The spatiotemporal and multilevel impacts of metro on urban environmental protection

This study establishes spatial difference-in-differences models to investigate the causal correlation between metro and urban environment, and introduces marginal effects analysis method to examine the direct and indirect effects of metro on urban air quality. The empirical results indicate that metro is able to improve the air quality of the whole city remarkably through promoting citizens to transfer transportation modes. Nevertheless, this effect is imbalance in both time and space. The ability of metro improving air quality is more significant during morning peak and evening peak along the metro line. Furthermore, due to the influence of induced traffic volumes, the ability of metro to improve urban air quality is weakened as time goes by. Therefore, policymakers should make the route of the metro coincide with the commuting direction of citizens to the greatest extent when making a rational metro development plan. Moreover, policies limiting the growth of private cars ought to be formulated synchronously to maximize the positive externality of the metro on urban environment.