Abstract
Unlighted highway signs, which use newly developed retroreflective materials, were installed along the major expressway in an urban area by the local department of transportation. Photometric measurements of the signs were used to assess their legibility applying the relative visual performance model, in comparison to lighted signs, con‐ forming to recommended illumination practices. The calculated visibility of the measured unlighted signs was similar to that of the signs equipped with exterior sign illumination. The practical significance and limitations of the relative visual performance approach are discussed.
Highlights
Overhead highway signs constitute an important part of the roadway visibility system (Van Derlofske et al 2001)
Exterior illumination would normally be used for highway signage in such locations, but if newer, unlighted materials could result in a similar visibility as lighted signs, savings in terms of energy and maintenance might be achieved as a result, as well as reductions in light pollution
The measured luminances and luminance contrasts, and the resulting relative visual performance (RVP) and visual response time values, generally indicate that the unlighted highway signs constructed from new retroreflective materials installed along the expressway were similar, in terms of visual performance, to exterior-lighted signs meeting AASHTO (Roadway Lighting Design Guide 2005) recommendations for sign illumination, when viewed from 100 m away
Summary
Overhead highway signs constitute an important part of the roadway visibility system (Van Derlofske et al 2001). Since the angle between a driver’s sight line to a sign and the light rays from the driver’s headlamps is usually small, especially for far viewing distances, retroreflective materials are good at directing headlamp illumination toward drivers As this angle increases, the luminance of a retroreflective material will generally decrease. Many scientists have investigated the effectiveness of different materials on sign visibility either through measurement of sign luminance or through studies of visual performance or subjective evaluation These studies have corroborated the promise of several newer reflective materials (e.g., types VII, VIII and IX) to provide superior performance to more commonly used materials (e.g., types I and III) in terms of higher luminances or longer legibility distances Exterior illumination would normally be used for highway signage in such locations, but if newer, unlighted materials could result in a similar visibility as lighted signs, savings in terms of energy and maintenance might be achieved as a result, as well as reductions in light pollution
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