Marco Gonzalez-Navarro 2020 #Infra4Dev Conference: Decarbonizing Infrastructure www.worldbank.org/infra4dev2

1. Subways and Urban Air Pollution
Marco Gonzalez-Navarro, UC-Berkeley

2. Does the construction of a new subway transportation
system lead to city-wide air quality improvements?

3. What do we know?
• Positive automobile traffic and air pollution relationship has been well
documented (Chen et al 2016)
• Subways reduce vehicular traffic (Anderson 2014)
• Chen and Whalley (2012) studies subway opening and air quality for a
single city (Taipei)

4. Subway data
• Average number of stations opened 14
• Daily average ridership 18 months after opening 130,000
• Average city population 4 million

5. Subway data

6. Air pollution data

7. City level air pollution data: Palma, Spain

8. Research design
• We compare air quality outcomes before to after the subway begins
service in each city to causally estimate the impacts of subways on air
quality

9. Average effects

10. Heterogeneous effects by initial air quality
Above median air pollution cities Below median air pollution cities
Coefficient estimate Coefficient estimate
Standarderror
Standarderror

11. Other dimensions of heterogeneity
• Subways with high levels of ridership reduce pollution more
• New subway openings reduce more pollution than expansions of existing
systems.

12. Estimated longer term impacts among high pollution cities
-7%
-6%
-5%
-4%
-3%
-2%
-1%
0%
year 1 year 2 year 3 year 4

13. How much should we value these pollution reductions?
• Take 1: We rely on studies estimating the relationship between pollution
levels and infant mortality (e.g. Arceo, Hanna and Oliva 2016)
• In an average high AOD city, we estimate an improvement of 3.2μg/m3 in
PM10 levels ≈ saving 22.5 infant lives per year ≈ 43 million USD per year.
• Using a 5% discount rate and summing up 5 years of effects means this
health externality is valued at 195 million USD

14. How much should we value these pollution reductions?
• Take 2: Use the Global Burden of Disease Methodology: Maps between
levels of PM2.5 and mortality risk ratios for all ages.
• In an average high AOD city, we estimate an improvement of 1.7μg/m3 in
PM2.5 levels ≈ saving 500 lives per year ≈ 1 billion USD per year.
• Using a 5% discount rate and summing up 5 years of effects means this
health externality is valued at 4.5 billion USD

15. Conclusions
• Existing estimate of the effects of particulates on mortality suggest they
are sufficiently harmful that for initially high pollution cities (Above 0.36
AOD ≈ 28μg/m3 PM2.5 annual average) the air quality improvements are
economically important.
• The effects are large enough to justify subsidies for subway construction
and operation.