1. School of Mathematics and Physics, Anhui Jianzhu University, Hefei 230601, P. R. China
2. MOE Key Laboratory for Urban Transportation, Complex Systems Theory and Technology, Beijing Jiaotong University, Beijing 100044, P. R. China
3. School of Automotive and Transportation Engineering, Hefei University of Technology, Hefei 230009, P. R. China
4. Department of Modern Physics, University of Science and Technology of China, Hefei 230026, P. R. China