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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.

ABSTRACT. An experiment using artificial snow was conducted to clarify the influences of temperature and normal load on temporal variations in the shear strength of snow. Artificial snow consisting of dendritic crystals was allowed to accumulate to $ $60 cm depth for the experiment. The shear strength, temperature and weight of the overlying snow were measured at three different depths in the accumulated snow. For the measurement of shear strength, the shear frame index (SFI) was found using a shear frame by placing weights with different masses on the snow contained within the frame. The measured SFI values were treated with the Mohr-Coulomb failure criterion to find the snow cohesion factor C and the internal friction factor tan . The results highlighted similar trends for SFI and C values by which their rate of increase over time was greater with higher snow temperatures and normal load caused by overlying snow. This indicates that C contributes significantly to increased SFI values. tan decreased over time with higher snow temperatures and increased with lower snow temperatures. In low-temperature conditions, in particular, it is likely that snow crystals are compacted but maintain their dendritic morphology.