Were traffic restrictions in Madrid effective at reducing NO2 levels?

Salas R., Perez-Villadoniga M. J., Prieto-Rodriguez J., Russo A.
Transportation Research Part D: Transport and Environment, 91, 102689.

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In this paper we assess the effectiveness of the introduction of Madrid Central, a low-emission zone (LEZ), to improve air quality in the centre of Madrid. We take advantage of this policy change to identify the effect of the application of an LEZ on NO2 concentrations. Our findings provide evidence that the introduction of Madrid LEZ has had a positive impact on reducing NO2 emissions, as measured by Plaza del Carmenís monitoring station. Furthermore, monitoring stations located near to Madrid LEZ also exhibit significant, though smaller, reductions in NO2 levels. These results suggest that there was a positive spillover effect and that pollution was not transferred from the city centre to other nearby areas. Instead, it seems that citizens in Madrid actually changed their transportation habits.