Authors：Rui Xie, Dihan Wei, Feng Han, Yue Lu, Jiayu Fang, Yu Liu, Junfeng Wang

Abstract：Urban traffic congestion and smog pollution are critical urban development issues. In this study, the influencing mechanism of traffic density on smog pollution in cities is described from the perspectives of direct emissions, spatial agglomeration, and technology spillover effects. Based on an improved STIRPAT model, we examine a panel of 283 prefecture-level cities in China from 2003 to 2015 and find an inverted U-shaped relationship between traffic density and urban smog pollution in large and medium cities and no significant relationship in small cities. Furthermore, the traffic densities in large and medium cities are on the left side of the curve, so direct emissions remain important. The reduction in smog pollution caused by spatial agglomeration and technology spillovers is not sufficient to offset the increase caused by direct emissions. In advancing urbanization, the government should relax its household registration policy that restricts migration to large cities and should avoid any bias in its construction land distributiontoward the mainland and small towns. By doing so, the government will further enhance the economic density and scale, shifting the traffic density to the right side of the inverted U-shaped curve so that spatial agglomeration and technology spillover effectscan mitigate smog pollution.

Key Words：Traffic density;Urban smog pollution;PM2.5;STIRPAT model