When the Streets have no Men: Urban Traffic Restrictions and Air Pollution

In this paper, we analyze two quasi-experiments to assess how urban traffic restrictions and social distancing norms affect pollution levels in the municipality São Paulo, one of the largest metropolitan areas in the world. First, using hourly air pollution levels measured in thirty-three monitoring stations in the state of São Paulo, we exploit exogenous variation in quarantine rules following the COVID-19 outbreak to estimate how social distancing norms affected pollution levels across different municipalities. We find an average decrease of 22.4% in air pollution after the first days of the capital's quarantine announcement, with heterogeneous effects across pollutants, driven by decreases in the vehicle fleet and urban mobility. Second, we compare this effect with another quasi-experiment that explores exogenous suspensions of traffic restriction rules between 2000-2018 in the municipality of São Paulo. We also document increases in pollution levels when more cars are allowed in the streets, with an average increase of 16.7% in air pollution. Finally, we use our estimates to show that a reduction of 4.7 times the estimated ATT in the quarantine period is necessary to reach the capital's long-term air quality goals.