1. Graduate Student, Institute of Public Safety Research, Dept. of Engineering Physics, Tsinghua Univ., Beijing 100084, China.
2. Ph.D. Candidate, Institute of Public Safety Research, Dept. of Engineering Physics, Tsinghua Univ., Beijing 100084, China.
3. Institute of Public Safety Research, Dept. of Engineering Physics, Tsinghua Univ., Beijing 100084, China.
4. Professor, Institute of Public Safety Research, Dept. of Engineering Physics, Tsinghua Univ., Beijing 100084, China (corresponding author).