The legibility of retroflective road markings, and in particularof signs, depends on many parameters, notably the luminance contrast between the message and the ground of the sign. For a given level of illumination, the luminance contrast itself depends on the retroreflection coefficients (R') of the materials used. There are a number of retroreflective products, which have different values of R', onthe market. The present study was aimed at determining how variations in R' affect visibility distance (d) at night for a driver at thewheel of his vehicle, the headlights of which illuminate the sign. In the first stage, a small-scale experiment (approximately 1/10th scale) was conducted in the laboratory to identify the variables. LEMegibility thresholds were determined by presenting different combinations of the alphabets and colors used on road signs to observers at different luminance contrast levels. In the second stage, working from these results, simulMMations were carried out to quantify the influence of the various parameters, in particular R' and d. It was found that R' is only one of the parameters that affect d. Most often, multiplying R' by 3 increases d by only 30 percent. But the dimensions of the letters used for the messages play a preponderant and limiting role because of their direct MZrelationship to visual acuity. According to earlier experiments, other parameters that are harder to quantify (incident illumination of the signs by vehicle headlamps; dirt on the headlamps, on the signs, on the windscreens; and weathe~r conditions) are important and may require a correction of d ranging from0 to 100 percent. The results are compared with those of similar studies carried out in the Federal Republic of Germany, the United States, and Japan.(A).
Abstract