Overhead guide sign lighting has been used by many agencies to improve visibility. However, the availability of newer and more efficient retroreflective materials has created a new challenge for state transportation agencies going through sign sheeting upgrade programs and considering the need for using sign lighting, as there is no existing answer regarding whether upgraded sign sheeting itself can meet drivers’ nighttime visibility demands without external sign lighting. The purpose of the paper is to investigate whether high intensity reflective sheeting can be used to replace overhead guide sign lighting. The conclusion of the study provided a guidance for agencies to adopt their current sign lighting policy. A luminance computation model is used in the paper to calculate overhead guide sign legend luminance under various situations, including different sign lighting technologies, different geometrics and overhead guide sign locations, and different amounts of sign dirt and sign aging. By comparing the calculated luminance of a specific overhead guide sign at a specific situation with the legibility luminance levels required by older drivers, sign lighting needs are assessed. In addition, a life-cycle cost spreadsheet is developed and used to calculate the cost of replacing the current sign sheeting with high reflective sheeting and the cost of installing/upgrading sign lighting. Eventually, the most cost effective approach to maintain overhead guide luminance is discussed.
sign lighting, visibility, retroreflective sign sheeting, overhead guide signs, sign legibility
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