Citizens in America began forming automobile clubs in the early 1900s. These groups took it upon themselves to mark their local sections of highway with signs to warn and direct drivers. This scattered effort resulted in a wide variety of sign designs and messages in different sections of highway, which caused confusion among motorists. In 1924, the first steps toward national uniformity in road signs were taken by the Bureau of Public Roads. Designers were faced with developing signs to guide a largely illiterate population of motorists. As a result, color and shape were especially important components of signs from the beginning.
Early signs lacked the reflectorized technology prevalent in signs today. In 1924 bright yellow was chosen as the background color for all warning signs, and white was the background color for all remaining signs. These lighter colors provided the greatest contrast with black lettering, especially when the signs were seen with the aid of headlights at night. Later signs used glass beads to produce a reflectorized effect at night. Beads—approximately 0.75 inch (20 mm) in diameter—were glued on the signs in the shape of numbers (such as the speed limit) or symbols to inform and warn nighttime drivers.
The development of retroreflective sheeting by the 3M company in the 1940s changed the face of traffic signs forever. This material, with reflective elements like glass beads on or under a transparent plastic film, enabled better visibility of signs at night. Unlike diffuse reflection and mirror, or specular, reflection, retroreflection allows a surface to return a portion of light to the original source. In diffuse reflection, the reflected light is scattered in all directions, as when sunlight bounces off a car. In mirror reflection, light bounces and reflects off the surface at an angle opposite to the source; this is similar to a pool ball striking the table cushion at a shallow angle and bouncing to the other end of the table. Retroreflective material, on the other hand, allows light beams to “bend” and return toward the original light source.
The first traffic sign using reflective sheeting was installed on the outskirts of Minneapolis in 1939. The surface of the sheeting was covered with tiny glass beads that produced the desired retroreflectivity. However, dirt tended to accumulate on the grainy surface and during wet weather, the water would coat the surface and diminish the reflective effects of the beads.
These problems were solved within a couple of years. An enclosed lens system was developed, essentially by covering the beaded sheeting with a transparent film that maintained the surface’s retroreflective qualities. This type of sheeting, referred to at the time as “flat-top sheeting,” is now known as engineering grade sheeting. It is the most economical grade and can be used on signs in areas with light traffic and slow speeds.
The next major development came in the late 1960s when encapsulated lens sheeting was invented, basically by adding a resin base and an additional reflector coat behind the glass beads. This high intensity material is three to four times as bright as engineering grade, and it retains its reflectivity longer; it is now the most commonly used type of reflective sheeting.
Another significant innovation came in 1989 with the substitution of microscopic prismatic reflectors for the traditional glass beads. There are about 7,000 microprisms per square inch (about 10 per sq mm) of this type of sheeting, producing about three times the brightness of the encapsulated lens variety. This is the most durable and most costly type of high performance sheeting currently available.
Source: How Products Are Made