Subways and Road Congestion
研究中国45条地铁线路开通对道路拥堵的影响，发现开通后第一年附近道路高峰时速提升约4%，对原本拥堵道路效果更明显，并计算每次通勤节省时间价值约0.10美元。
We study whether subways alleviate road congestion by examining 45 subway line launches in China and by using detailed data on road speed. Our difference-in-differences estimation finds that in the first year after a subway line is launched, rush hour speed on nearby roads increases by about 4 percent. The effect is most prominent in initially congested roads and declines over distance to the new subway line. Evidence on road speed is corroborated with substitution patterns among modes of transportation. Using auxiliary data from Beijing, we calculate that the time savings for each automobile or bus commute from faster speed is worth US$0.10.