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.