Explained: ASTM standards for traffic signs

In the United States, the Department of Transportation Federal Highway Administration issues the Manual on Uniform Traffic Control Devices (MUTCD). The MUTCD sets minimum standards, provides guidance, and ensures uniformity of traffic control devices across the states² . States may issue additional regulations or a supplemental manual on traffic control devices. In Minnesota this cumulates in the Minnesota Manual on Uniform Traffic Control Devices (PDF, 60.89 MB), also known as the MN MUTCD.

For example, the MN MUTCD classifies three types of signs (section 2A.5 Classifications of Signs, page 2A-2 (PDF, 60.89 MB))³:

A. A regulatory sign shows traffic laws or regulations such as stop signs and speed limits.

B. A warning sign calls attention to an unexpected condition, such as a sharp turn in the road or a school zone.

C. A guide sign provides directions to road users, including routes, destinations, and distances.

All these signs “shall be retroreflective … or illuminated … of the same shape and similar color by both day and night …” (section 2A.7 Retroreflectivity and Illumination, page 2A-3 (PDF, 60.89 MB))⁴. In addition, the material of these signs shall be one of two categories: Beaded Sheeting for ASTM Types I, II or III, and Prismatic Sheeting for ASTM Types III, IV, VI, VII, VIII, IX, or XI⁵. For more detailed information on this, see the MN MUTCD, Table 2A.3 Sign Color, Sheeting Type, Additional Criteria, on page 2A-6 (PDF, 60.89 MB).

In practice, the light from a source is reflected back in a cone-like shape, centered around the light’s incidental path. This is a desirable feature of sheeting; the reflected light should be going to the driver, as any of the light that is going back to a car’s headlight—as it travels along the incidental path—is considered a waste in retroreflectivity terms.

Sheeting differs in how the cone of the retroreflected light is shaped. In general, the light intensity of the cone is the highest at the incidental path, and decreases as the angle increases. To measure differences between sheeting, the cone of retroreflection is evaluated by measuring the light intensity at several places inside the cone, known as the observation angle. By convention, sheeting is evaluated at the observations angles of .2, .5 and 1 degrees relative to the incidental path.

In addition, a more qualitative summary of the retroreflective sheeting types, based on the MN MUTCD, is provided in the Retroreflective Sheeting Identification Guide (PDF, 3.36 MB)⁶.

• Type I is an engineering grade, retroreflective sheeting. Historically it has been composed of enclosed-lens glass-bead material. Modern versions may be of a prismatic construction. Type I sheeting has the lowest performance of all Types. Agencies specify it for signs where brightness during nighttime is less critical, such as “No Parking” signs. Type I should not be used for black-on-yellow warning signs or any legend on overhead or shoulder-mounted white-on-green signs.
• Type II is a super-engineering grade, medium-to-high intensity retroreflective sheeting typically composed of enclosed-lens glass-bead material. Common applications for Type II include permanent highway signage, construction zone devices and delineators.
• Type III is a high-intensity retroreflective sheeting comprising encapsulated glass-bead retroreflective material or unmetallized, microprismatic retroreflective-element material. Type III is used where an intermediate amount of retroreflectivity performance is required, including permanent highway signage, construction zone devices and delineators.

• Type IV, also known as high-intensity prismatic, is a sheeting typically composed of an unmetallized, microprismatic retroreflective-element material. Type IV uses include permanent highway signage, construction zone devices and delineators.
• Type V is a super-high-intensity retroreflective sheeting typically made of metallized microprismatic retroreflective-element material. This sheeting is commonly used for delineators.
• Type VI is an elastomeric, high-intensity retroreflective sheeting without adhesive, typically composed of vinyl microprismatic retroreflective material. This sheeting is commonly used for orange temporary roll-up warning signs, traffic cone collars and post bands.
• Type VIII is a super-high-intensity retroreflective sheeting typically comprising an unmetallized, cube-corner microprismatic retroreflective-element material. Type VIII performs best at long sight distances where narrow angles are present. Common applications include permanent highway signage, construction zone devices and delineators.
• Type IX Very-high-intensity retroreflective sheeting typically made of an unmetallized cube corner microprismatic retroreflective-element material. Type IX sheeting has its highest retroreflectivity characteristics at short sight distances with higher observation angles. Notice that 1.0-degree observation angle is a requirement of ASTM standards for Type IX and Type XI sheeting only. Common applications for this material include permanent highway signage, construction zone devices and delineators.
• Type XI is a retroreflective sheeting typically manufactured as an unmetallized cube corner microprismatic retroflective element material. This super-high efficiency retroreflective sheeting is designed to perform best at both medium and short sight distances. This makes Type XI a versatile sheeting that agencies use to help improve road safety. Many agencies also use Type XI sheeting for overhead guide signs. Its high performance eliminates the need for external lighting of overhead signs, which is a huge cost savings on energy and maintenance costs. 

It should be noted that ASTM D4956’s method of quantifying sheeting performance is only part of a process to determine a sign’s road performance. This leaves many agencies looking for help to better understand how choices between sheeting types may impact the drivers on their roads.

To meet this important need, a workgroup at ASTM was initiated with the aim to develop a standard practice to evaluate sheeting materials based on road performance, including the impact of headlights, sign placement, vehicle type, and road geometry. The workgroup plans to use a method that quantifies the change in sign brightness as a function of the distance to the sign. These luminance curves aim to capture the real-world brightness of a sign as a driver approaches it, which can be compared to driver needs to make an assessment of performance.

While the above listed sheeting types provide minimum sheeting retroreflectivity coefficients, luminance is the end result of retroreflectivity in conjunction with headlights, sign placements and other factors, and is easier to evaluate and understand by the practitioner. The task group intends to develop a set of standard scenarios for evaluating luminance of various sheeting to provide guidance to the users of the standard by comparing the performance of these sheetings in each scenario.

Subscribe to receive transportation safety emails and stay up-to-date on the latest events, news and promotions for the transportation safety industry.