LUMINANCE: A SYSTEMS APPROACH TO THE VISIBILITY OF TRAFFIC CONTROL DEVICES

Traffic control devices serve to guide roadway users, to warn of actual or potential hazards, and to announce relevant regulations. For the purposes of this paper, traffic control devices are defined as permanent or temporary signs, road markings, and work zone delineation devices. In order to serve all their functions, traffic control devices must be seen both day and night. The primary factor in nighttime visibility is brightness, or luminance, contrast. Luminance contrast can be achieved by externally or internally illuminating the sign, or more typically by relying on automobile headlamps to illuminate the device. To utilize headlamp illumination, the traffic control device must be fabricated using special retroreflective material to direct the light back to the driver's eye. Because headlamps provide dynamic illumination from a moving vehicle, the amount of illumination is not constant. Retroreflection refers to the physical principle of returning light to its source. Light is returned in a cone, with virtually all of the reflected light within 3 deg (0.05 rad) of the light source. The amount of light returned depends on the geometry of the situation and the retroreflective properties of the material. The coefficient of retroreflection characterizes materials at a certain geometry. This number is often used in specifications for traffic control devices.