In today's road administrations it is very important to keep track of costs and the amount of chemicals and abrasives used in winter road maintenance. Variations in snow and ice conditions from year to year make it difficult to compare figures. In order to tackle this problem, the Swedish National Road Administration (SNRA) has developed a new Winter Index. The Winter Index is based on data from our 680 Road Weather Information System (RWIS) stations as well as data provided by the Swedish Meteorological and Hydrological Institute (SMHI). Basically, we use the former for information on the air and road surface temperatures, humidity, wind and type of precipitation and SMHI data for the amount of precipitation. The RWIS data is collected every half-hour and the SMHI data every third hour. To calculate the amount of precipitation, SMHI uses a model called Mesan, which is an operational Mesoscale Analysis System. This model subdivides Sweden into a 22 by 22 kilometre grid net, and calculations are performed for each grid individually. The system provides data on slippery roads, snowfall and snowdrifts expressed in number of occasions; e.g., two icy road surface occasions will be registered if it is known that a skid control measure will be effective for 5 hours, and the RWIS data shows that there still is a risk of slippery roads after 6 hours. The same principle applies to snowfalls and snowdrifts. The system can detect four kinds of slippery surfaces ranging from light frost to freezing rain, and three kinds of snowfall and snowdrift, from light to heavy. The amount of material used and the cost involved is entered in the final step of the calculation. This provides a good basis for comparing salt consumption from year to year. When calculating the Salt Index in this model, the length of road treated with salt, type of road (standard class) and our "Guidelines for Salt" are used. A Salt Index of 1.0 indicates that the contractor (or county, regional, or national road manager) has used the optimum salt dosage. The system is also a good tool for benchmarking costs. (Author/publisher)
Samenvatting