Road signs use shapes, colors, words, and symbols to communicate a message to drivers. Without such signs, the movement of traffic would be disorderly and unpredictable. Virtually all traffic signs use retroreflective sheeting, which is designed to reflect some of the light from vehicle headlights back to the driver so that the sign will be visible at night. Color and shape can also provide cues to motorists even when the words or symbols on the sign are unintelligible. Regulatory signs, such as speed limit signs, are usually rectangular and use a white background. Stop signs, on the other hand, have a distinct octagonal shape and a red background in order to catch the driver's eye.

Designers must utilize elements like shape and consider material properties in creating signs that drivers can see and understand in time to react appropriately. Contrast, which is a measure of the brightness of the message in relation to its background, is an important property of any sign. The environmental backdrop—usually green vegetation and blue sky—must also be considered in the design process. A border is placed around all signs to distinguish them as geometric shapes in contrast to nature.

In order to maintain similar appearances among traffic signs, the federally approved Manual on Uniform Traffic Control Devices (MUTCD) provides specifications for sign dimensions and the use of symbols. In addition, the MUTCD prescribes that all signs be either reflectorized or illuminated.

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

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.

Traffic signs consist of three basic components: a blank, background sheeting, and sign copy. Blanks, usually constructed of plywood, aluminum, or steel, serve as the framework of the sign. Plywood is the least expensive blank material. It is fairly strong but is susceptible to weather damage since it is porous; plywood blanks must be overlaid with a thin layer of plastic. Aluminum will not rust, but it is very lightweight and must be reinforced with metal braces along the back. It is the most expensive blank option. Steel is a more economical alternative to aluminum; it is also more sturdy and does not need reinforcement. Rusting can be prevented by applying a coat of zinc to the steel blank.

Background sheeting and the letter and symbols for the sign copy are cut from retroreflective sheeting. This sheeting consists of tiny glass beads or microprisms embedded in a flexible plastic surface; this construction allows light from car headlights to be reflected off the sign and back to the driver. Colored light is reflected from the sign if the sheeting is dyed with a pigment. For instance, to make "STOP" signs, red dye can be added to the sheeting mixture when it is in a liquid form.