A basic understanding of the materials that make up a sign is needed to carry out a good maintenance program. This section provides a brief overview of the materials for the sign face and the sign backing.

Sign Face Sheeting

The sign face consists of film material, called sheeting, which is comprised of glass beads or microprisms to provide visibility at night. Highway signs, that are not illuminated by external lights, are visible at night because they have sheeting made of retroreflective material. Retro-reflection is a type of reflection that redirects incident light from the sign face back to the source—the vehicle headlights.

Retroreflection is achieved through either glass beads or microprisms imbedded in the sheeting.

Over time manufacturers have developed different types of sheeting to accomplish retroreflection, initially using small glass beads, but now more commonly using microprisms with various angular designs. Sheeting manufacturers have specific brand names for their materials. ASTM International has classified the different types by conformance to the retroreflectance properties, color, and durability. The types for rigid surface signs are as follows:

• Type I — commonly referred to as engineering grade and typically made as an enclosed lens, glass beads sheeting.
• Type II — commonly referred to as super-engineering grade and also typically made of glass beads.
• Type III — commonly referred to as high intensity and can be made of encapsulated glass beads or microprisms.
• Types IV, VIII, IX and XI — no common identifiers but referred to as prismatic with the differences related to the varying angles of the microprisms.

ASTM has established new sheeting specifications for minimum retroreflectivity values, called ‘coefficient of retroreflection’ (RA) for each sheeting type. RA is a quantitative measure and is expressed in metric units of candelas per lux per square meter (cd/lx/m2). The specification provides an initial RA value for a prescribed entrance angle and observation angle—common angles within an instrument that measures RA and relates somewhat to roadway/vehicle geometries. Minimum RAs for the various sheeting types of white, yellow, fluorescent yellow, green and red color sheeting used for permanent rigid signs at the common angle readings of -4 degree entrance angle and 0.2 degree observation angle—angles typically set for measuring RA in a portable instrument (see pg. 27) are shown in Table 1.

Table 1. Minimum Coefficient of Retroreflectivity (RA) for Several Types of New Sheeting Used for Rigid Signs

The values shown in Table 1 are for newly purchased sign sheeting. Exposed to the sun and weather over time, the colors will fade and the retroreflective qualities of the sign sheeting will deteriorate to such an extent that the sign will no longer be readable, both day and night, at a distance needed by the driver. The 2009 MUTCD includes a requirement for minimum maintained retroreflectivity levels for most traffic signs. These values are shown in Table 2 for certain types of permanent signs most commonly found on local, non-freeway roads.

Table 2. Minimum Maintained Retroreflectivity Levels for Specified Signs.

Based on the values in Table 2, Type I sheeting material should not be used for warning signs and for the white legend on green background guide signs because it does not provide the retroreflectance level required by the driver. The higher level sheeting types will provide a brighter sign and in general have a longer service life, allowing for an overall cost savings.

Agencies will have until January 2012 to establish and implement a sign assessment or management method in compliance with the 2009 MUTCD to maintain minimum levels of retroreflectivity. The compliance date for replacing signs that do not meet minimum requirements is January 2015, except for street name signs, which is January 2018.

To learn more about sign materials and especially sign retroreflectivity, visit the following FHWA web site: www.fhwa.dot.gov/retro.

Sign Backing

The retroreflective sheeting material is applied to the rigid sign backing (also known as the substrate), which can be aluminum, wood, or a composite plastic. Aluminum is by far the most commonly used material. The aluminum backing can be reused often, reducing the cost of signing and can be recycled easily to recoup a small portion of the cost.

Plywood is used occasionally for temporary signs because it can be cheaper and is lighter than aluminum. Fiberglass-reinforced plastic materials are used infrequently because of their handling and recycling issues.

Most agencies use aluminum for sign backing.