The Assessment of Alternative Overhead Sign Lighting

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National Surface Transportation Safety Center for Excellence


This report evaluates an alternative method for lighting highway signs that takes advantage of their retroreflective properties. This method uses a single luminaire mounted some distance upstream of the sign, with a focused, but evenly distributed beam so that the sign always receives the same amount of illumination. To evaluate the proposed system, a human-subjects experiment was performed on the Virginia Smart Road to test two sign configurations: an overhead sign mounted on a gantry above the highway and a sign mounted at the side of the road on a horizontal curve. For the overhead sign, there were no statistical differences between the upstream lighting, traditional style lighting, and headlamp-only conditions, though there was an increase in the mean legibility distance of the sign (~14 m) when the upstream luminaire was located on the shoulder of the road at the highest luminance setting. For the horizontal curve, upstream lighting provided no benefit for sign legibility due to the ambient lighting in the vicinity, but placing the luminaire closer to the sign (approximately 20 m) resulted in more consistent luminance from a wider range of viewing angles. Although upstream lighting provided legibility distances similar to traditional sign lighting, it may offer advantages due to reduced maintenance and energy costs.



transportation safety, illumination, lighting, energy efficiency, highway maintenance, highway signs