Subways and urban air pollution

Urbanisation will be an important part of the development process for most low income countries, and demands for urban infrastructure construction will outpace the ability of policy makers to provide it. Given this, it is critical to develop an empirical basis for prioritising construction projects. What should policymakers do first? Build highways, subways, water treatment facilities, or open a title office?  At present, the empirical foundations for these sorts of decisions are poor.  By quantifying the effects of subways on air quality, the proposed project will contribute to our ability to choose infrastructure construction projects in a way that contributes more effectively to sustainable economic development.

This research investigates the effects of subways on urban air pollution, which is important for three reasons:

• In 2010, 138 large cities on Earth were home to 7,886 subway stations and about 10,700 km of subway routes. Subway construction and expansion projects range from merely expensive to truly  breathtaking; $3 trillion is a reasonable guess of construction costs for the current stock. Understanding the effect of subways on cities is important if we are to evaluate proposals to build or extend subway systems.
• Subway projects generally require large subsidies. To justify these subsidies, proponents often assert the ability of a subway system to have a transformative effect on the city. Understanding the effect of subways on cities is important if we are to evaluate such subsidies and the arguments used to support them.
• The urban rural wage gap is often larger than 100% in developing countries. This wage gap suggests that if we could only induce more of the rural population to move to cities, we could cause a dramatic increase in their income. The ‘unhealthiness’ of cities may place a constraint on how quickly this migration can occur and a component of this unhealthiness is poor air quality. Therefore, understanding the effect of infrastructure  policy on air quality  is an important part of an agenda to promote sustainable economic growth in developing countries.

To our knowledge, the literature examining the relationship between subways and air pollution consists of a single paper. This paper examines a single subway expansion in central Taipei. Since we will examine all subway expansions between 2002 and 2013, we expect to provide a more definitive result.

This study is organised around data describing the location and date of opening of every subway station in the world, together with remote sensing data describing air pollution everywhere in the world -- monthly, at a 3 km spatial resolution -- from 2002 to 2015. These data are underutilised in the academic literature studying pollution, and we expect that an important contribution of this project will  be to draw attention to these data.

Together these two data sources allow us to construct monthly data describing city level measures of air quality and subway extent. We will proceed by examining how urban air quality changes as the extent of the city’s subway network changes and will also be able to investigate whether subways change the distribution of pollution within a city.