Public Transit and Air Pollution

Advocates of public transit frequently tout improved air quality as a primary benefit. Yet little is known about the causal impacts of public transit on local air pollution. Exploiting variation in transit availability resulting from work stoppages in 18 Canadian cities between 1974-2011, this study identifies the effect of public transit on air pollution. Our findings indicate that transit leads to a 3.5 part-per-billion increase in nitrogen oxides while having no statistically significant effect on carbon monoxide or fine particulate matter. Estimates are robust to a series of specification and placebo tests and magnitudes are consistent with a calibrated simulation model. Overall, the results suggest that expanding the current configuration of public transit in North American cities is unlikely to yield improvements in local air quality