How do the physics of retroreflectivity impact traffic signs? We explain retroreflectivity, sign sheeting technologies, and how various factors impact visibility.
Retroreflection occurs when a surface returns a large portion of directed light beam back to its source.
Retroreflective materials appear brightest to observers nearest the light source (such as a motorist). The object’s brightness depends on the intensity of the light striking the object and the materials the object is made of.
Traffic sign technology has changed dramatically in the last few years, as breakthroughs in retroreflective science have been adopted by many government agencies around the world.
Significantly brighter signs, like the one on the far right, are manufactured with 3M’s Full Cube Technology. This represents optimal performance in retroreflective technology, with optic elements that are nearly 100% efficient, resulting in sign sheeting that returns almost 60% of available light.
Compared to beaded engineer grade sign sheeting on the left and “truncated cube” High Intensity Prismatic sign material in the middle, 3M DG³ reflective sign sheeting is noticeably brighter and more legible at a greater distance.
Glass-bead retroreflection. An incoming light beam bends as it passes through a glass bead, reflects off a mirrored surface behind the bead, then the light bends again as it passes back through the bead and returns to the light source.
Cube corner retroreflection. This technology returns light more efficiently than glass beads. With this technology, each cube corner has three carefully angled reflective surfaces. Incoming light bounces off all three surfaces and returns to its source.
See a side-by-side comparison of 7 differences between our High Intensity Prismatic and Diamond Grade™ reflective sheetings.
To answer that question, let’s look at the principle of retroreflection and how it works with 3M’s advanced sign technologies help to make our roadways safer.
When headlights strike a traffic sign, the light is reflected back to the driver in a cone-shaped pattern.
This “cone of reflectivity” or “cone of returned light” allows the driver to see the sign. The closer the driver is to the center of the cone, the brighter the sign appears. For drivers on the edge of or outside the cone, signs are dimmer and more difficult to see at night.
Most retroreflective technologies also have some diffuse reflection, which is essentially wasted light that is not reflected to the driver. The difference between low and high efficiency sign sheeting technologies is how much light they return toward the light source. Higher efficiency sign sheetings return more light back to the driver.
Technology makes it possible to expand the cone of reflectivity, both in terms of directing more light to drivers, and by returning light in a larger cone of reflectivity.
A larger cone of retroreflected light means that even drivers in SUVs, pickups and semi-trucks, with high observation angles, are still able to see bright, clearly legible roadway signs.
Observation angle is the angle between the line formed by a headlight beam striking a sign surface and the line formed by the retroreflected light at the driver’s eye. The observation angle is a critical factor when determining how bright a sign appears—as the observation angle increases, the sign will appear less bright.
Large trucks can have twice the observation angle of passenger vehicles at the same distance. That means there’s much less reflected light available for these drivers to see road signs. For even more info, read here about vehicle size, observation angle and traffic sign visibility.
The coefficient of retroreflection describes the amount of light returned and is the result of lab measurements at certain specified entrance and observation angles. By always measuring materials at the same angle with the same light source, sign sheetings can be compared with one another.
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The design of the optical elements is also important to entrance angularity—or the angle at which a vehicle’s headlights illuminate a sign. Higher degrees of entrance angularity occur on curved or multi-lane roads, at intersections, where signs are set further off the road, or through the simple physics of distance—the closer a vehicle gets to a sign, the greater the entrance angularity.
Signs made with lower performing, glass bead sheeting, as shown here, don’t return much of the reflected light to the driver, because the sheeting is inefficient and the cone of reflectivity is small.
Higher efficiency prismatic sheeting addresses this problem by creating a larger, brighter cone of reflectivity. Signs made with prismatic technology typically last longer. This results in a longer service life, which reduces maintenance and the total “cost of ownership” over time.
Durable, fluorescent sign sheeting is another significant technology.
“Human Factors” studies indicate that drivers recognize fluorescent traffic signs from greater distances than standard-colored signs.
These advancements in traffic safety technology make it possible for government agencies around the world to overcome the challenges of a changing driving environment.
From high-performance prismatic retroreflective and fluorescent sign sheeting to advanced pavement markings—including wet reflective optics—3M technology gives drivers the positive, unambiguous guidance they need to drive more safely. Day and night. And in all weather conditions.