1. Beijing Jiaotong University, No 9. South Liu Li Qiao Road, Fengtai District, Beijing, China 100073, and Beijing Transport Institute (email: )
2. Dyson School of Applied Economics and Management, Cornell University, Warren Hall, Ithaca, NY 20854 (email: )
3. Dyson School of Applied Economics and Management, Cornell University, 405 Warren Hall, Ithaca, NY 20854, and NBER (email: )