Air pollution hindering a transit-oriented city: Examining the association of particulate matter concentration with public transit ridership and road traffic in Seoul, South Korea

In high-density metropolitan areas prone to air pollution, public transit may be less preferred when air quality is poor due to the difficulty of avoiding outdoor air exposure during their use. By applying seemingly unrelated regression models, we examined the association of particulate matter (PM) with both public transit ridership and, as an alternative to sharing vehicular traffic demands, private car road traffic in Seoul from 2015 to 2018. To control for fluctuations in ridership and road traffic associated with seasonalities such as the day of the week, we used nine-term moving averages transformed into residual form. Results show that higher PM concentration was negatively associated with not only transit ridership, but also road traffic volume and congestion level; yet, the reduction in subway ridership was found to be larger than that in road traffic. This suggests that (1) when PM concentration is severe, people could reduce overall travel rather than change travel modes from public transit to private cars, thus implying that both modes are complementary rather than substitutionary in terms of responding to air pollution; and (2) public transit still seems more susceptible to air pollution than private cars. The findings help us understand the adverse effects of air pollution on public transit use as well as better predict and respond to demand for public transit in poor atmospheric conditions, thereby providing future policy directions for sustainable transportation planning.