Relationships between road slipperiness, traffic accident risk and winter road maintenance activity

A method for deriving quantitative relationships between road slipperiness, traffic accident risk and winter road maintenance (WRM) activity is described. The method is also applied to data from an area in southern Sweden. If a specific type of road slipperiness represents a large accident risk despite high WRM activity it is important to increase public awareness during such periods. If the type of slipperiness represents a large accident risk but is accompanied by low WRM activity, it is also important to increase the WRM to reduce the accident risk. In the method, a road slipperiness classification, based on atmospheric processes, is used to classify the road conditions at the time an accident occurred. The road condition is classified either as non-slippery or as one out of 10 types of slipperiness. Data for the slipperiness classification are taken from the Swedish Road Weather Information System (RWIS). Results from this study show that the traffic accident risk was different for different types of road slipperiness. Highest accident risk was associated with road slipperiness due to rain or sleet on a frozen road surface. When accidents occurred in these situations, there was always high WRM activity. This indicates that, in order to reduce the accident rate during these situations, public awareness must be increased by providing information to drivers. The study also demonstrates the benefits of applying a standardized road slipperiness classification to all kinds of sources of road safety information, such as a RWIS, traffic accident reports and WRM reports. With a standardized and objective classification of the road conditions and digitally stored data, all evaluations are easily conducted.