The Nighttime Legibility of Highway Signs as a Function of Their Luminance Characteristics

Abstract

A laboratory study was carried out to define the effects of luminance, contrast, color, and driver visual characteristics on sign 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 drivers. In general, the predictions were within 10% of the measured legibility distances. The results indicate that 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 non-reflective backgrounds. The contrast provided by the legend is very important. Luminance contrast requirements are lowest for highly reflective backgrounds and increase as background reflectivity decreases.