1. Ph.D. Candidate, School of Traffic and Transportation, Beijing Jiaotong Univ., No. 3 Shangyuancun, Haidian District, Beijing 100044, China (corresponding author).
2. Professor, School of Traffic and Transportation, Beijing Jiaotong Univ., No. 3 Shangyuancun, Haidian District, Beijing 100044, China.