A frequent dilemma facing highway agencies is deciding how to provide effective nighttime performance for overhead guide signs. While the Manual on Uniform Traffic Control Devices (MUTCD) provides minimum retroreflectivity standards for overhead guide signs, there are few guidelines that agencies can reference to decide how to provide sufficient nighttime performance of overhead guide signs in site-specific situations.
For instance, the common wisdom is that it is adequate to turn off overhead guide sign lighting in rural areas if highly reflective sheeting materials are used. However, there is little consensus about the use of lighting in suburban and urban areas where visual backgrounds and roadway geometries are more complex. Minimum sign retroreflectivity requirements are designed to provide conspicuity and legibility for all signs including overhead guide signs. In selected visual environments, such as urban areas with high visual complexity, higher levels of retroreflectivity or illumination or both may be needed to provide adequate conspicuity and legibility. (read more)