zone devices, and 

delineators 

III High-intensity encapsulated glass 

beads none provided 

permanent highway signing, 

construction zone devices, and 

delineators 

IV High-intensity microprismatic none provided 

permanent highway signing, 

construction zone devices, and 

delineators 

V High-intensity metallized 

microprismatic none provided delineators 

VI Elastomeric 

high-intensity 

vinyl 

microprismatic none provided 

orange temporary roll-up warning 

signs, traffic cone collars, and post 

bands 

VII Super-highintensity microprismatic medium and long 

road distances 

permanent highway signing, 

construction zone devices, and 

delineators 

VIII Super-highintensity microprismatic medium and long 

road distances 

permanent highway signing, 

construction zone devices, and 

delineators 

IX Very-highintensity microprismatic short road 

distances 

permanent highway signing, 

construction zone devices, and 

delineators 

X Super-highintensity microprismatic medium road 

distances 

permanent highway signing, 

construction zone devices, and 

delineators 

The FHWA Retroreflective Sheeting Identification Guide–September 2005, which lists the 

materials sold under each type designation as of the date of the guide, can be accessed at 

http://safety.fhwa.dot.gov/roadway_dept/docs/retrore_sheet_id.pdf. 

3 
4 

REPORT ORGANIZATION 

The FHWA has outlined maintenance methods that are intended to provide agencies with a 

flexible means of conformance with the MUTCD requirements for minimum retroreflectivity of 

traffic signs and provide protection from potential tort claims. The purpose of this report is to 

describe the methods shown in table 2 that can be used to maintain minimum retroreflectivity 

levels. 

Table 2. Outline of Methods for Maintaining Minimum Retroreflectivity of Traffic Signs. 

Chapter Method Description Advantages Disadvantages 

2 Combination of 

Methods or Other 

Method 

Agency blends 

different methods or 

adopts customized 

method (based on 

engineering study). 

Customized method by 

agency to achieve 

effectiveness and 

efficiency. 

Potentially labor and 

time intensive 

depending on level of 

engineering study. 

3 Nighttime Visual 

Inspection 

Assessment 

conducted according 

to procedure by 

trained inspector. 

Less time consuming 

and the overall 

appearance of signs are 

evaluated. 

Subjective and overtime 

pay for late-evening 

labor. 

3 Measured Sign 

Retroreflectivity 

Signs are measured 

with an instrument 

according to 

procedure. 

Direct measurement 

without subjectivity and 

objectively evaluate 

questionable signs 

Time consuming and 

unable to evaluate other 

factors affecting sign’s 

appearance. 

4 Expected Sign Life Signs replaced based 

on age, warranty, or 

degradation of sign 

sheeting. 

Develop local expected 

service life and easily 

implemented. 

Sign sheeting type and 

expected life needs to 

be known and tracking 

installation date of sign. 

4 Blanket 

Replacement 

Replacement of all 

signs at specified 

intervals based on 

shortest life of 

material used. 

Effectively replaces all 

signs at once. 

Potential waste of 

relatively new signs. 

4 Control Signs Replacement of signs 

based on a sample set 

of control signs. 

Less labor intensive 

and develops local sign 

life expectancy. 

Control sign sample set 

must be representative 

and monitored. 
CHAPTER 2. RETROREFLECTIVITY MAINTENANCE METHODS 

Traditionally, it has been up to Federal, State, and local agencies to manage and maintain their 

traffic signs in accordance with the MUTCD standards. As a result, agencies have implemented 

different methods to manage traffic signs that reflect local conditions, needs, and priorities. The 

management process begins with agency policies and practices regarding the use of 

retroreflective materials in the fabrication of new signs. Agency policies have often been driven 

by the costs of the various retroreflective materials. Once new signs have been deployed, there 

has been less attention paid to the adequacy of the retroreflectivity provided by an individual 

sign. By and large, the most common method used to trigger maintenance of traffic sign 

retroreflectivity has been visual inspection. However, other methods have been tested and 

implemented including measurement of retroreflectivity and scheduled replacements based upon 

sign age. 

The establishment of minimum maintained traffic sign retroreflectivity levels in the MUTCD 

requires that agencies adopt one or more acceptable methods. This provision was intended to 

assure that agencies use methods that will be effective in maintaining nighttime visibility for 

their deployed traffic signs. This minimum standard has raised concerns among State and local 

agencies. 

One of the main concerns is associated with the potential increase in tort exposure once 

numerical values are tied to minimum retroreflectivity levels for traffic signs. In 1998, a national 

survey was conducted asking if agencies expected “an increase in tort claim lawsuits as a result 

of the minimum retroreflectivity values.”(10)

 Two-thirds of the respondents replied that their 

agencies did expect an increase in tort claim lawsuits if minimum retroreflectivity levels for 

traffic signs were implemented. The survey respondents claimed that “whether the 

retroreflectivity contributed to the accident or not, the lawyers will be aware of the minimum 

values and will use them against the State.” 

In order to minimize the risk to an agency of being found negligent in meeting the requirements 

for minimum traffic sign retroreflectivity, a sign maintenance program must be provided in order 

to ensure the nighttime visibility of signs. This approach has been effective in related tort claims 

against agencies. Conducting and maintaining an inventory of devices, replacing devices at the 

end of their effective lives, knowing the laws relating to traffic control devices, and applying 

State traffic control device specifications and standards are four basic principles suggested by the 

ITE Traffic Sign Handbook to “significantly reduce tort liability lawsuits involving traffic control 

devices.”(11)

 It follows that sign maintenance methods need to be developed and implemented to 

provide protection from tort liability. 

There have also been concerns that the implementation of new methods would impose new 

burdens on agencies. It was noted that the MUTCD should provide flexibility for agencies in 

terms of complying with minimum maintained traffic sign retroreflectivity levels. 

This need for maintenance methods was cited often during the FHWA sponsored workshops on 

minimum levels of in-service retroreflectivity for signs in the summer of 2002.(12)

 These 

workshops were conducted to present the most recent research findings on minimum levels of 

5 
retroreflectivity and to solicit input from public agency officials prior to developing a proposed 

rule on minimum levels of retroreflectivity. A total of 99 individuals participated in the four 

invitation-only workshops. One of the most consistent key findings of the workshops was that 

the public agency participants wanted the MUTCD to provide several methods that could be used 

to meet the minimum retroreflectivity requirements. 

DEFINITIONS OF MAINTENANCE METHODS 

A significant portion of the 2002 FHWA workshops on nighttime traffic sign visibility was 

devoted to discussions of options that are available to agencies to improve the nighttime 

visibility of signs. Several different terms were used to describe the options, including evaluation 

methods, assessment procedures, implementation options, and management processes. All of the 

terms were intended to describe actions that an agency can take to provide a reasonable level of 

nighttime sign visibility to road users. 

There was essentially unanimous agreement among the public sector participants that the 

MUTCD should not dictate the methods or processes to be used to determine whether signs meet 

the goal of reasonable nighttime visibility. Instead, the MUTCD should describe various 

evaluation methods that agencies can choose from to provide reasonable nighttime sign 

visibility. The ability to choose from several options will allow agencies to adopt a method that 

best fits the resources and current practices of individual agencies. 

Different methods were presented and discussed during the various workshops. Most methods 

can be divided into one of two categories—evaluation or management methods. Evaluation 

methods involve some type of assessment of the nighttime visibility of individual signs (e.g., 

visual inspection or retroreflectivity measurement). Management methods are based on the 

expected retroreflective life of the overall sign inventory, based on factors such as warranties, 

demonstrated performance, or control sign assessments. The following accepted methods are 

described in greater detail in this report. 

• Nighttime Visual Inspection. The retroreflectivity of an existing sign is assessed by a 

trained sign inspector following a formal visual inspection procedure from a moving 

vehicle during nighttime conditions. Signs that are visually identified by the inspector to 

have retroreflectivity below the minimum levels should be replaced. 

• Measured Sign Retroreflectivity. Sign retroreflectivity is measured using a 

retroreflectometer. Signs with retroreflectivity below the minimum levels should be 

replaced. 

• Expected Sign Life. The installation date is labeled or recorded when a sign is installed, 

so that the age of any given sign is known. The age of the sign is compared to the 

expected sign life. The expected sign life is based on the experience of sign 

retroreflectivity degradation in a geographic area. Signs older than the expected life 

should be replaced. 

• Blanket Replacement. All signs in an area/corridor or of a given type are replaced at 

specified intervals. This eliminates the need to assess retroreflectivity or track the life of 

individual signs. The replacement interval is based on the expected sign life for the 

shortest-life material used in the area/corridor or on a given sign type. 

6 
• Control Signs. Replacement of signs in the field is based on the performance of a sample 

set of signs. The control signs might be a small sample located in a maintenance yard or a 

selection of signs in the field. The control signs are monitored to determine the end of 

retroreflective life for the associated signs. All signs represented by a specific set of 

control signs should be replaced before the retroreflectivity levels of the control signs 

reach the minimum retroreflectivity levels. 

Other methods (e.g., sample comparisons, Q-beam methods (illuminating a sign with a spot light 

during a daytime visual inspection), and visual inspection techniques tied to evaluation distances 

dependent on sign type and size) were identified in the workshops but were not considered 

practical or effective in determining the adequacy of nighttime visibility. Workshop participants 

also indicated that a sign management system could also be used as one of the evaluation 

methods. However, an evaluation method is a tool that supports a sign management system. A 

sign management system does not provide a means for evaluating nighttime sign visibility; it 

provides a means of managing information from one or more evaluation systems used to predict 

when a sign should be replaced. 

The MUTCD minimum retroreflectivity requirements are not intended to imply that agencies 

must measure the retroreflectivity of every sign in their jurisdictions. The various maintenance 

methods provide agencies with options that will improve nighttime sign visibility. 

The sign retroreflectivity maintenance methods described above are divided into two groups, 

assessment methods and management methods, as noted in table 3. Agencies have flexibility to 

adapt these methods for maintaining sign retroreflectivity into existing sign management 

processes or may upgrade their sign management process by incorporating an approved 

maintenance method. 

Table 3. Retroreflectivity Maintanence Methods. 

Assessment Methods Management Methods 

Nighttime Visual Inspections 

Retroreflectivity Measurements 

Expected Sign Life 

Blanket Replacement 

Control Signs 

COMBINING MAINTENANCE METHODS 

Combinations of two or more methods may be viable for some agencies. In addition, agencies 

are not limited to the proposed maintenance methods. Agencies may develop their own methods 

using documented engineering studies that demonstrate that deviations are appropriate. 

Agencies may combine different methods or parts of different methods to achieve sign 

retroreflectivity maintenance practices that best fit the agency’s needs and budget. Generally, a 

combination method would include a management method complemented with an assessment 

method used to provide supplemental data. This method provides a means to track individual 

signs but without the need to inspect or measure every sign. Any number of combinations can be 

implemented to logically integrate with other aspects of the sign management process and best fit 

7 
an agency’s limited resources. Also note that the proposed methods can be used exclusively with 

effective results. 

One possible combination is the use of a management method with both daytime and nighttime 

visual inspections. The expected life of a sign is a management method and is based on the age 

and degradation of the sheeting types used. This management method in combination with 

daytime visual inspections may allow an agency to track how many signs they have, how old 

they are, and where they are located. It also provides field crews with a list or summary of 

deployed signs that can be easily used to note the need for sign replacements or repairs when 

conducting nighttime visual inspections. The information may be downloaded to electronic 

devices (e.g., laptop computers or PDAs) to further facilitate field inspections and documentation 

of sign conditions and replacement needs. Combining the expected sign life management method 

with both daytime and nighttime visual inspections is one example of adapting methods that 

meet an agency’s needs. 

Another possibility is to combine expected sign life with measured retroreflectivity. Under this 

method, it is not required to measure the retroreflectivity of all signs. Measurement of a small 

sample from across a region allows the agency to compare the expected and measured 

retroreflectivity. The measurements allow the agency to validate, and revise if necessary, the 

service life of each sign sheeting material and color used by the agency. 

In summary, these methods can be used in different ways but will provide a consistent evaluation 

of the nighttime visibility of in-place traffic signs. Additional details on these methods and their 

applications are provided in the following chapters. 

OBJECTIVES OF SIGN RETROREFLECTIVITY MAINTENANCE METHODS 

The intent of the methods is to provide a systematic means for agencies to maintain traffic sign 

retroreflectivity at or above the minimum levels. The FHWA has determined that agencies that 

use an approved method to maintain traffic sign retroreflectivity are in conformance with the 

minimum maintained retroreflectivity requirements established in the MUTCD. 

Substantial conformance with the MUTCD Section 2A.09 is achieved by having a method in 

place to maintain the minimum retroreflectivity levels. Conformance does not require or 

guarantee that every individual sign will meet or exceed the minimum retroreflectivity levels at 

every point in time. For example, if an agency chooses to implement the visual nighttime 

inspection method, there is no guarantee that the retroreflectivity of all signs will be satisfied 

during the entire period that the signs are in service. Assuming that an agency successfully 

completes the annual visual nighttime inspections and that signs failing the subjective evaluation 

or signs rated as marginal are scheduled for replacement or reassessment within a reasonable 

time period, then there is clearly a period of time when these signs might be below the minimum 

retroreflectivity levels while the sign is awaiting replacement or reassessment. Having a method 

in place to maintain the minimum retroreflectivity levels is a valuable way for agencies to 

prioritize how to spend limited resources on those signs that should be replaced sooner, 

ultimately contributing to improved safety for the motoring public. 

8 
9 

t 

There are other conditions where signs might be rated as being satisfactory while temporarily 

falling below the minimum retroreflectivity levels. For example, dew and frost on signs have 

been shown to significantly reduce retroreflectivity.(13)

 In addition, while research has shown 

that the visual nighttime inspection is a reasonable method in terms of identifying signs that need

to be replaced because of marginal retroreflectivity, the nighttime visual inspection method is no

100 percent reliable.(14,15)

 When sign inventories are not available for use during visual 

inspections, it is not unreasonable to miss a small percentage of signs along a densely-signed 

corridor, especially if a sign was knocked down or missing for some other reason at the time of 

the inspection. It is also possible that a sign or a group of signs meets the retroreflectivity 

minimums for a predetermined number of years, but because of factors such as manufacturing 

defects or inadvertent mishandling during installation, a certain percentage might fall below the 

minimum retroreflectivity levels sooner than expected. 

Regardless of which maintenance method is adopted by an agency, documentation of the sign 

management process is important in assisting agencies to achieve conformance with the MUTCD 

standard to maintain minimum retroreflectivity levels of traffic signs. Written procedures ensure 

that agency personnel properly follow the selected method, while maintenance records provide 

the agency with a systematic process for sign replacements and justification for the allocation of 

limited resources. As an example, measurements of traffic sign retroreflectivity might show that 

certain signs are near or below the thresholds in the table of minimum retroreflectivity levels 

even before they reach the end of their expected life. The records provide documentation that an 

appropriate maintenance method was followed and permit the agency to assess and revise, if 

necessary, the method for a given type or group of signs. As long as an agency has a reasonable 

method in place to manage or assess its signs and establishes a reasonable schedule for sign 

replacement as needed, the agency will be deemed to be in conformance. 

CHAPTER 3. ASSESSMENT METHODS 

This chapter contains information about the evaluation or assessment methods for maintaining 

traffic sign retroreflectivity. The methods described in this chapter include 

• Nighttime Visual Inspections. 

• Measuring Traffic Sign Retroreflectivity. 

The basic concept of an assessment method is that the condition of each individual sign is 

assessed or evaluated on a periodic basis. The MUTCD does not set specific intervals, but many 

agencies currently assess their signs every one to two years. 

VISUAL NIGHTTIME INSPECTIONS 

Visual inspections are perceived to be the most likely means to find nighttime visibility problems 

with signs. Using this approach, it is possible to assess more than just the retroreflectivity of a 

sign. Damage, obstructions, poor placement, and other factors that might detract from the 

nighttime visibility of the sign can be observed. The MUTCD currently includes language that 

encourages agencies to undertake periodic daytime and nighttime visual inspections. Many 

agencies already perform some type of periodic sign inspection, although not all inspections are 

performed at nighttime. This method requires a minimal investment of resources on the part of 

the agency, although there is a need for a record-keeping system for inspection data and the 

potential for higher labor costs where overtime pay is required. While visual inspections will 

reveal night visibility problems not discernable under any other method, they are subjective and 

hence more difficult to tie to a benchmark value of retroreflectivity. Agencies using visual 

inspections must establish procedures to provide consistency in inspections. This implies the 

need for training programs and certification of inspectors to assure consistency of inspections. 

Inspection procedures should address the type of vehicle used, type of headlamps on the 

inspection vehicle, headlamp aiming, and age and visual acuity of the inspector(s). 

Background 

Probably the most common type of sign maintenance program is the visual inspection method. 

Guidelines have been available for at least 50 years concerning the details of how to conduct a 

proper nighttime sign inspection.(16,17)

 While there are some concerns about the reliability of the 

visual nighttime inspection, research has shown that trained inspectors can do a reasonable job of 

determining which signs need to be replaced because of inadequate retroreflectivity.(14,15)

 

The visual inspection technique uses trained personnel to observe traffic signs during the 

nighttime to assess the overall appearance of a sign and determine if it meets the required 

minimum retroreflectivity level. The observation is typically done through the windshield of the 

vehicle at or near the speed limit of the roadway. 

The key to this method is having trained inspectors. While there is no nationally-recognized 

training course or certification for sign inspectors, agencies should provide some form of training 

before sign inspections are performed. One way to perform the training is to have the inspectors 

11 
12 

observe sample signs at a variety of known retroreflectivity levels before conducting the 

inspections. Training helps facilitate an inspector’s ability to discern sign retroreflectivity levels 

that are at the minimum levels prior to conducting inspections. Preferably, there should be 

sample signs that are at or near the minimum retroreflectivity levels associated with each sign 

type and color. The inspector should view the sample signs under similar conditions to those 

under which inspections will be performed. This includes using the appropriate vehicle and 

placing the sample signs at typical positions that will be encountered during an inspection. For 

this method to be effective, the training must prepare the inspector in advance, using correct 

sample signs that represent retroreflectivity levels at or near the MUTCD minimum 

retroreflectivity levels. 

Procedures 

The usual method of inspecting signs at night is to use a two-person crew. While the driver 

focuses on the driving task, the passenger evaluates the signs and records the appropriate 

information. An alternative to a two-person crew is to use one person with a tape recorder or 

camcorder. If an inventory is available, signs that have been knocked down or missing for some 

other reason can be identified during the nighttime inspection. If no inventory exists, an 

inventory of existing signs can be created while conducting the nighttime inspection, but it may 

not account for missing signs. A nighttime inspection procedure can be performed without a sign 

inventory. 

The nighttime visual inspection method should only use the low-beam headlamps of the vehicle 

as the source of illumination for the signs. The interior light of the vehicle should remain off to 

the extent feasible. The inspection should be performed at highway speeds and from the travel 

lanes and not the shoulder. As the vehicle approaches the sign, the sign’s overall appearance in 

terms of brightness and legibility is assessed. Usually the sign is given a rating defined by the 

agency. At a minimum, the scale should include three designations: good, fair, and poor. The 

inspector records the information for each sign and the rating that it is given. Signs rated as poor 

should be scheduled for replacement as soon as possible. Depending on the inspection schedule, 

signs rated as fair can be noted as requiring attention during the next set of scheduled inspections 

or can be identified for additional assessment, such as measurement at a later date using a handheld retroreflectometer. 

The vehicle and inspector combination should be selected to provide a conservative estimate of 

sign retroreflectivity. The increased sales of pickup trucks and sport utility vehicles, which result 

in larger observation angles, make these types of vehicles appropriate for use in many regions. 

Relatively new vehicles, with visually/optically aimable (VOA) headlamps, should be 

considered. Ideally, the inspector should be older, with nighttime visual capabilities similar to 

older drivers. The vision of the inspector should be tested to ensure that it is within the legal 

limits of the State. It is important that an agency develop consistent guidelines to decrease the 

subjectivity of inspections. For instance, some items to consider are procedures to clean the 

headlamps and windshield before each night of inspections and to periodically check the 

headlamp aiming. A procedure to check the headlamp aim of VOA headlamps is provided in 

table 4. 
Table 4. Headlamp Aiming Procedure. 

What you will need: 

9 A level area with a distance of approximately 7.625 m (25 ft) plus the length of the 

vehicle from a flat lightly colored wall 

9 A tape measure 

9 Masking tape 

Instructions: 

 Park the vehicle so that the headlamps are precisely 7.625 m (25 ft) from a flat lightly colored 

wall. The vehicle should have at least 1⁄2 of a tank of gas and should be loaded as it would be 

when inspecting signs. This includes the weight of the driver (and passenger present). 

 Measure the exact middle of both the windshield and rear window, and mark them with strips 

of tape, creating vertical centerlines, front and rear. 

 Standing behind the car, sight along the centerlines, and have an assist mark the position of 

the vehicle centerline on the wall with a vertical strip of tape. 

 Measure the distance between the vehicle centerline and the headlamp lenses. Mark that 

distance to the right and left of the centerline on the wall with vertical strips of tape. 

 Measure the height of each headlamp from the ground (measuring to the center of the lens). 

Using those measurements, place horizontal strips of tape on the wall where the vertical strips 

have been applied. There should now be two crosses on the wall, with centers that correspond 

to the center of each headlamp lens. 

 For headlamps with a left-side cutoff (VOL), mark a horizontal line that is 53.34 mm (2.1 

inches) below the headlamp centers with a horizontal strip of tape. For headlamps with a 

right-side cutoff (VOR), mark a horizontal line that runs through the headlamp centers. 

 Turn the vehicle headlamps on low beam. The left edge of the bright spots on the wall should 

just touch the vertical bars of the crosses. The top edge of the strongest gradient of light 

should just touch the horizontal line. Adjust the headlamp aim per manufacturer’s 

instructions, if required. 

Probably the most important element of the nighttime inspection is documenting the process and 

results. This can be done with a voice or video recorder, or even with paper and pencil. 

Whichever method is selected, it is important that inspections are properly documented and 

archived to provide tort protection. 

Current Practices 

Visual nighttime inspections are typically used in conjunction with a signage replacement 

schedule to make sure that the signs are legible and to find signs that may have been passed over 

or accidentally skipped during the last replacement schedule. Inspections are usually performed 

every one to two years and rotate between predefined sections of roads under the agency’s 

jurisdiction. The inspection plans should include specific routings to ensure full coverage of the 

road network and that the inspections can be safely conducted with the levels of traffic on the 

road. A variety of practices exist for documenting inspection results and initiating actions to 

replace signs that are at or near the minimum levels. 

13 
Concerns 

One concern associated with nighttime visual inspections is that it is the most subjective of all 

the methods. Another concern is funding overtime pay to conduct the inspections during lateevening or early-morning hours. It is also important that inspectors are properly trained. 

Linking Nighttime Visual Inspections to Minimum Retroreflectivity Levels 

Minimum retroreflectivity levels are incorporated into this method by training the inspectors and 

using procedures that allow them to correlate their observations through the use of sample signs. 

A good practice is for inspectors to observe the sample signs prior to each inspection run. The 

use of appropriate sample signs at or near minimum retroreflectivity levels is a key element to 

training that links the nighttime visual inspection method to the minimum retroreflectivity levels. 

Advantages and Disadvantages 

One of the major benefits of using the visual inspection method is that it has the least 

administrative and fiscal burden of all the methods. Many agencies already perform some type of 

periodic sign inspection, although not all inspections are performed at night. This method also 

has a unique feature in that the signs are viewed in their natural surroundings. Thus, the overall 

appearance of the sign and the ability of the sign to provide information to the driving public can 

be assessed. 

Another advantage of the visual inspection method is that it has the lowest level of sign 

replacement and sign waste. Only those signs identified as needing to be replaced because of low 

retroreflectivity levels are replaced, assuming that the inspection frequency is appropriate. With 

management methods, it is probable that some signs will be replaced before their full life is 

achieved. This may imply that the visual inspection method (as compared to the measured 

retroreflectivity method) maximizes sign life. 

While this method may be more subjective than other methods, research has shown that trained 

observers can reasonably and repeatedly detect signs with marginal retroreflectivity. There is 

some risk involved while doing these inspections, particularly if the driver is also the evaluator 

and recorder. Ideally, nighttime inspections should be conducted with two people for safety 

reasons. 

MEASURED SIGN RETROREFLECTIVITY 

In general, there are two ways that sign retroreflectivity can be measured in the field: with handheld contact instruments or with non-contact instruments. Contact instruments require the 

measurement device to be in physical contact with the sign surface. Non-contact instruments, 

which measure the retroreflectivity from a distance, include both a hand-held device and vehiclebased systems. The use of the measurement method as an exclusive process to maintain sign 

retroreflectivity has not historically appealed to agencies, as will be discussed in the following 

sections. However, when combined with another method, the measured sign retroreflectivity 

method adds an element of accuracy to the overall program. This combination of methods may 

maximize maintenance budgets and provide additional protection from tort claims. 

14 
Background 

There are several commercially available hand-held retroreflectometers that can be used to 

measure sign retroreflectivity. While the contact instruments are believed to provide relatively 

low levels of uncertainty for a given measurement, using contact instruments can be time 

consuming. Non-contact devices offer flexibility and speed-up the measurement process, but the 

trade-off is a higher level of uncertainty. The uncertainty associated with field measurement of 

sign retroreflectivity has not been well established. ASTM procedures for the measurement of 

sign retroreflectivity require the averaging of multiple measurements on the face and legend of 

the sign. The selection of the measurement points and the calibration of the device can lead to 

different results, even when measuring the same sign. This can create an issue if there are small 

differences between measured values and the required minimum levels—while the uncertainty in 

the precise value may not be significant from a performance standpoint, it has some potential to 

create tort liability for marginal signs. 

More information about retroreflectometers can be found on the following Web sites: 

Contact Devices, Hand-Held 

• Delta Retrosign GR3, http://www.flinttrading.com. 

• Roadvista 922, http://www.roadvista.com. 

• Zehntner ZRS 5060, 

http://www.zehntner.com/html/download/prospekt_zrs5060_d_e.pdf. 

• Mechatronic RC200, http://www.mechatronic.de/04traffic/en/01/rc2000.html. 

Non-Contact, Hand-Held 

• Impulse RM, http://www.pwsglobalinc.com. 

Non-Contact, Mobile 

• Mandli Digilog road systems, http://www.mandli.com/. 

Note that the FHWA does not endorse the use of any specific instrument. While the above list is, 

to the best knowledge of the authors, complete as of the date of publication, other instruments 

may be available or may become available in the future. Each agency is encouraged to review 

the specifications for the various instruments and determine for themselves which instrument is 

most appropriate for their application. 

Procedures 

Measuring retroreflectivity using a contact instrument should be performed as specified in 

ASTM Standard Test Method E1709-00e1, which requires a minimum of four retroreflectivity 

measurements to be taken of the sign background and legend, if applicable. The four 

measurements for each color are averaged to obtain an overall measurement of the 

retroreflectivity for each color on the sign. These values are compared to the minimum 

retroreflectivity values to determine whether or not the sign should be replaced. 

15 
Hand-Held Contact Instruments 

One important distinction needs to be made between the hand-held contact instruments: ASTM 

E1709 describes two types of sign retroreflectometers: point instruments and annular 

instruments, which are defined in the test method as follows: 

“The instrument may be either a “point instrument” or an “annular instrument,” 

depending on the shape of the receiver aperture. Point and annular instruments make 

geometrically different measurements of RA, which may produce values differing on the 

order of 10 percent. Both measurements are valid for most purposes, but the user should 

learn the type of his instrument from its specifications sheet and be aware of certain 

differences in operation and interpretation. For both instrument types, the “up” position 

should be known. The point instrument makes an RA measurement virtually identical to 

an RA measurement made on a range instrument following the procedure of Test Method 

E810. The annular instrument makes an RA measurement similar to an average of a great 

number of RA measurements on a range instrument with presentation angle (γ) varying 

between -180 ° and 180 °.” 

The geometries for the two types of hand-held contact instruments are illustrated in figure 1, 

while figure 2 provides an illustration of the instrument layout. 

Figure 1. Annular and Point Aperture Instrument Angles. 1

 

1

 Reprinted, with permission, from ASTM E1709-00e1 Standard Test Method for Measurement of Retroreflective Signs Using a 

Portable Retroreflectometer, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of 

the complete standard may be obtained from ASTM (http://www.astm.org).

16 
Figure 2. Upright Optical Schematics.2

Differences in Point and Annular Instrument Measurements 

Glass bead sheeting materials tend to be rotationally insensitive. Therefore, point and annular 

instruments should produce similar RA values for these materials. The RA values for prismatic 

sheeting, however, are rotationally sensitive, and the RA values produced by point and annular 

instruments can differ on the order of 10 percent, with differences of up to 25 percent possible. 

Annular instruments cannot accurately gauge how the RA of prismatic sheeting varies with 

rotation angle. 

Current Practice 

Few agencies have reported making retroreflectivity measurements of traffic signs on a regular 

basis. Most of those agencies use the retroreflectivity measurements to supplement visual or 

other inspection methods. The remainder use measured retroreflectivity values from a sample set 

of signs as an assessment of their total sign inventory. 

There are private companies that specialize in retroreflectivity measurements. These companies 

offer retroreflectivity measurements for both signs and pavement markings, but the majority of 

the work that has been performed by private companies pertains to retroreflectivity 

measurements of pavement markings. 

Concerns 

The main concern with the measured sign retroreflectivity method is that retroreflectivity only 

accounts for one aspect of a sign’s appearance. Other factors should be considered when 

determining whether or not a sign is adequate for continued use at a particular location. These 

factors include ambient light levels, presence of glare, location relative to the road, and the 

 

2

 Reprinted, with permission, from ASTM E1709-00e1 Standard Test Method for Measurement of Retroreflective Signs Using a 

Portable Retroreflectometer, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of 

the complete standard may be obtained from ASTM (http://www.astm.org). 

17 
18 

complexity of the visual background. A sign that is acceptable in a rural environment may not be 

acceptable in a complex urban environment. 

Another concern with this method is the amount of time it takes to measure the retroreflectivity 

of a traffic sign using hand-held devices. Given the current methods and technology available to 

obtain a sign’s retroreflectivity, the time commitment required to take retroreflectivity readings 

of all signs within an agency’s jurisdiction may be labor intensive and cost prohibitive. 

Linking Measurements to Minimum Retroreflectivity Levels 

This method uses measured retroreflectivity as the basis for the decision of whether or not a sign 

meets the required minimum level of retroreflectivity. The measured retroreflectivity values are 

compared to the minimum retroreflectivity levels specified in the MUTCD. A sign should be 

scheduled for replacement if the measured retroreflectivity is at or very close to the minimum 

required level. This method provides the most direct comparison of the sign’s in-service 

retroreflectivity relative to the minimum maintained retroreflectivity levels. 

Advantages and Disadvantages 

Measured retroreflectivity provides the most direct means of monitoring the maintained 

retroreflectivity levels of deployed traffic signs. This removes all subjectivity that exists in other 

methods. However, a limit must be established on how close a sign’s retroreflectivity levels can 

be to the required minimum levels before they are replaced. Measurement uncertainty and the 

variance between the retroreflectivity at the prescribed measurement geometry versus the 

retroreflectivity at the actual observation geometry may result in a sign that meets the minimum 

requirements but does not meet the needs of the driver, and vice versa. 

The main disadvantage of using this method is that measuring all of the signs in a jurisdiction is 

time consuming. Measured sign retroreflectivity may be best used to support one of the other 

methods or as a means of evaluating marginal signs. Another disadvantage is that using the 

retroreflectivity of the sign as the only indicator of whether or not a sign should be replaced may 

end up neglecting other attributes of the sign’s overall appearance. Other factors should be 

considered, including the overall appearance and legibility of the sign, as well as environmental 

concerns, such as areas with high levels of visual clutter or glare, that may require a brighter 

sign. Agencies need access to instruments and trained personnel to use this method. 
CHAPTER 4. MANAGEMENT METHODS 

This chapter describes the management methods that can be used for maintaining traffic sign 

retroreflectivity. The methods described in this chapter include 

• Expected Sign Life. 

• Blanket Replacement Method. 

• Control Sign Method. 

EXPECTED SIGN LIFE 

In this method, signs are replaced before they reach the end of their expected service life. The 

expected service life is based on the time required for the retroreflective material to degrade to 

the minimum retroreflectivity levels. The expected service life of a sign can be based on sign 

sheeting warranties, test deck measurements, measurement of signs in the field (control signs) 

and measurement of signs taken out of service, or information from other agencies. The key to 

this method is being able to identify the age of individual signs. This is often accomplished by 

placing a sticker or other label on the sign that identifies the year of fabrication, installation, or 

planned replacement or by recording the date of installation in a sign management system. 

Various approaches or algorithms can be used to trigger an indication of the need to replace a 

sign. For example, one software system uses sign material type, color, age, and direction the sign 

faces in a model that predicts the level of retroreflectivity at any point in time. When the 

minimum levels are approached, the sign is flagged for replacement. The process must, however, 

be geared to flag signs that need replacement early enough to assure that the process of physical 

replacement can be completed before the signs drop below the minimum retroreflectivity levels. 

Procedures 

Although there are variations to this method, the basic idea is that the installation date of every 

sign in an agency’s jurisdiction is known, along with the type of retroreflective sheeting material 

used on the sign face. It is also necessary to define an expected sign (i.e., service) life for each 

type of retroreflective sheeting material. This can be done for individual signs or as a general 

parameter for the types of material used by the agency. Other information may also be of interest 

to the agency such as sign color, direction the sign is facing, and sign construction (silkscreening versus electro-cut (EC) film). This information is used in a systematic manner to 

“flag” signs that need to be replaced before their sign life expires. 

One way to use this method is through a computerized sign management system to keep track of 

an agency’s sign inventory and periodically extract information on signs that are reaching the age 

at which they need to be replaced. The degree of sophistication of the sign management system 

will dictate the options available to the agency. For example, most systems can generate lists of 

signs needing replacement, but some allow specific categories of sign type, size, or color to be 

focused upon. These systems may be able to generate individual work orders for each sign that 

needs to be replaced or can group replacements in a manner that provides an effective work 

schedule for sign crews. 

19 
If an agency has a computerized sign management system, it should be possible to query the sign 

database at regular intervals for a list of signs that are nearing the end of service life. Actual 

readings of sign retroreflectivity can be taken to determine if the degradation is occurring as 

expected. If the degradation is not occurring as fast as expected, then signs of that type could be 

left in the field longer (and an update to the planned replacement date subsequently made in the 

database). Conversely, if the deterioration is occurring faster than expected, the signs can be 

scheduled for replacement sooner. Monitoring changes in degradation can help ensure better 

nighttime visibility and increase the overall life cycle of an agency’s signs, resulting in cost 

savings. 

Another way this method can be used is by placing an installation or replacement date sticker on 

each sign to allow field crews to know when specific signs reach their replacement age. If a sign 

is found to be older than indicated by the maximum life noted on the sticker, then the sign should 

be replaced. This method can be time consuming if signs along a roadway vary significantly in 

age, but it can be executed during the day and requires no inspection or measurement of the sign. 

A complication of this method is related to the placement of the date stickers. When placed on 

the front of the sign, field crews can more readily view the date information. However, the 

information must be limited so as not to distract from the message on the sign. More information 

can be included on stickers placed on the back of the sign, but it is harder for field crews to see 

this information as they drive by, particularly on wide roadways. Figure 3 shows the Wyoming 

coding system that can be seen from the front of the sign. The number indicates the year the sign 

was installed. For instance, the stop sign was installed in 1995. Wyoming uses a 10-year 

replacement cycle that allows the Wyoming agency to use a one-digit coding system. (Note: The 

photograph was taken before 2005.) Minnesota uses a color-coded sticker on the back of the 

sign, as shown in figure 4, with a different color each year to make inspections easier. 

20 
Figure 3. Examples of Wyoming DOT Signs. 

21 
Figure 4. Example of Minnesota DOT Sign Sticker. 

Current Practice 

The use of expected sign life as a maintenance method is widely used because of its ease of 

implementation. Most agencies use the warranty period provided by the manufacturer to 

determine when a sign should be replaced. However, some agencies, like Indiana, are beginning 

to extend their expected sign life levels beyond the warranted sign life as a result of research 

documenting the durability of sign materials in their area.(18)

 This may create some complexity in 

sign management, as the expected sign life will vary across a jurisdiction depending on the 

geographic location of the sign, the amount of direct sun exposure, and other environmental 

factors that affect service life. The use of a single value for expected service life might result in 

some signs failing to meet the minimum maintained retroreflectivity levels prior to scheduled 

replacement or might result in replacement of signs that exceed the minimum levels. 

Delaware, Kansas, Maine, Missouri, and North Dakota all use a 10- to 12-year life cycle for all 

of their beaded high-intensity retroreflective materials (as of 2005). Some agencies such as 

Indiana(18)

, Michigan, and North Carolina(19) are learning that they can expect their in-service life 

expectancy to be 15 years for beaded high-intensity materials. 

Concerns 

The main concern with this method is that there are little data on how different types of sheeting 

deteriorate over time in a given climate. It can be a complex process to determine how long signs 

of a certain sheeting type and color will last in a given region of the country. Also, there are no 

definitive results on the role that the orientation of the sign face plays in the deterioration of the 

sign and whether or not signs facing different directions deteriorate at significantly different 

rates. While there have been many studies, these studies do not come to the same conclusions 

about the relationship between sign face orientation and deterioration rates. (See references 18, 

20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, and 31.) 

22 
One of the easiest ways to assign expected sign life to retroreflective sheeting materials is to use 

the manufacturer’s warranty. However, these warranties obviously include a certain factor of risk 

on the part of the manufacturer and therefore are often conservative. They may also vary 

depending on the region of the country. In general, however, it can be expected that 

retroreflective sheeting materials will have a warranty provided for the ASTM Type-designated 

materials as shown in table 5. Additional information on sign sheeting durability can be found in 

several research reports. (See references 18–31.) 

Table 5. Typical Warranty Life. 

ASTM D4956 Type Years of Warranty* 

I and II 7 

III and IV 10 

VII, VIII, IX, X 12 

* May be different for fluorescent sheeting materials 

Linking Expected Sign Life to Minimum Retroreflectivity Levels 

The minimum retroreflectivity levels provide the initial basis for the expected life criteria, but an 

understanding of the actual degradation rates of in-service signs is required to set appropriate 

triggers as retroreflectivity levels approach the minimum requirements. Degradation rates differ 

by region of the country, type and color of material, and orientation. Furthermore, under this 

method, the actual retroreflectivity of a sign is not assessed—only the age of the sign is 

monitored. 

There is a potential need to gather sample data on the true service life of signs to adjust the 

expected life measures. Some agencies accomplish this by the measurement of a sample of the 

removed signs; some monitor the performance of a small number of signs; and others measure 

the retroreflectivity of in-service signs with known installation dates.(19) 

Advantages and Disadvantages 

This method requires that agencies track the installation date of their signs. For the field 

replacement approach to this method, there is the benefit of associating the condition of a sign to 

its age. The use of a computerized sign management system may eliminate the need for a date 

sticker, but it also limits the means that may be used to analyze actual service lives because of 

the need for bar-code reading equipment or other technology-dependent equipment that might be 

used to code information on a sign. 

The expected sign life method allows agencies to help develop local service life requirements 

based on actual end-of-service-life retroreflectivity measurements and comparisons to minimum 

required levels. These comparisons can provide useful information on service life under local 

conditions, product performance, sign fabrication processes, and analysis of replacement 

strategies. This method requires that the type of sheeting used to fabricate a sign be known. 

Other pertinent information may also be necessary to take advantage of sophisticated sign life 

prediction algorithms. 

23 
One drawback to this method is that it can be fairly time consuming to check date stickers if the 

stickers are not easily viewable or identifiable on the sign. Another possible difficulty relates to 

marking signs that need to be replaced, although immediate replacement is possible for some 

sign types. If an agency uses a sign management system and functions with the use of portable 

computers in the field, the inspectors can easily note the signs that need to be replaced, and even 

generate work orders. 

BLANKET REPLACEMENT 

The blanket replacement method is essentially the expected sign life method executed on a 

spatial or strategic basis. On a spatial basis, all the signs in a specific area or corridor get slated 

for replacement at the same time, when the effective service life is reached. On a strategic basis, 

all the signs of a specific type get slated for replacement at the same time. Depending on the size 

of the jurisdiction, it may be possible to plan sign replacements that consider both geographic 

and strategic criteria. The blanket replacement is being used by various agencies around the 

country such as the City of Glendale, AZ. 

This method is probably the simplest of the management methods in that tracking the age of 

individual signs, either by physical labeling or in a database, is not necessary. It is only necessary 

to maintain a record of when the blanket actions were undertaken and when they need to be 

repeated. Usually this method is repeated after a set number of years, depending on the expected 

life of the signs. 

Procedures 

At set time periods, a sign maintenance crew will go to a specific area or corridor and replace all 

the designated traffic signs under its jurisdiction. This might be done such that regulatory signs 

are replaced in one cycle, warning signs in another cycle, and guide signs in a third cycle. The 

time interval between replacements is usually based on the expected sign life as discussed in the 

previous section. Under this method, all signs are replaced regardless of the amount of time they 

have been in the field or the condition at the time of replacement. Blanket replacements can be 

scheduled to coincide with major roadwork or repaving, resulting in the least impact on traffic. 

This is especially beneficial on routes with high traffic volumes. 

Current Practice 

This maintenance method is popular with State DOTs. Of the agencies that were contacted and 

that use a blanket replacement method, most replace their Type I signs every 7 to 10 years; Type 

III signs every 10 to 15 years; and Types VI, VIII, and IX signs every 15 years. The vast 

majority of the agencies contacted use Type III sheeting for the majority of their traffic signs, 

although some agencies are testing the use of Types VII, VIII, and IX for use on a wider scale. 

Some agencies currently use the microprismatic materials for signs that they believe require 

higher visibility. 

Replacement time depends on the region of the country as well as on past experience of how 

long signs last in the field. For example, Maine, Kansas, and New Hampshire replace signs with 

Type III sheeting every 10 years, while Minnesota replaces Type III signs every 12 years and 

24 
Michigan every 15 years. Minnesota also uses Type IX sheeting on guide signs and replaces 

these signs every 15 years. 

Concerns 

One of the issues with this method is that the replacement times can vary depending on the 

region of the country in which the agency is located, or even across a jurisdiction for large 

agencies. The replacement time also depends on the types of sheeting that are used to make the 

agency’s traffic signs. Therefore, an agency needs to have relevant data on the in-service life of 

all the sheeting materials it has in the field. Another concern is that this method potentially 

wastes resources by removing signs before their useful life has been reached. This is particularly 

true where signs have been added or replaced in an area after the last replacement cycle. When 

the replacement cycle comes around, these signs will be replaced regardless of their age. They 

can be reused if handled properly, but that would require that each sign that is replaced be 

inspected to determine the amount of useful sign life remaining. 

Linking Blanket Replacement to Minimum Retroreflectivity Levels 

The minimum retroreflectivity levels provide the initial basis for the expected life criteria, but an 

understanding of the actual degradation rates of in-service signs is required to set appropriate 

triggers as retroreflectivity levels approach the minimum requirements. Under this method, 

retroreflectivity levels of signs are not measured, and opportunities are limited for capturing data 

that may be useful in adjusting service lives, trigger points, or sign maintenance strategies. 

Advantages & Disadvantages 

The major benefit of using this method is that all signs are replaced; there is a low likelihood of a 

given sign being skipped over or not being replaced. This ensures that all replaced signs are 

visible and meet minimum retroreflectivity levels. 

The major drawback to this method is the potential amount of waste than can be generated if 

signs that are relatively new are removed during a normal replacement cycle. This can be 

particularly expensive when a blanket replacement method is first implemented. Follow-up 

replacement cycles can also be wasteful if signs are replaced between the expected service life 

periods because of knockdowns, graffiti, etc. 

CONTROL SIGNS 

The control sign method is based on measurements made of a subset of signs that represent an 

agency’s inventory. The subset of signs represents a population of signs made with the same 

material for which the retroreflectivity performance over time is monitored by actual 

measurements. As the retroreflectivity levels of the control signs approach the minimum levels, 

it triggers action to begin replacement of the entire associated population. The control signs can 

be located at one or more of the agency’s maintenance yards or can be traffic signs that are 

deployed at various locations in the jurisdiction. The control signs are measured periodically to 

monitor actual degradation of retroreflectivity. This method requires only the management of the 

control sign information and the retroreflectivity measurements of those signs over time. 

25 
Procedures 

The use of this method requires the installation of signs in a maintenance yard or the definition 

of specific control signs from the population of deployed signs. Periodic measurements of 

control signs are made following ASTM E1709 or other accepted procedures. Measurements or 

other observations are tracked over time to monitor changes in retroreflectivity and nighttime 

visibility. Once these signs, as a whole, start to approach the minimum retroreflectivity levels, all 

the traffic signs in the field that these control signs represent are replaced. 

Current Practice 

None of the agencies contacted reported using this method to maintain their traffic signs. 

However, some agencies do take retroreflectivity readings on a sample set of signs to estimate 

how the overall sign population is performing. This is used primarily as a verification method for 

agency sign management polices and practices. 

Concerns 

The effectiveness of this method is dependent upon the size of the control sign sample. The 

larger the sample, the better the estimation of the retroreflectivity levels of the sign populations it 

represents. There is no specific guidance on the number or percentage of the population the 

sample represents. However, a minimum of three signs per type of sheeting and color should be 

monitored. 

Another question relates to how often a set of control signs is needed. Each new sign material or 

deployment of a major product order would warrant a set of control signs, as there are likely to 

be differences in retroreflectivity performance. It may be appropriate to install controls when 

new sign fabrication processes are implemented or other major changes in the sign management 

process occur. It may also be appropriate for a large agency that deploys signs continually to set 

up control signs as materials age on the shelf and personnel change. Too short a time period 

between adding control signs may cause the agency to have a large number of control signs to 

monitor, which negates the simplicity of this method. Too much time between control signs 

could result in errors estimating the service life of signs installed in the time interval between the 

control signs. 

Another consideration is how often the control signs should be checked for their retroreflectivity 

levels and appearance. If the time interval between measurements is too short, then this may 

needlessly waste time and personnel resources. On the other hand, if the time interval is too long, 

signs may be left in the field that are not adequate for continued use and may pose a possible 

safety risk. An annual inspection of the signs, including retroreflectivity measurements, may be 

appropriate. 

Linking Control Signs to Minimum Retroreflectivity Levels 

The control signs must be measured at given intervals with a retroreflectometer to determine how 

they are performing. These values are then compared to the minimum retroreflectivity levels in 

order to trigger sign replacement actions. The precise retroreflectivity levels of the majority of 

deployed signs are not known using this method. 

26 
27 

Advantages and Disadvantages 

The main benefit of this method is that it is not nearly as labor intensive as taking 

retroreflectivity readings on every sign in an agency’s jurisdiction. Because a sample set of signs 

is used to monitor the retroreflectivity levels, it is easier and less labor intensive to get an 

estimate on how the traffic signs, represented by the control signs, are performing in the field. 

Another benefit of using this method is that signs that do meet the required minimum 

retroreflectivity levels are not removed prematurely, allowing for an efficient use of the signs 

and their material. This may be particularly advantageous when the life of a new sign material 

exceeds the warranties provided by the manufacturer. 

This method requires agencies to have the capability to measure the retroreflectivity of the 

control signs. Without an appropriate sampling process, the control signs may not be 

representative of the larger sign population they are intended to represent. This could lead to 

replacing signs that do not need replacement or not replacing signs that do need replacement. 

Therefore, agencies must evaluate the number of signs of each type within their jurisdiction and 

establish guidelines on the number of control signs that are needed to appropriately represent 

signs in the field. 

APPENDIX A. ESTABLISHMENT OF MINIMUM MAINTAINED 

TRAFFIC SIGN RETROREFLECTIVITY LEVELS

Earlier versions of the MUTCD (in Section 2A.22) have stated, “All traffic signs should be kept 

properly positioned, clean, and legible, and should have adequate retroreflectivity.” However, the 

MUTCD did not define what was meant by “adequate retroreflectivity” until recently. While the 

ASTM Types have been used to specify the types of materials that agencies use, there were no 

criteria in the United States for “maintained retroreflectivity.” This need for common 

benchmarks to serve as the criteria for maintaining nighttime traffic sign visibility performance 

and to link it to sign maintenance processes was recognized in the late 1980s. Since then, 

considerable efforts have been devoted to developing maintenance criteria that can be used to 

assess the nighttime performance of traffic signs. These efforts have been based upon 

retroreflectivity, a convenient surrogate measure of nighttime visibility. This chapter describes 

the work that has been completed to establish minimum maintained traffic sign retroreflectivity 

levels. In addition, information relevant to the application of the minimum maintained 

retroreflectivity levels is provided. 

HISTORICAL BACKGROUND 

The establishment of minimum maintained traffic sign retroreflectivity levels (also referred to as 

end-of-service-life values) is one of the latest steps in the evolution of providing a safe and 

efficient road transportation system. The progression of this concept in the United States was 

accelerated in 1984 when the Center for Auto Safety (CAS) petitioned the FHWA to establish 

retroreflectivity standards for signs and markings.(32)

 Congress then directed the Secretary of 

Transportation in 1992 to revise the Manual on Uniform Traffic Control Devices to include “a 

standard for a minimum level of retroreflectivity that must be maintained for pavement markings 

and signs which apply to all roads open to public travel.” (33) 

Even before the CAS petition for retroreflectivity standards, the FHWA had an active research 

program addressing the nighttime visibility requirements of traffic control devices. This research 

program continued through the 1980s and into the 1990s, resulting in several research reports 

(see references 34, 35, 36, 37, and 38.), and culminating in 1993 with research 

recommendations for minimum in-service sign retroreflectivity levels.(39, 40)

 The minimum 

retroreflectivity levels were distributed for review and comment among highway agencies

several national workshops were held. Concerns about the levels and the associated impacts o

implementing them became apparent. In 1995, a research effort was funded to revise the initial 

1993 levels and to assess the impacts of the requirements on State and local agencies. The 

research efforts resulted in a pair of reports that were published by the FHWA.(

, and 

f 

sed 

e reports. 

41, 42)

 The revi

1998 set of minimum levels was presented in thes

The most evident change of the 1998 revisions was the removal of minimum levels for overhead 

signs because of significant variability associated with headlamp luminous intensity directed 

toward overhead signs. As the 1998 revised levels were being finalized, the Federal Motor 

Vehicle Safety Standard Number 108, “Lamps, Reflective Devices, and Associated Equipment” 

(FMVSS 108) was revised so that vehicle owners could easily aim their headlamps and therefore 

29 
reduce the variability associated with headlamp aim. (FMVSS 108 is the document that sets the 

minimum and maximum luminous intensities for headlamps, headlamp mounting heights, and 

standardization of headlamps on new vehicles sold in the United States.) Because of these 

changes, the FHWA sponsored additional research to develop minimum in-service 

retroreflectivity levels for overhead and street name signs, which were not accounted for by the 

initial 1993 or revised 1998 levels. The research for overhead and street name signs was 

completed in early 2001.(43)

 Although the overall approach was different than earlier research 

related to minimum retroreflectivity levels, several of the same assumptions were maintained. 

One of the significant contributions of the overhead and street name sign research was a 

demonstrated need to update some of the fundamental assumptions associated with the earlier 

development of minimum in-service retroreflectivity levels. From early 2001 until 2003, 

research continued on the development of updated minimum maintained traffic sign 

retroreflectivity levels. 

It should also be noted that in late 1998, the FHWA was close to issuing a proposed rule on 

minimum levels of sign retroreflectivity when the Board of Directors of the American 

Association of State Highway and Transportation Officials (AASHTO) requested that the 

FHWA delay any future action of minimum retroreflectivity levels until an AASHTO task force 

could review the proposed minimum requirements and their impacts on highway agencies. The 

AASHTO Standing Committee on Highways created the Special Task Force on Retroreflectivity 

in 1999, and it included representatives from Federal, State, city, and county transportation 

agencies, plus industry, research, and private sector entities. The efforts of the task force led to a 

resolution that was adopted by the AASHTO Board at its December 2000 meeting. The key 

points of the resolution were 

• Traffic signs should be visible at night. 

• Processes are needed for agencies to determine signs that are visible at night. 

• The processes used to provide sign visibility at night should not impose undue burdens on 

transportation agencies. 

• Agencies should be able to choose from several different processes that can be used to 

provide sign visibility at night. 

• Minimum visibility requirements should be simple and unambiguous so that they can be 

easily and properly applied. 

• Minimum retroreflectivity values should not be included as part of the MUTCD. 

• Agencies should have six years to implement the methods. 

30 
TECHNICAL BACKGROUND ON RETROREFLECTIVITY LEVELS 

A. 1993 MINIMUM LEVELS 

The initial set of minimum retroreflectivity levels published in 1993 was derived from analyses 

based upon a theoretical computer model called Computer Analysis of Retroreflectance of 

Traffic Signs (CARTS).(39)

 The CARTS model is comprised of several sub models that work in 

series to determine retroreflectivity needs based on user selected inputs. The first sub model 

determines the minimum distance at which a sign must be legible in order for a motorist to 

respond appropriately and safely. This distance is termed the Minimum Required Visibility 

Distance (MRVD) and is the sum of distances associated with the following factors: detecting 

the sign, recognizing or reading the sign, deciding on the appropriate action, initiating the 

response, and completing the required maneuver (depending on the sign message, the latter 

factors may not be needed). 

Using the computed MRVD value, the next sub model estimates the threshold legibility 

luminance needed for the sign. The major componenet of this sub model is a visibility model 

called PCDETECT,(44) which is based on data from the classical Blackwell experiments of the 

1940s where subjects were tasked with the identification of circular targets against uniform 

backgrounds.(45)

 The last sub model takes the MRVD and estimates threshold legibility 

luminance and back-calculates the retroreflectivity needed at the standard measurement 

geometry of 0.2 and -4.0 degrees for the observation and entrance angles, respectively. 

Because of the infinite number of possible scenarios in terms of the combination of sign types, 

sign locations, driver needs, headlamp performance variations, and the like, several scenarios 

were selected to represent typical or design conditions. For instance, the driver was assumed to 

be 47 years old, and the dimensions of the vehicle approximated a large passenger sedan. The 

assumed headlamp was a composite headlamp representing the median performance of 26 

headlamps from passenger cars with model years ranging from 1985 to 1990. 

The results of this initial work were summarized in four tables of minimum retroreflectivity 

levels, distinguished by the color of sign. There were separate tables for white signs, yellow and 

orange signs, green signs, and red signs. Depending on which of the four tables one was 

considering, the minimum retroreflectivity levels also depended on at least some of the following 

factors: roadway speed, sign size, type of retroreflective sheeting, sign location for green signs, 

and type of legend (symbol versus text). There was also a minimum contrast ratio of 4:1 required 

for white on red and white on green signs. Because this research was conducted in the early 

1990s, the only types of microprismatic sheeting included in the recommendations were ASTM 

Types IV and VII. 

B. 1998 MINIMUM LEVELS 

After the 1993 proposed minimum retroreflectivity levels were published, the developers of 

CARTS received many comments questioning the modeling assumption of one headlamp with 

the driver directly above the headlamp (also called Cyclops modeling). In reality, this modeling 

represents a motorcycle rather than a four-wheeled vehicle. Because of retroreflective sheeting 

31 
materials’ sensitivity to observation angle, a Cyclops modeling assumption can produce 

significantly different results than a model with the proper positioning of the headlamps with 

respect to the driver’s eye position. In July 1994, the developers of CARTS provided a refined 

version that accounted for the effect of two headlamps on the observation angle.(46)

 

Shortly thereafter, the FHWA sponsored two research projects to determine the adequacy of the 

1993 minimum retroreflectivity levels.(47, 48)

 During the same period, the FHWA also sponsored 

three national workshops to solicit input regarding the 1993 minimum retroreflectivity levels. In 

1998, a report by McGee and Paniati listed the following reasons for revising the minimum 

retroreflectivity levels:(41)

 

• The results from research that utilized a human factors and mathematical modeling 

approach to consider the range of visual, cognitive, and psychomotor capabilities of the 

driving population and the complexity of the relationships between the driver, the 

vehicle, the roadway environment, and the sign (in other words, the refined version of 

CARTS). 

• The results of human factors research to evaluate the percent of drivers than would be 

accommodated by signs with varying levels of retroreflectivity (in other words, the 

Mercier et al. research).(46, 47) 

• The results from measurements made on over 20,000 in-service signs in over 50 States 

and local jurisdictions (data from three different reports).(42, 49, 50) 

• Input received from the more than 40 State and local jurisdictions represented at the three 

regional workshops held in Baltimore, MD, Kansas City, MO, and Denver, CO, in late 

1995. 

• Input from public agency and private industry representatives received at numerous 

presentations. 

The revisions in 1998 resulted in several changes, but the most evident change was the removal 

of all minimum retroreflectivity levels for overhead signs because of many unresolved issues 

with vehicle headlamp performance specifications and the difficulty in measuring overhead sign 

retroreflectivity.(49)

 The minimum retroreflectivity levels for red, yellow, and orange signs were 

slightly reduced. Most of the minimum retroreflectivity levels for white signs were reduced, but 

a few were raised. The minimum retroreflectivity levels for ground mounted green signs, which 

did not include street name signs, stayed the same. 

C. 2001 MINIMUM LEVELS FOR OVERHEAD AND STREET NAME SIGNS 

In March 1997, the NHTSA implemented a final rule that revised FMVSS 108 in order to 

address the issue of headlamp misaim, which was believed to be a significant factor related to the 

amount of glare and the variability of headlamp luminous intensity directed toward overhead 

signs. The final rule reflects the consensus of the negotiated rulemaking concerning the 

improvement of headlamp aim ability performance and visual/optical headlamp aiming. 

The final rule established improved headlamp aiming features that provide more reliable and 

accurate aiming and help vehicle operators more easily determine the need for correcting aim. 

The rule introduced visually/optically aimed headlamps to the United States. The term “VOA” 

32 
generically describes two types of visually/optically aimed headlamps: VOR and VOL 

headlamps. The VOL headlamp is a low beam with a horizontal cutoff to the left side of the 

beam. The VOR headlamp is a low beam and has a horizontal cutoff to the right side of the 

beam. VOL headlamps can reduce glare to oncoming drivers compared to conventional U.S. low 

beams. VOR headlamps have less ability to reduce oncoming glare but produce luminous 

intensity distributions more similar to conventional U.S. low beams. 

Because of the NHTSA’s revision to FMVSS 108, the FHWA sponsored a research project to 

develop minimum retroreflectivity levels for overhead signs. In order to complete the initial set 

of minimum retroreflectivity recommendations, the FHWA also included minimum 

retroreflectivity levels for street name signs in the scope of the project. The research included the 

development of an analytical process to determine minimum retroreflectivity levels from a host 

of factors including demand luminance. To determine the adequate demand luminance values, 

the researchers performed a legibility study with full-scale guide signs and street name signs. 

Special emphasis was devoted to accommodating older drivers. The results of the study were 

published in 2003 and included research recommendations for a set of minimum retroreflectivity 

levels for overhead and street name signs.(51)

 

Besides providing recommendations for minimum retroreflectivity levels for overhead and street 

name signs, the researchers also performed sensitivity analyses to determine the relative impact 

of factors such as the assumed design driver capabilities, the headlamp type, and the vehicle 

type. This research identified a need to update some of the key assumptions of the initial 1993 

and revised 1998 minimum retroreflectivity levels. In addition, there was a need to develop 

minimum retroreflectivity levels for the various types of retroreflective sheeting that had been 

introduced into the market since the earlier works’ completion. 

D. 2003 MINIMUM LEVELS 

Because of a demonstrated need to update the minimum retroreflectivity levels for traffic signs, 

FHWA sponsored additional research focused on the investigation and sensitivity of updated 

factors such as the driver age, headlamps, vehicle types, and retroreflective sheeting. More 

specifically, this work included the use of an older design driver, newer style headlamps, larger 

vehicle types such as sport-utility vehicles (SUVs), and much more robust retroreflectivity 

prediction tools. The specifics of the update are reported in an FHWA report, and a summary 

paper is provided in the Transportation Research Record 1824.(52, 53)

 

The key updates included using an older driver in the analyses, a larger vehicle that is more 

representative of large SUVs, a more modern headlamp profile, and additional sign sheeting 

materials that were not included in the initial analyses. 

33 
RECOMMENDED MINIMUM RETROREFLECTIVITY LEVELS 

The results of all this research led to the minimum maintained traffic sign retroreflectivity levels 

shown in table A1. 

Table A1. Minimum Maintained Traffic Sign Retroreflectivity Levels.c

Sheeting Type (ASTM D4956-04) 

Sign Color Beaded Sheeting Prismatic Sheeting 

I II III III, IV, VI, VII, VIII, IX, X 

Additional 

Criteria 

W*; G ≥ 7 W*; G ≥ 15 W*; G ≥ 25 W ≥ 250; G ≥ 25 Overhead White on Green 

W*; G ≥ 7 W ≥ 120; G ≥ 15 Ground-mounted 

Black on Yellow Y*; O* Y ≥ 50; O ≥ 50 d

or 

Black on Orange Y*; O* Y ≥ 75; O ≥ 75 e

White on Red W ≥ 35; R ≥ 7 f

Black on White W ≥ 50 ⎯ 

c The minimum maintained retroreflectivity levels shown in this table are in units of cd/lx/m2 measured at an 

observation angle of 0.2 ° and an entrance angle of -4.0 °. 

d For text and fine symbol signs measuring at least 1200 mm (48 inches) and for all sizes of bold symbol signs 

e For text and fine symbol signs measuring less than 1200 mm (48 inches) 

f Minimum Sign Contrast Ratio ≥ 3:1 (white retroreflectivity ÷ red retroreflectivity) 

* This sheeting type should not be used for this color for this application. 

Bold Symbol Signs

• W1-1, -2 – Turn and Curve 

• W1-3, -4 – Reverse Turn and 

Curve 

• W1-5 – Winding Road 

• W1-6, -7 – Large Arrow 

• W1-8 – Chevron 

• W1-10 – Intersection in Curve 

• W1-11 – Hairpin Curve 

• W1-15 – 270 Degree Loop 

• W2-1 – Cross Road 

• W2-2, -3 – Side Road 

• W2-4, -5 – T and Y Intersection 

• W2-6 – Circular Intersection 

• W3-1 – Stop Ahead 

• W3-2 – Yield Ahead 

• W3-3 – Signal Ahead 

• W4-1 – Merge 

• W4-2 – Lane Ends 

• W4-3 – Added Lane 

• W4-5 – Entering Roadway Merge 

• W4-6 – Entering Roadway Added 

Lane 

• W6-1, -2 – Divided Highway Begins 

and Ends 

• W6-3 – Two-Way Traffic 

• W10-1, -2, -3, -4, -11, -12 – HighwayRailroad Advance Warning 

• W11-2 – Pedestrian Crossing 

• W11-3 – Deer Crossing 

• W11-4 – Cattle Crossing 

• W11-5 – Farm Equipment 

• W11-6 – Snowmobile 

Crossing 

• W11-7 – Equestrian Crossing 

• W11-8 – Fire Station 

• W11-10 – Truck Crossing 

• W12-1 – Double Arrow 

• W16-5p, -6p, -7p – Pointing 

Arrow Plaques 

• W20-7a – Flagger 

• W21-1a – Worker 

Fine Symbol Signs – Symbol signs not listed as Bold Symbol Signs. 

Special Cases 

• W3-1 – Stop Ahead: Red retroreflectivity ≥ 7 

• W3-2 – Yield Ahead: Red retroreflectivity ≥ 7; White retroreflectivity ≥ 35 

• W3-3 – Signal Ahead: Red retroreflectivity ≥ 7; Green retroreflectivity ≥ 7 

• W3-5 – Speed Reduction: White retroreflectivity ≥ 50 

• For non-diamond shaped signs such W14-3 (No Passing Zone), W4-4p (Cross Traffic Does Not Stop), or W13-1, 

-2, -3, -5 (Speed Advisory Plaques), use largest sign dimension to determine proper minimum retroreflectivity 

level.

34 
35 

It was noted that there may be conditions where the minimum retroreflectivity levels shown in 

table A1 would not provide adequate nighttime visibility. It was also noted that if the worst case 

scenario was chosen for the analyses, there would be no type of retroreflective sheeting that 

could provide adequate luminance levels to achieve detection and legibility for all drivers. 

The development of the updated minimum retroreflectivity levels consisted of many different 

scenarios comprised of a variety of practical and typical speeds, roadway cross sections, vehicle 

types, sign positions, sign sizes, headlamp types, etc. Ultimately, the minimum retroreflectivity 

levels were derived from the equilibrium point of demand and supply luminance levels, which 

also vary as a function of the aforementioned factors. (It is important to note that luminance and 

retroreflectivity are not synonymous terms. Briefly, luminance is the perceived brightness of a 

sign, and retroreflectivity is a property of the sign that describes its efficiency to return headlamp 

illuminance back toward the driver.) Technically, each specific scenario for each specific driver 

has a unique minimum luminance and therefore a unique minimum retroreflectivity level 

associated with that situation. However, from a practical point of view, the minimum 

retroreflectivity levels need to be easy to measure and manage. This requires that the infinite 

number of minimum retroreflectivity levels associated with the infinite number of driving 

scenarios be consolidated to a practical and manageable number. 

The level of complexity of the framework of the minimum retroreflectivity levels of 1993 and 

1998 was a particularly significant issue as seen by the AASHTO Special Retroreflectivity Task 

Force. As the research effort to update the minimum retroreflectivity levels was nearing 

completion, the researchers focused on consolidating the recommendations into an easy-to-use 

format. In consolidating the minimum retroreflectivity levels, certain decisions were made 

regarding the resolution of the levels. For example, factors such as sign size and roadway speed 

were collapsed into one level representing the majority of typical driving scenarios for a given 

sign type. 

The consolidation efforts were progressing as a second round of national minimum 

retroreflectivity workshops was underway.(54)

 The most current research recommendations 

regarding the consolidation of the minimum retroreflectivity levels were presented to the 

participants of each of the four workshops. The input received from the workshop participants 

helped shape the outcome of the recommended minimum retroreflectivity levels presented in this 

paper. 

The consolidation efforts ultimately resulted in some degree of compromise between the 

precision and the brevity of the minimum retroreflectivity levels. The final research report 

provides a detailed description of how the minimum retroreflectivity levels were consolidated to 

an implementable format.(52)
KEY ASSUMPTIONS 

The minimum retroreflectivity levels shown in table A1 represent the most recent results of a 

series of research studies that have been undertaken over the past two decades. They also 

represent the latest efforts in a long series of considerations related to providing safe and 

efficient roadways. The key assumptions associated with table A1 are 

• These minimum retroreflectivity levels represent driver needs in dark rural conditions 

with essentially no ambient lighting, no glare except from the vehicle instrument panel, 

and no visual complexity. 

• The nighttime driver needs used to develop the minimum retroreflectivity levels can be 

expressed as accommodating sign legibility or recognition for 50 percent of drivers over 

55 years of age. 

• The required legibility distances were based on a legibility index of 12.2 meters (40 feet) 

per inch of letter height. 

• The required recognition distances were based on CARTS MRVD values. 

• In conditions where the required threshold luminance levels were below 1.0 cd/m2, a 

minimum of 1.0 cd/m2

 was assumed for maintenance of sign conspicuity. 

• The supply sign luminance was modeled assuming that the only contribution of 

illuminance originated from the design vehicle. In other words, no contribution from 

other vehicles in the proximity of the design vehicle was considered. There was also no 

consideration of pavement reflection adding to the luminance of the sign.(55) 

• The supply luminance did consider windshield transmissivity (0.72/km) and atmospheric 

transmissivity (0.86/km). 

• The headlamp luminous intensity matrix used for developing the minimum 

retroreflectivity levels representing a market-weighted model year 2000 passenger car. 

The data were derived from measurements made with perfect aim, no scattering of light 

caused by lens wear or dirt, and a voltage of 12.8 volts. 

• The retroreflectivity data used for the analysis and modeling was the same as is included 

in the ERGO2001 program. While the retroreflective sheeting materials used throughout 

this paper are classified using the ASTM D-4956-04 classification scheme, it is important 

to note that the retroreflectivity data from the EGRO2001 model do not necessarily 

represent all manufacturers’ sheeting performance within each ASTM Type designation. 

For instance, there are several manufacturers of high intensity retroreflective material 

(ASTM Type III). Each brand performs differently. This is particularly true for 

microprismatic sheeting materials. 

• Minimum retroreflectivity levels developed for straight and flat roadways (i.e., no 

curves). 

• Vehicle dimensions represented a contemporary-styled sport utility vehicle. 

• Signs were installed normal to the roadway. 

36 
REFERENCES 

1. “FHWA Fiscal Year 2003 Performance Plan.” This document can be viewed at the Internet 

Web site: http://www.fhwa.dot.gov/reports/2003plan/index.htm. 

2. “Traffic Safety Facts 2005: A Compilation of Motor Vehicle Crash Data from the Fatality 

Analysis Reporting System and the General Estimates System.” Publication NO. DOT HS 

809484 December 2006. This document can be viewed at the Internet Web site: http://wwwnrd.nhtsa.dot.gov/pdf/nrd-30/NCSA/TSFAnn/TSF2005.pdf 

3. Federal Highway Administration and The Bureau of Transportation Statistics, 2001 National 

Household Travel Survey, (23.3% of vehicle miles traveled occur between 7:00 pm and 6:00 

am). This document can be viewed at the Internet Web site: http://nhts.ornl.gov. 

4. Staplin, L., Lococo, K., and Byington, S., Older Driver Highway Design Handbook. FHWARD-97-135, FHWA, Washington, D.C., 1998 

5. Manual on Uniform Traffic Control Devices. U.S. Department of Transportation, Federal 

Highway Administration, Washington, D.C., 2003. 

6. Carlson, P.J. and H.G. Hawkins, Jr. Updated Minimum Retroreflectivity Levels for Traffic 

Signs. FHWA-RD-03-81. U.S. Department of Transportation, Federal Highway 

Administration, Washington, D.C., 2004. 

7. Carlson, P.J., G.H. Hawkins, Jr., G.F. Schertz, D.J. Mace, and K.S. Opiela. “Developing 

Updated Minimum In-Service Retroreflectivity Levels for Traffic Signs.” In Transportation 

Research Record 1824, TRB, National Research Council, Washington, D.C., 2003, pp. 133–

143. 

8. Carlson, P.J. and H.G. Hawkins, Jr. Minimum Retroreflectivity Values for Overhead Guide 

Signs and Street Name Signs. FHWA-RD-03-82. U.S. Department of Transportation, Federal 

Highway Administration, Washington, D.C., 2003. 

9. American Society for Testing and Materials. Standard Specification for Retroreflective 

Sheeting for Traffic Control. ASTM D4956-04, West Conshohocken, PA, 2004. 

10. McGee, H.W. and S. Taori. Impacts on State and Local Agencies for Maintaining Traffic 

Signs within Minimum Retroreflectivity Guidelines. FHWA-RD-97-053, U.S. Department of 

Transportation, Federal Highway Administration, Washington, D.C., 1998. 

11. Traffic Control Device Handbook. Institute of Transportation Engineers, Washington, D.C., 

1999. 

12. Hawkins, H.G., P.J. Carlson, G.F. Schertz, and K.S. Opiela. Workshops on Nighttime 

Visibility of Traffic Signs: Summary of Workshop Findings. FHWA-SA-03-002, U.S. 

Department of Transportation, Federal Highway Administration, Washington, D.C., 2003. 

37 
38 

13. Hildebrand, E. “Reduction in Traffic Sign Retroreflectivity Caused by Dew and Frost.” 

Proceedings from TRB’s 82nd Annual Meeting, Washington, DC, January 2003. 

14. Lagergren, E.A. Traffic Sign Retroreflectivity Measurements Using Human Observers. 

Report No. WA-RD-140.1, Washington State Transportation Center, Seattle, WA, 1987. 

15. Hawkins, H.G. and P.J. Carlson.“Results of Visual Evaluations of Sign Retroreflectivity 

Compared with Minimum Retroreflectivity Recommendations.” In Transportation Research 

Record 1754, TRB, National Research Council, Washington, DC, 2001, pp. 11–20. 

16. How to Check Traffic Signs for Correct Reflectivity. 3M Company, St. Paul, MN, 1957. 

17. McGee, H.W. and D.L. Mace. Retroreflectivity of Roadway Signs for Adequate Visibility: A 

Guide. FHWA-DF-88-001, FHWA, Washington, D.C., 1987. 

18. Bischoff, A. and D. Bullock. Sign Retroreflectivity Study. FHWA-IN-JTRP-2002-22, FHWA, 

Washington, D.C., 2003. 

19. Rasdorf, W.J., J.E. Hummer, E.A. Harris, V.P. Immaneni, and C. Yeom. Designing an 

Efficient Nighttime Inspection Procedure to Ensure Motorist Safety. FHWA/NC/2006-08, 

North Carolina State University, Raleigh, NC, 2006. 

20. Black, K.L., H.W. McGee, S.F. Hussain, and J.J. Rennilson. Service Life of Retroreflective 

Signs. Report No. FHWA-RD-90-101, U.S. Department of Transportation, Federal Highway 

Administration, Washington, D.C., 1991. 

21. Hawkins, H.G., P. J. Carlson, J.B. McCaleb, and C.R. McIlroy. Impact of Minimum 

Retroreflectivity Values On Sign Replacement Practices. FHWA/TX-97/1275-1F, College 

Station, TX, 1996. 

22. W.D. Ketola. “Durability Testing for Retroreflective Sheetings.” In Transportation Research 

Record 1230, TRB, Washington, D.C., 1989. 

23. Black, K.L., S.F. Hussain, and J.F. Paniati. “Deterioration of Retroreflection Traffic Signs.” 

ITE Journal, July 1992. 

24. D.A. Andersen. Sign Retroreflectivity Study. North Dakota State University, North Dakota 

DOT, 1993. 

25. W.D. Ketola. “Laboratory-Accelerated Versus Outdoor Weathering for Retroreflective 

Sheeting Specifications.” In Transportation Research Record 1657, TRB, Washington, D.C., 

1999. 

26. Montebello, D. and J. Schroeder. Cost Effectiveness of Traffic Sign Materials. MN/RC-2000-

12, Minnesota DOT, St. Paul, MN, 2000. 

27. Kirk, A.R., E.A. Hunt, E.W. Brooks. Factors Affecting Sign Retroreflectivity. SR514, Oregon 

DOT, Salem, OR, 2001. 
39 

28. Nuber, L. and D. Bullock. Comparison of Observed Sign Retroreflectivity Values with 

Proposed FHWA Minimums. TRB Annual Meeting, Washington, D.C., 2002. 

29. Awadallah, F.Durability of Retroreflective Signs. Doctoral Dissertation–University of 

Maryland–Department of Civil Engineering, College Park, MD. 1987. 

30. Kirk, A. R., E.A. Hunt, and E.W. Brooks. Factors Affecting Sign Retroreflectivity. Final 

report submitted to OR Department of Transportation, Salem, OR. 2001. 

31. Wolshon, B., R. Degeyter, and J. Swargam. Analysis and Predictive Modeling of Road Sign 

Retroreflectivity Performance.16th Biennial Symposium on Visibility and Simulation, Iowa 

City, IA. 2002. 

32. Advance Notice of Proposed Amendments to the MUTCD: Minimum In-Service 

Retroreflective Performance of Traffic Control Devices. Federal Register, Volume 50, 

Number 81, pp. 16,515–16,517, April 26, 1985. 

33. United States Department of Transportation and Related Agencies Appropriations Act of 

1992, Public Law 102–388, 106 Statute 1520, Section 406. 

34. Mace, D.J., K. Perchonok, L.E. Pollack. Traffic Signs in Complex Visual Environments. 

FHWA-RD-82-102, U.S. Department of Transportation, Federal Highway Administration, 

Washington, D.C., 1982. 

35. Mace, D.J., R.B. King, and G.W. Dauber. Sign Luminance Requirements for Various 

Background Complexities. FHWA-RD-85-056, U.S. Department of Transportation, Federal 

Highway Administration, Washington, D.C., 1985. 

36. Mace, D.J., R.S. Hostetter, L.E. Pollack, and W.D. Zweig. Minimal Luminance Requirements 

for Official Highway Signs. FHWA-RD-86-151, U.S. Department of Transportation, Federal 

Highway Administration, Washington, D.C., 1986. 

37. McGee, H.W. and D.J. Mace. Retroreflectivity of Roadway Signs for Adequate Visibility: A 

Guide. FHWA-DF-88-001, U.S. Department of Transportation, Federal Highway 

Administration, Washington, D.C., 1988. 

38. Mace, D.J., P.M. Garvey, and R.F. Heckard. Relative Visibility of Increased Legend Size vs. 

Brighter Materials for Traffic Signs. FHWA-RD-94-035, U.S. Department of Transportation, 

Federal Highway Administration, Washington, D.C., 1994. 

39. Paniati, J.F. and D.J. Mace. Minimum Retroreflectivity Requirements for Traffic Signs. 

FHWA-RD-93-077, U.S. Department of Transportation, Federal Highway Administration, 

Washington, D.C., October 1993. 

40. Paniati, J.F. and D.J. Mace. Minimum Retroreflectivity Requirements for Traffic Signs, 

Summary Report. FHWA-RD-93-152, U.S. Department of Transportation, Federal Highway 

Administration, Washington, D.C., October 1993. 
40 

41. McGee, H.W. and J.F. Paniati. An Implementation Guide for Minimum Retroreflectivity 

Requirements for Traffic Signs. FHWA-RD-97-052, U.S. Department of Transportation, 

Federal Highway Administration, Washington, D.C., 1998. 

42. McGee, H.W. and S. Taori. Impacts on State and Local Agencies for Maintaining Traffic 

Signs within Minimum Retroreflectivity Guidelines. FHWA-RD-97-053, U.S. Department of 

Transportation, Federal Highway Administration, Washington, D.C., 1998. 

43. Carlson, P.J. and H.G. Hawkins, Jr. “Minimum Retroreflectivity Values for Overhead and 

Street Name Signs.” In Transportation Research Record 1794, TRB, National Research 

Council, Washington, D.C., 2002, pp.38–48. 

44. Farber, E. and C. Matle. PCDETECT: A Revised Version of the DETECT Seeing Distance 

Model. Transportation Research Record 1213. Transportation Research Board, National 

Research Council, Washington, D.C., 1989. 

45. Blackwell, H.R. “Contrast Thresholds of the Human Eye.” Journal of the Optical Society of 

America, Volume 36, Number 11, November 1946. 

46. Mercier, C.R., C. Goodspeed, C.J. Simmons, and J.F. Paniati. “Evaluation of Proposed 

Minimum Retroreflectivity Requirements for Traffic Signs.” In Transportation Research 

Record 1495, TRB, National Research Council, Washington, D.C., 1995. 

47. Goodspeed, C.H. and C.R. Mercier. Nighttime Sign Visibility Performance. Unpublished 

Draft Final Report, Federal Highway Administration, Washington, D.C., November 1993. 

48. A.F. Bildstein, Jr. The Effect of Environmental Illumination on Traffic Sign Conspicuity and 

Retroreflectivity. Master of Science Thesis, College of Engineering and Mineral Resources, 

West Virginia University, Morgantown, WV, 2001. 

49. Black, K.L., H.W. McGee, S.F. Hussain, and J.J. Rennilson. Service Life of Retroreflective 

Signs. Report No. FHWA-RD-90-101, U.S. Department of Transportation, Federal Highway 

Administration, Washington, D.C., 1991. 

50. Black, K.L., H.W. McGee, and S.F. Hussain. Implementation Strategies for Sign 

Retroreflectivity Standards. NCHRP Report 346, Transportation Research Board, 

Washington, D.C., 1992. 

51. Carlson, P.J. and H.G. Hawkins, Jr. Minimum Retroreflectivity Values for Overhead Guide 

Signs and Street Name Signs. FHWA-RD-03-82. U.S. Department of Transportation, Federal 

Highway Administration, Washington, D.C., 2003. 

52. Carlson, P.J. and H.G. Hawkins, Jr. Updated Minimum Retroreflectivity Levels for Traffic 

Signs. FHWA-RD-03-81. U.S. Department of Transportation, Federal Highway 

Administration, Washington, D.C., 2004. 
41 

53. Carlson, P.J., G.H. Hawkins, Jr., G.F. Schertz, D.J. Mace, and K.S. Opiela. “Developing 

Updated Minimum In-Service Retroreflectivity Levels for Traffic Signs.” In Transportation 

Research Record 1824, TRB, National Research Council, Washington, D.C., 2003, pp. 133–

143. 

54. Hawkins, H.G., P.J. Carlson, G.F. Schertz, and K.S. Opiela. Workshops on Nighttime 

Visibility of Traffic Signs: Summary of Workshop Findings. FHWA-SA-03-002, U.S. 

Department of Transportation, Federal Highway Administration, Washington, D.C., 2003. 

55. P.J. Carlson. A Model for Estimating Traffic Sign Illuminance and Luminance as a Function 

of Pavement Reflectance. Doctorate of Philosophy, Texas A&M University, College Station, 

Texas, 2002. 
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