To alleviate traffic congestion, one of the most pressing urban challenges in developing countries, Beijing’s government has been investing increasingly in subway infrastructure. In this study, using fine-scale daily traffic records from 2009 to 2013, we perform a regression discontinuity design to examine the average treatment effects of subway openings on traffic conditions in Beijing from 2009 to 2013. Three findings emerge from our empirical analysis. First, the opening of a new subway line resulted in a significant decline of daily passenger bus ridership, by 452,400 on average.