At the beginning and end of a winter season, there is a greater risk of car accidents in ice and snow conditions than during mid-winter. This was found in a Swedish study where accident data for four winters was correlated to road condition data obtained through visual observations. The purposeof the study was to see how the car accident risk on ice and snow varies over the winter. To examine this, the winter season was divided into threeperiods: Early winter, Mid-winter, and Late winter. Accident rates were calculated for each period and the values for Early and Late winter were compared with the corresponding Mid-winter values. The accident rate (car accident risk) is here defined as the number of accidents divided by the vehicle mileage, where the accident data includes all the car accidents on Swedish state roads recorded by the police, during the winter seasons from 1993/94 through 1996/97, excluding collisions with wildlife. Accident rateswere calculated for five different types of road condition assessed by the police: Dry bare ground, Moist or Wet bare ground, Packed snow or Thick ice, Black ice or Hoar-frost, and Loose snow or Slush, where the last three are ice and snow conditions. The vehicle mileage for each road conditionwas calculated on the basis of visual observations performed to monitor the road condition. The analyses show that icy and snowy road conditions constitute a greater risk during Early and Late winter periods than during Mid-winter. The reason for this could be that drivers are more likely to expect these kinds of road condition during Mid-winter than at the beginningand end of the winter season. Moreover, the accident risk is greater during Late winter than during Early winter, and the shorter the Early and Late winter period, the higher the risk. For the covering abstract see ITRD E143097.
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