The mathematical description of low visibility over highways is intruduced. It proves the necessity of monitoring and forecasting of low visibility on the basis of the theory of phase change and the actual sudden change of visibility and the braking distance requirement. The monitoring and forecasting system of low visibility that has been used on the Shanghai-Nanjing highway in Wuxi for more than a year are discribed. Some examples are given to show that this system can forecast the heavy fog, including the regional fog and the sudden fog so that serious traffic accidents can be lessened or avoided.