This paper investigates the relationship between the opening of a city’s subway network and its air quality. It finds that particulate concentrations drop by about 5 percent in a 10km disk surrounding the city center during the year following the opening of a new subway system. The reduction in particulates is larger nearer the city center, but extends over the whole metropolitan area. The reduction persists over the longest time horizon that the study’s data can measure, about eight years, although these estimates are less reliable further from the subway opening date. The 5 percent reduction is consistent with observed ridership in the subway system and induced reductions in automobile travel.
The results of the analysis also point to decreasing returns to subway expansions, both in terms of particulate reduction and ridership. Using estimates from the literature on the relationship between particulates and infant mortality suggests that each subway rider provides an external health benefit of about 0.64 dollars per trip in the average city over at least the first 56 years after system opening. In the absence of pollution pricing, this provides a welfare basis for nontrivial public subsidies to subway ridership.
This paper is a part of a Global Research Programme on Spatial Development of Cities, funded by the Multi Donor Trust Fund on Sustainable Urbanization of the World Bank and supported by the UK Department for International Development
Nicolas Gendron-Carrier, Marco Gonzalez-Navarro, Stefano Polloni, Matthew A. Turner (2017) Subways and Urban Air Pollution
Subways and Urban Air Pollution