1. Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109 USA.
2. Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 48105 USA. He is now with the Department of Civil Engineering, The University of Hong Kong, Hong Kong.
3. Didi Chuxing, Beijing 100000, China.
4. Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 48109 USA, and also with the Transportation Research Institute, University of Michigan, Ann Arbor, MI 48109 USA (e-mail: henryliu@umich.edu)