In this paper, 4 expressway sections under relatively good meteorological observation condition are selected as samples from the top 10 expressways in China with frequent traffic accidents in fog days. The main characteristics of traffic accidents in fog days are analyzed, and the distribution of traffic accidents in different visibility intervals is quantitatively studied. Compared with the traffic accidents in sunny days, the traffic accidents in fog days are more serious and the casualty rate is increased significantly. The main types of accidents are collision on fixture and scraping. The proportion of vehicles of heavy trucks, medium trucks and large buses is higher than that on sunny days. The main causes of traffic accidents in fog days are the driving in the same lane without keeping the safe distance from the front vehicle, improper operation, and driving not according to regulations under low visibility. Statistics show that, traffic accidents in fog days mainly occur in the early morning of fall or winter seasons, and the accident peak is around 7:00 in the morning. This is consistent with the temporal distribution of fog days. 70%-90% of traffic accidents in fog days occur when visibility is less than 1km. And the proportion (25%-50%) of traffic accidents occurring when visibility below 100m is the highest. When the visibility is less than 1 km, the probability density of traffic accidents decreases with the increase of the visibility. Especially when the visibility is less than 300m, the probability density of traffic accidents grows rapidly. When the visibility is less than 100m, the probability density of traffic accidents increases to about 30%. According to the distribution of traffic accidents in different visibility intervals, the influence of the visibility on the traffic safety is divided into 4 grades, and the meteorological visibility can be directly related with the traffic safety level.