How Drivers Perceive Visibility in Blowing Snow: Human Subject Experiments on Visibility by Viewing Videos of Blowing Snow.

Road meteorological observatories measure visibility using visibility meters. However visibility values measured by visibility meters stem from themeteorological definition, which comes from a perspective different from that of road use. Subject experiments were therefore conducted using roadvideos in blowing snow conditions to clarify the difference between visibility perceived by drivers and that measured using a visibility meter. Theexperiments revealed that the visibility perceived by drivers in blowing snow is approximately 70 meters lower than conventionally measured visibility. Also, a high correlation is observed between the visibility perceived by drivers and the sum total of projected area of snow particles passed through a unit area in a unit time. It was also learned that the visibility perceived by drivers during blowing snow is affected by the intensity of visibility fluctuation, the presence or absence of snowfall, road surface conditions, the surrounding environment (i.e. urban or suburban) and thetime of day, and is hardly influenced at all by the direction of the snowstorm. Based on the above results, the need for a visibility index for road traffic in blowing snow was suggested.