Optimization of Combined Horizontal and Vertical Curves Design of Mountain Road Based on Vehicle-Road Coordination Model

Through the analysis of driving stability of the vehicle on combined horizontal and vertical curves (CHVC), the safety model based on vehicle-road coordination was established with the critical state of accident as the limited condition. The influence of various factors on the design parameters such as the minimum radius and the maximum slope length were discussed. The safety model showed that the minimum radius of CHVC was proportional to the driving speed and road gradient, but inversely proportional to the value of the superelevation and crosswise affixing coefficient. Besides, the maximum slope length of CHVC was proportional to the road rolling resistance coefficient and slip ratio, but it was inversely proportional to driving speed, road gradient, and vehicle type. Furthermore, the maximum slope length was less affected by the superelevation. In addition, in order to improve the safety level of mountainous roads, this paper discussed the threshold value of the design index of CHVC considering the actual operating speed of the vehicle.