Solid-State Roadway Lighting Design Guide: Volume 1: Guidance (2020)

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CHAPTER 9 Roadway Sign Lighting Current Guide The current AASHTO Roadway Lighting Design Guide (AASHTO 2018) was updated to include the luminance levels of signs on the basis of the visual complexity of the roadway surroundings. Uniformity levels are also given for signs. The color of the light source as well as its placement are also discussed. Additional Considerations for LED Sources Using an LED light source can provide additional options for sign lighting that are not available with traditional light sources. Light sources can be placed to use the sign sheeting retroreflectivity to enhance the sign luminance and reduce the overlighting and waste of light that occurs with traditional sources. LEDs can provide tight optical control that will limit the light output to the sign face itself and thus limit the amount of light spread past the sign. An alternative location for the light source is also to be considered. Use of a very tightly controlled LED luminaire allows placement of the light source up to 100 ft from the location of the sign. The light source should be located in the general direction of the driver’s approach and aimed at the sign to achieve any retroreflective benefit. This installation method uses the sign’s reflectivity to provide better sign luminance than typical installations provide and is more closely aligned with the headlamp performance, which is the design mode of the sign sheeting. This approach to lighting may also reduce maintenance issues, because the LED can be serviced from the side of the road rather than over the roadway on the sign truss. Key Issues for Roadway Sign Lighting • Choose the LED source color to properly render sign colors. An LED with a higher color- rendering index (CRI) will provide the best color appearance. • Minimize overlighting and waste of light beyond the sign itself by using a tight optical control to limit the light from the luminaire to within the size and shape of the sign. 41