This paper describes how road visibility information is important for safe driving, and that there is a particularly great need for such information in winter. Road administrators provide drivers with road images that were gathered using closed-circuit television (CCTV) cameras installed along national highways. However, when road condition information is provided by image, the provision media are limited. The Hokkaido Regional Development Bureau has been looking for ways to quantify road image information. If it can be proven that the estimation of road visibility for the same road image does not differ among people, and if the drivers' assessments of visibility correlate with assessments given by a new, road-image-based scale, that scale can be used as a scale of road visibility. The scale is called the Visibility Scale (VS). In this study, VS is calculated from the Weighted Intensity of Power Spectrum (WIPS) developed by Hagiwara et al. The paper surveyed subjective assessments of road images that were taken by CCTV cameras. For daytime images, the assessments had little dispersion among subjects and they correlated closely with VS. For nighttime images, the assessments had little dispersion among subjects but they did not correlate closely with VS. To determine the best way of providing VS information, subjects' with the understanding of that information given according to various provision media (text, numerical rank, symbol) were surveyed, and it was found that there was little difference in understanding according to provision media.
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