The heat balance of road snow was analyzed to examine ice-film formation and to determine the characteristics of heat balance on a natural snow cover. The main heat source used to melt natural snow cover is solar radiation. However, the results of the analysis indicated that the absorbed solar radiation is reduced due to the high albedo of snow and therefore cannot compensate for the heat loss by long-wave radiation. This occurrence results in negative net radiation during winter. To evaluate the heat balance on road snow, the infrared radiation from vehicles was estimated by measuring vehicle temperatures. These measurements indicated that heat input applied to snow decreased with vehicle speed and increased with traffic volume. A new index—the shading time—was developed and is defined as the amount of time a section of road is shadowed by vehicles. It was established that incoming long-wave radiation increased by 50 percent during 20 minutes of shading time and that the net radiation became positive, suggesting that snow melting occurred on roads with heavy traffic flow, even in winter.
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.
Many factors are thought to relate to the occurrence of multivehicular collisions during snowstorm. These factors were analyzed by using a database and observations of multivehicular collisions. In addition to severe road weather conditions and surrounding environment during snowstorm, the drivers’ driving attributes and the traffic factors also were found to relate closely to such accidents. From these results the risk factor level was examined to assess the occurrence risk of multivehicular collision. When these risk factor levels were applied to collision cases, they closely expressed the occurrence risk. Although many challenges remain in developing a quantitative evaluation method of risk assessment, the factors identified in this study enable objective assessment of the risk of multivehicular collisions during snowstorm by consideration of the road weather, environmental, and traffic conditions of the site.
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