School of Traffic and Transportation, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, China;
Qinghai Research Institute of Transportation, Xining, Qinghai 810001, China;
China School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China;
Qinghai Research Institute of Transportation, Xining, Qinghai 810001, China;