The scope of the report is to discuss the efficiency effect of different methods for friction control of roads on the safety and accessibility due to the climatic conditions, and to establish climatic parameters that may be helpful in selecting the strategies for winter maintenance. The report is based on the analysis of the data assembled for the VTI project “Winter Model”, and presents mainly comparisons between accident data, operational standards for winter maintenance and climatic conditions in the different regions of Sweden. In addition, recommendations for selecting strategies for winter maintenance are presented.
Salt and gritting are the main methods for friction control on winter roads. In the latest years the method of using warm wetted sand has been developed and the method represents an alternative to salting in stable cold climates. The favourable weather situation for the warm wetted sand method is assumed to be road surface temperatures below -1°C and less than 3 mm water equivalent (WE) within 6 hours. The corresponding values for recommending salt are assumed to be above -8°C and less than 1 mm WE/hr.
The basis for the data used in the present analyses is all police recorded accidents, except wildlife accidents, on the state roads in Sweden and data of road surface conditions recorded by the road autho¬rities in the 1993/1994 to 1996/1997 winters. The accident data are broken down into:
o Types of accidents and severity of the accidents.
o Road conditions recorded by the police at the time of the accidents.
o Vehicle mileage on the different road conditions.
o Maintenance standard for the roads.
The accident analyses include only fatalities and severe injuries and are made separately for:
o Four climatic regions; Southern, Central, Lower Northern and Upper Northern Sweden.
o Three maintenance standard classes; A1+A2 and A3+A4, which require salting and B1+B2, which is based on gritting.
o Two road conditions: bare roads and snow/ice covered roads.
The accident rates for driving on snow/ice covered roads are dependent on the proportion of vehicle mileage on snow and ice relative to bare roads and the rates increase for lower proportions. The accident rate for snow/ice covered roads compared to bare roads is as high as 8–10 for vehicle mileage proportions of approximately 0.05 and 2–3 for proportions exceeding 0.5. For instance, 30–40 per cent of the accidents recorded on the A1+A2 roads in Southern and Central Sweden occurred during the 4 per cent of the time the roads were covered by snow or ice.
The calculated numbers of accidents related to driving on snow and ice are lowest for the salted roads in the three southern regions of Sweden, but the opposite for Upper Northern Sweden, where there is a significantly higher number of accidents on the salted road network.
The data material indicates that there is a maximum of accidents when the vehicle mileage ratios on snow and ice are between 0.3 and 0.4. If these indications are correct one should preferably introduce salting if the ratio of snow and ice on unsalted roads is 0.3–0.5 and the use of salt may reduce the ratio to less than 0.2. On the other hand, by introducing salt in climatic areas having ratios exceeding 0.5 and ending up with ratios close to 0.3 will probably increase the number of accidents.
The ratio of the summer/winter accident rates for fatalities and severe injuries is higher for salted than for unsalted roads in all four climatic regions. The ratio is 1.2 for salted roads in the three southern regions and 1.4 for Upper Northern Sweden. The corresponding ratios for unsalted roads are 0.95 and 1.05. The data material makes it further possible to calculate the accident rates for driving on bare roads in the winter and compare these rates with the corresponding summer accident rates. The calculations indicate that the accident rates for driving on bare roads in winter are on average 0.55 for the unsalted roads and 0.75 for salted roads. Probably drivers expect on unsalted roads the roads to be partly covered with snow or ice in the winter and they reduce their speed even when the roads are bare. Similarly, drivers on salted roads expect the roads to be free of snow and have thus a higher speed when the roads are bare.
The development of the warm wetted sand method and the experience with high accident rates in very cold climates should be reflected in the selection of strategies for the winter maintenance of roads. The report has introduced three climatic parameters, which are mainly recommended to be used on a monthly basis:
- Winter Severity Index, Wsev; proportion of recordings < -8°C
- Winter Stability Index, Wstab; proportion of days favourable for the warm wetted sand method
- Winter Instability Index, Winst; proportion of fluctuations around 0°C per day.
The high accident rates found for the salted road network in Upper Northern Sweden form the background for recommending that salt should not be used in areas with winter months having Wsev exceeding 0.2, which means that the 20 per cent of the recordings are below -8°C. In areas with such climate, the warm wetted sand method is usually a reliable alternative, even for relatively high traffic loads.
The warm wetted sand method is most favourable in cold stable climates, but since the increase in cost compared to traditional gritting is relatively small, one may justify using warm wetted sand even in conditions when the probability of the long lasting effect is low. Warm wetted sand is only recommended for use in periods, and on roads with AADT <2,000, when the Winter Stability and the Instability Indexes are within the area represented by the following points in the Wstab-Winst diagram:
o Wstab =1 and Winst =0
o Wstab =0.2 and Winst =0
o Wstab =0.4 and Winst =1.2
In areas and periods with lower values of Winst represented by these limits, traditional gritting is the only alternative to salt at present.
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created: Ferenc Rajcsanyi, 02.07.2009 11:41:00 last modified: Ferenc Rajcsanyi, 23.07.2009 11:37:00
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