Research on factors affecting the conspicuity of signs and signals, especially the conspicuity of road traffic control devices, has been summarized by Jenkins and Cole (1986) and the Commission Internationale de L'Eclairage (CIE, 2000). The principal factors affecting the conspicuity of signs are angular size, the boldness of the internal graphics, the luminance contrast of the sign with its immediate background, the visual complexity of the background, and the angle between the direction of the sign and the direction of gaze. At night the luminance of the sign is also a factor.
The color of a sign has not emerged as an important determinant of conspicuity. The CIE report (2000) observed that the effect of color on sign conspicuity has not been studied rigorously, although some trends have emerged as a by-product of studies of other parameters. The CIE report tentatively concluded that white signs seen at night may need to have a higher brightness than do red, orange, green, and blue signs to achieve the same conspicuity. This suggests that these colors contribute to sign conspicuity.
Odeschalchi (1960) asked a panel of observers to subjectively judge the effect of sign area on conspicuity of road signs in a rural environment in daylight. He found that white, yellow, and red signs need to be about the same size to give what was judged to be adequate conspicuity (although yellow signs may give slightly greater conspicuity than do white signs), whereas green and blue signs need to be larger.
Forbes, Pain, Joyce, and Fry (1968) had observers rate the relative attention-getting property of pairs of differently colored simulated signs seen against four different-colored background scenes. These researchers found that white, yellow, and red signs were rated equally in attracting "first attention," whereas, in decreasing order, green, blue, and black attracted attention less well. However, Jenkins and Cole (1979) found that red, blue, and yellow discs superimposed onto projected colored scenes of the urban road traffic environment were no more conspicuous than grey discs of the same luminance.
It is surprising that color has not been identified as an important determinant of the conspicuity of signs, considering that people's everyday experience is that color -- especially vivid color -- can attract attention. Moreover, it has been shown that search time for colored objects is significantly less than that for the same achromatic objects when the color is unique to the target and the observer knows the color in advance (Christ, 1975). Under some circumstances color has been shown to have a pop-out characteristic, meaning that the colored object is immediately obvious, a quality that is independent of the number of distractor elements in the background. This is evidence of a parallel-processing search mechanism, which D'Zmura (1991) attempted to elucidate. However, target color seems to have little benefit to visual search when the color is not known to the observer in advance and when the proportion of nontargets (distractors) that have the same color as the target exceeds 7. Christ (1975) presented an elegant analysis of data on the role of color in target location and identification.
The advent of color-coded electronic visual displays provoked further research on the role of redundant color coding in information acquisition for complex electronic displays, in which color is not the primary source of the information displayed but is used only to reinforce the message or to provide visual organization. Although Tullis (1981) found no improvement in response time for a redundantly color-coded graphic display, both Luder and Barber (1984) and Macdonald and Cole (1988) found better performance for color-coded electronic aviation displays than for the same display without color.
This paper reports a study that aims to determine whether redundant color coding contributes to the conspicuity of road traffic signs and signals. …