DETERMINE THE LUMINUS REQUIREMENTS OF RETROREFLECTIVE HIGHWAY SIGNING

The purpose of this study was to define the relationship between sign luminance and legibility in a way that would assist in selecting optimum material choices for various signing applications as well as aid in decisions concerning maintenance and replacement. A laboratory study was carried out to define the effects of luminance, contrast, color and driver visual characteristics on legibility distance. At the same time a computer model was developed which could predict the legibility distance of a sign, based on the laboratory data as well as geometric and photometric variables. A field study was then conducted in which legibility distance predicted by the model was compared with legibility distance measured on a number of real and simulated signs using a sample of normal drives. In general, the model's predictions were within 10% of the measured legibility distances. Data have been developed which show graphically the relationship between legibility distance and the photometeric properties of background and legend materials. In general, more highly reflective backgrounds permit somewhat greater legibility distances. Perhaps more important, reflectorized backgrounds reduce the effect of changes in viewing conditions, which can be quite substantial in the case of a non-reflective background. The contrast provided by the legend is very important. The optimum choice of legend depends on the reflectivity of the background and the sign location. Luminance contrast requirements are lowest for highly reflective backgrounds and increase as background reflectivity decreases. /Author/