1. Institute of System Science, School of Traffic and Transportation, Beijing Jiaotong Univ., Beijing 100044, P.R. China; and School of Civil and Environmental Engineering, Nanyang Technological Univ., Singapore 639798, Singapore (corresponding author).
2. Professor, Institute of System Science, School of Traffic and Transportation, Beijing Jiaotong Univ., Beijing 100044, P.R. China.
3. Associate Professor, Institute of System Science, School of Traffic and Transportation, Beijing Jiaotong Univ., Beijing 100044, P.R. China.
4. Assistant Professor, School of Civil and Environmental Engineering, Nanyang Technological Univ., Singapore 639798, Singapore.