Abstract: In order to fully protect the safety of drivers with different characteristics, and to improve the economic performance, a night time model and an effective dynamic distance model are proposed in this article, considering the night environments such as environmental illumination, visibility, weather and glare which affect the characteristics of the driver. Then, according to the main objective optimization idea, the night speed limit method of mountainous expressway is proposed based on driving economy and security. In this method, the time and the minimum of vehicle fuel cost per hour are chosen as the objective function, combining with constraint conditions of the safety requirements for obstacle recognition, vehicle driving stability, and night time reaction time model and parking sight distance model. In this way, an optimal model of the night time safety speed of freeway in mountainous area is established. The maximum safe speed value of the model is the maximum speed limit value of the freeway. Finally, to verify the rationality and validity of the model, a simulation is conducted by using Matlab. The simulation results show that the maximum safe speed decreases with the decrease of the road adhesion coefficient when the visibility and time are not changed, and the maximum safe speed decreases with the decrease of the road adhesion coefficient when the road adhesion coefficient and time are unchanged. The results show that the law is in line with the current research results, and the maximum speed limit value is 99.390km/h, which is consistent with the design speed of 100km/h of the highway. Therefore, the speed limit model provides a reference for studying the safety speed and improving the safety of highway in mountainous area.

Key words: mountainous freeway, night time speed limit, driver’s characteristics, night time response time model, main objective optimization algorithm