关键词: 地铁, 交通限行, 空气质量, Heckman两步法, 合成控制法

Abstract: Many cities in China are faced with serious environmental pressure. Policy makers have tried many means, such as developing metro to ease traffic pressure or introducing policies to restrict private car travel. These two methods can be regarded as a quasi-natural experiment.The study points out that the dredging policy (metro) to solve the traffic and environmental problems is more favorable, but the effect will gradually weaken, and the restriction policy will have a limited impact on air quality. However, for some cities, the effect of the two methods will be more prominent.It is pointed out that policy makers should control the intensity and contingency effect of the two methods.

Key words: The metr, Traffic restrictions, Air quality, Heckman two-step method, Synthetic control