Minimum Luminance of Highway Signs Required by Older Drivers

Abstract

No standard for a minimum level of highway sign luminance or retroreflectivity exists. The purpose of this research was to establish such a level. Because of the increasing number of older drivers and their diminished visual capabilities, the focus is on sign luminance requirements of these drivers. Forty-two subjects 65 years of age or more and 19 subjects 25 years of age or less participated in this project. All subjects had a valid driver’s license and wore corrective lenses during the field test if they normally wore them while driving. The field study determined the minimum sign luminance required for the subjects to read a sign with black numerical data on a yellow background. The subjects viewed seven yellow warning signs on a simulated roadway in a large asphaltpaved parking lot. The signs were viewed at distances of 30, 40, 50, 60, 70, 80, and 90 m (32.7, 43.6, 54.5, 65.4, 76.3, 87.2, and 98.1 yd). The sign luminance was varied by using signs of differing retroreflectivity. All tests were conducted at night with low-beam headlights and no fixed roadway lighting. The sign luminance requirements for the two groups has been determined and compared.