Road meteorological observatories measure visibility with visibility meters. However, visibility values measured by visibility meters stem from the meteorological definition, which comes from a perspective different from that of road use. Subject experiments were therefore conducted with road videos in blowing snow conditions to clarify the difference between visibility perceived by drivers and that measured with a visibility meter. The experiments revealed that visibility perceived by drivers in blowing snow was approximately 70 m lower than conventionally measured visibility. Also, a high correlation was 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 was 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 the time of day, and is hardly influenced at all by the direction of the snowstorm. On the basis of the above results, the need for a visibility index for road traffic in blowing snow was suggested.

Afterimages of snow particles in blowing snow may cause lower visibility of the human eye compared to images captured by a digital video camera. However, the impact of the afterimage of snow particles on the visibility of the human eye in blowing snow has not been investigated quantitatively. Here we examined visibility taking the effect of afterimages of snow particles on the human eye into consideration. By considering the duration range of afterimages, and changing the windspeed while the snow density remains constant, our numerical model of visibility for the visual duration range of 0.12 to 0.04 s could accurately explain the visibility of the human eye previously observed in blowing snow.

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