Two factors: darkness and slippery road surfaces have a multiplicative effect on accident RISK (accidents/kilometres driven).
The amount of EXPOSURE (kilometres driven) on slippery conditions has a major effect on how much the accident risk is going to increase. There is a tendency that the share of accidents on slippery road surfaces is constant irrespective of the share of exposure driven in such conditions; that describes the risk differences.
The development of vehicle technology has improved traffic safety, especially in winter conditions during the last decade. The main single technical development has been the increase of front wheel driven cars. The effect seems to be indirect. Earlier, other technical factors or varying distribution of user categories could increase the accident risk of one vehicle (car) model. Front wheel drive “forgives” the negative effects of these kinds of factors.