Skidding resistance on snowy and icy roads is much less than that on clear roads; hence, it may be said that, in many cases, traffic accidents under such road conditions are closely related to skidding. We investigated the road conditions, skidding-resistance value, and their relationship to traffic-accident rates during winter. The model that estimates the number of days of slippery road conditions is constructed. Monthly days of slippery road conditions in 1982, 1983, and 1984 are calculated by using this model. We carried out measurements of the skid-resistance number on slippery road surfaces on Hokkaido, comprising approximately 5000 individual skid tests on slippery road surfaces. An accident-rate predictive model for winter is designed from the road conditions and the meteorological conditions. Road-surface conditions under no-snow conditions during winter is assumed, and reduced number of accidents are estimated.