Did the London congestion charge reduce pollution?

Recent vehicle charging schemes aim to reduce pollution and other congestion externalities. We reexamine the London congestion charge introduced in 2003 and demonstrate significant reductions in several pollutants relative to controls. We even find evidence of reductions per mile driven suggesting amelioration of a congestion externality. Yet, we find a more robust countervailing increase in harmful NO2 likely reflecting the disproportionate share of diesel vehicles exempt from the congestion charge. This unintended consequence informs ongoing concern about pollution from diesel-based vehicles and provides a cautionary note regarding substitution effects implicit in many congestion charging schemes.