Impacts of Urban Rail Transit on Local Air Quality in Hong Kong: A Quasi-experimental Test Based on the Anti-extradition Bill Protests

Abstract

<p>We examine the impacts of disrupted urban rail services on air quality in Hong Kong—or the pollution-abatement potential of urban rail transit—with a quasi-experimental design. In our generalized difference-in-difference-in-differences approach, unplanned, semi-random rail-service disruption events during Hong Kong's 2019/20 Anti-Extradition Law Amendment Bill movement are treated as an exogenous shock. Our results show that on average, nitrogen oxides concentrations near a given metro station increased by 7.8% when the station was shut down due to on-going protests elsewhere within the city. During rush hours, the shutdown-induced pollution-intensifying effects further increased to ≤29.7%, suggesting imminent need for a rail-to-road mode shift among commuters. The magnitude of the effects, however, tends to decline over time, with a 1% decline for an additional hour lasted during the shutdown. This declining trend seems to reflect adaptation at both network (e.g., increased supply of alternative transit services) and individual (e.g., alternative route choice) levels.<br></p>