Nighttime legibility of traffic signs: conditions eliminating the effects of driver age and disability glare

Abstract

The effects of observer age and environmental glare on nighttime legibility of traffic signs were investigated in two field experiments with the subjects driving or riding in a car towards a sign. Experiment 1 showed that equating older and younger subjects in terms of their low luminance/high contrast visual acuity resulted in elimination of any age effects on legibility. Furthermore, the presence of a glare source with an illuminance of 0.17 or 0.017 lux offset 2[deg] from the sign legend improved legibility distance significantly. The results of Experiment 2 indicated that a glare source with an illuminance of 0.0098 lux had no effect on legibility when presented at an offset angle of 1.5[deg] or 0.6[deg], but it had a significant detrimental effect at an offset angle of 0.2[deg]. The present findings suggest that (1) the usually observed age-related performance decrement on nighttime legibility tasks is the result of visual-acuity deficits, and not shortcomings in information-processing ability; (2) legibility is relatively unaffected by glare, unless the glare angle is very small or glare level very high; and (3) glare sources positioned outside of the fovea might improve nighttime legibility performance under certain conditions.