Traveling from state to state, a driver encounters signs of the same shape and similar color. However, in some states signs are more difficult to read than in others, especially at night. We discuss how differences in nighttime performance can all be traced back to a combination of international, national and state standards for the sheeting materials used on 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).
When driving at night, a car’s headlights illuminate objects in the surrounding environment. The light scatters from these objects and is reflected in all possible directions. A fraction of the scattered light will end up at the person driving the car and make the surrounding environment visible.
Fortunately, the critical surfaces of our roads and the important traffic signs along it are marked with retroreflective materials. Retroreflection is the property of an object or surface to reflect light back along or close along its incidental path, regardless of where the light is coming from (i.e. regardless of the entrance angle). This gives the objects with a retroreflective surface a bright appearance to the driver, which makes signs stand out much more prominently than the other objects. However, not all retroreflective sheeting materials are equally bright.
Historically, retroreflective signs used a sheeting that was made of enclosed glass beads. By current standards, beaded sheeting has a very low retroreflective performance. The performance of today’s sheeting is up to seven times as great, thanks to substantial changes in technology and construction. Much of the reflective sheeting used today employs truncated or full cube corners. Full cube corner sheeting, including 3M’s Diamond Grade™ Sheeting, can be twice as efficient in returning light as traditional cube corner sheeting.
The ratio of the perceived brightness (luminance, cd/m2) to that of the intensity of the incident light (lx) measured on the surface of a retroreflective material is known as the “Coefficient of Retroreflection (cd/lx/m2 )”. A “perfect” retroreflector would reflect the light back to where it came from; that is exactly opposite the incidental path regardless of the relative orientation between light source and sheeting. In practice, this would send the light back to the headlight and would not work for a driver. Therefore, sign sheeting should return the incident light in the general direction of the light source where the driver is also expected to be. Retroreflective sheeting must also retain this property regardless of the direction of the light source as the approaching vehicles could be in a variety of approach paths. To check this former property in the lab, the Coefficient of Retroreflection is evaluated for light coming from different places, for instance at two Entrance angles of -4 and 30 degrees.
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.
ASTM International is one of the major consensus-based standard setting organizations in the world. ASTM has no enforcement or authority to impose its standards; however, it provides a platform to help develop consensus based industry standards, some of which are adopted by government agencies worldwide and are often written into regulations and law. In general, manufacturers self-certify the compliance of their products with the ASTM standards.
ASTM’s Standard Specification for Retroreflective Sheeting for Traffic Control (D4956) covers sheeting properties such as retroreflection, color and durability. The standard does not specifically address additional materials and imaging methods that may be applied to the sheeting to fabricate a specific traffic sign, such as inks, and overlays1.
The ASTM determines sheeting types by conformance to requirements as can be evaluated in a lab setting. The retroreflective requirements capture performance at a standard set of angles between light source, sheeting surface and light receptor, that is the Entrance and Observation angle as we defined above. Below is a summary table with minimum values of white sheeting for several different sheeting types.
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.
1 ASTM International, ASTM D4956-16b, Standard Specification for Retroreflective Sheeting for Traffic Control, retrieved Sep 7, 2017.
2 U.S. Department of Transportation Federal Highway Administration, Manual on Uniform Traffic Control Devices (MUTCD), retrieved September 7, 2017
3 Minnesota Manual on Uniform Traffic Control Devices (MN MUTCD), Section 2A.5 Classifications of Signs, page 2A-2 (PDF,60.89 MB), retrieved September 7, 2017
4 Minnesota Manual on Uniform Traffic Control Devices (MN MUTCD), Section 2A.7 Retroreflectivity and Illumination, page 2A-3 (PDf, 60.89 MB), retrieved September 7, 2017.
5 Minnesota Manual on Uniform Traffic Control Devices (MN MUTCD), Table 2A.3 Sign Color, Sheeting Type, Additional Criteria, page 2A-6 (PDF, 60.89 KB), , retrieved September 7, 2017
6 Minnesota Manual on Uniform Traffic Control Devices (MN MUTCD), Appendix A3 Retroreflective Sheeting Identification Guide (PDF, 3.36 MB), retrieved September 7, 2017