Laboratory and field studies were conducted to assess the minimum luminance levels of signs that ensures that they will be detected and identified at adequate distances under nighttime driving conditions. Thirty subjects participated in the field study, driving a car on public roads and reporting when they could identify the test signs that were positioned at random points along the side of the road. Surround complexity, subject age, retroreflective efficiency, and sign color were considered. A study was also carried out to measure the effect of subject expectancy. All of the independent variables, including color, were found to have an effect on sign conspicuity. Forexample, sign retroreflectivity had to be increased by a factor of about 10 to achieve equivalent conspicuity when going from areas of low to high complexity and by a factor of about 3 to compensate for the effect of subject age. The colors red, orange, green, and blue had substantially greater conspicuity than did yellow with equivalentretroreflectivity. Possible reasons for the latter finding are discussed. Minimum retroreflectivity recommendations and the rationale for their development are presented for stop signs, construction areawarning signs, warning signs, and overhead guide signs. This paper appears in transportation research record no. 1247, Visibility criteria for signs, signals, and roadway lighting.
Abstract