1. State Key Laboratory of Fire Science, University of Science and Technology of China, Collaborative Innovation Center for Urban Public Safety, Hefei 230026, P. R. China
2. Institut fuer Theoretische Physik, Universitaet zu Koeln, D-50937 Koeln, Germany
3. College of Urban Railway Transportation, Shanghai University of Engineering Science, Shanghai 201620, P. R. China
4. School of Transportation and Logistics, National United Engineering Laboratory of Integrated and Intelligent Transportation, Southwest Jiaotong University, Chengdu 610031, P. R. China
5. College of Urban Construction and Safety Engineering, Shanghai Institute of Technology, Shanghai 201418, P. R. China