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About reflectivity

In 1924, warning signs were all bright yellow, while all other traffic signs were white. These lighter shades improved visibility by bringing out the black text and messages on the sign face, particularly at night, when illuminated at night by headlights. Simply using light colors, however, was not ideal, and manufacturers raced to invent an effective method for illuminating signs at night.

The first innovation in road sign reflectivity came from the UK. The inventor was Percy Shaw, a road mender from Yorkshire. Anecdotally, the idea occurred to Shaw one night in 1933 when he was driving home in a heavy fog and hit a dangerous stretch of road with a sheer drop on each side. Like most drivers of the time, Shaw would have used the reflection of his car’s headlights in the nearby tram tracks to guide his way, but those had been removed for repair. According to the story, Shaw saw two bright spots in the path of his headlights as he drove: the eyes of a cat sitting near the road.


What Shaw then invented, reflective glass spheres called cats eyes, became used all over the world. These studs were set into a white rubber dome and protected by cast iron. Shaw’s design had two pairs of the beads facing in opposite directions. These were buried into the road; if drivers ran them over, the rubber would contract and the cats eyes would slip safely below the road’s surface. These road studs even cleaned themselves. The cast iron base would collect rainwater, and whenever the rubber contracted from pressure, the water would pour out and wash the surface of the beads.


Another watershed moment for traffic sign reflectivity also occurred in the 1930s. A mining and manufacturing company, 3M (now a multinational conglomerate corporation), invented reflective sheeting. This product contained thousands of tiny reflective spheres that can rebound light. The effect is tremendously brightening; if a light is shone on a road sign with reflective sheeting at night, the sign almost appears to have its own light source. Reflective sheeting is so effective that it is now Federally required for most traffic signage.


3M recently announced that future signs will be even more visible for drivers. Updated materials will allow for nearly twice as much reflected light, produced by the brightest and most visible materials employed on the roads today. High intensity sheeting with an encapsulated lens is the standard for these new road signs, which will have a service life of ten years.


This technology is more widely applicable than consumers might initially think. Consider, for example, the Scotchlite 680, a retroreflective bicycle. The bike appears black, but turns a bright white when light shines on it.
Signs of traffic
Above, this stop sign embedded with catseves is shown without a direct light source. Below, on the same sign, you can see the beads brighten an illuminate when they are exposed to light.
Signs of traffic

When it comes to road signs, it’s simple: more reflectivity means more safety. Listed below are a few of the MUTCD’s recommendations for upholding sign reflectivity:


1. Professional inspection


A trained sign inspector is responsible for assessing existing signage, with underperforming signs marked for replacement. These inspections are performed in a moving vehicle.


2. Measuring retroreflectivity


Certified inspectors use retroreflectometers, and signs whose levels fall under the minimum are marked for replacement.


3. Sign life expectation


After every sign installation, the date of the installation is recorded directly on to the sign. Every sign has a known expected life, and if the sign is nearing its end, or showing early wear, it is replaced.

4. Simultaneous replacements


Scores of signs are typically installed at one time in particular areas, and should be replaced as such. There are designated intervals for sign replacements based on the expected sign life.