IES RP-19-01

Roadway signs that are designed to be eligible under daylight conditions can also be lighted to fulfill their basic purpose at night. Well-designed sign lighting can aid the driver in rapid, accurate recognition and uderstanding of the sign's message. This serves to improve safety by reducing the possibility that motorists will stop or drastically reduce speed at signs that may otherwise be difficult to read, and provides the sign legibility necessary for decisive driver action. Lighting of roadway signs becomes more significant as volume of traffic increases, complexity of highway design increases, adverse weather in specific locations is prevalent, and ambient luminance increases. Elements of roadway sign lighting, both internally and externally lighted signs, are discussed, as well as maintenance factors, sign surface reflectance and color change considerations. This publication does not cover aspects of signs (shape, letters, symbols, size or location), which are included in applicable federal regulations and by highway agencies. It also does not address changeable message signs.