A nighttime field study was conducted to assess the effects of retroreflective material area, distribution, and color on judgments of conspicuity. Participants, seated in a stationary vehicle, took part in a pairwise comparison of the stimuli. The independent variables included retroreflective power, area and distribution of the retroreflective material, color of the retroreflective material, participant age, and participant gender. The results indicate that color (white, fluorescent yellow-green, and fluorescent red-orange) was a significant factor in the judgment of conspicuity, as might be predicted from the Helmholtz-Kohlrausch effect. In addition, color interacted with the distribution of material at the high level of retroreflective power. The area of the retroreflective material was also significant. The present study, in agreement with a number of previous studies, indicates that color influences the conspicuity of retroreflective stimuli, but that the results are not always in agreement with color correction factors prescribed in ASTM E 1501. The discrepancy between empirically derived color correction factors seems to be attributable to an interaction of the stimulus size (subtended angle) and color, which previous studies have not extensively examined. To a lesser degree, the retroreflective power of a material also appears to influence conspicuity. While the ASTM correction factors may be appropriate for intermediate subtended solid angles, particularly for nonsaturated colors, smaller correction factors appear appropriate for markings subtending small angles (approaching point sources), and larger factors for larger subtended angles of saturated stimuli
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