I mpact of fog on the safety and smooth flow of expressways: 1. Reduce visibility b ecause fog scatters light and absorbs light, the transparency of the sight is reduced, and the driver cannot see the situation in front and surrounding area . It is necessary to judge the distance between vehicles and the variable information boards, signs and markings and other traffic safety facilities. It is difficult to distinguish. In addition, because the intensity of heavy fog may be not the same on different road section s, it i s difficult for the drivers to adjust their speed and distance between vehicles in time according to the different visibility of each road section. In this case, traffic accidents are vulnerable to happen . 2. Reduce the friction coefficient between the vehicle and the road surface. Due to the mixture of fog, and dust, the sticking coefficient between the tire and the road surface is reduced. Especially in the northern winter, the ice fog will form a thin lay