Methods for Maintaining Traffic Sign Retroreflectivity

November 2007
Publication No. FHWA-HRT-08-026

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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 one-half 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) 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 (R_A), 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 https://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.

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 high-intensity	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-high-intensity	microprismatic	medium and long road distances	permanent highway signing, construction zone devices, and delineators
VIII	Super-high-intensity	microprismatic	medium and long road distances	permanent highway signing, construction zone devices, and delineators
IX	Very-high-intensity	microprismatic	short road distances	permanent highway signing, construction zone devices, and delineators
X	Super-high-intensity	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/night_visib/sign_visib/sheetguide/.

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.

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