Causal association between metro transits and air quality: China's evidence.

In policy remedies, transportation infrastructure such as metro transit is widely considered to be an important and effective means to reduce air pollution. However, the policy prediction that metro transits reduce air pollution depends on driver responses. China provides an appropriate context to explore the answer since its major cities have expanded their metro transit systems in recent years, which enables us to exploit a natural experiment. Accordingly, a sharp regression discontinuity is employed to evaluate the impact of 112 metro lines (with an accumulation of 3286 km) on air quality. Evidence shows that the opening of metro transits has a negative causal effect on air pollution. The results are robust to several alternative specifications. Furthermore, a heterogeneity analysis demonstrates that metro openings have a time-varying impact on air pollution, which is larger during rush hours. We anticipate that the air quality improvement in China caused by metro transits can generate large welfare gains.