As an example, Davis (2008) empirically studies the effect of a driving restrictions program on air quality in Mexico City. In the urban economics literature, the impact of urbanisation on air quality has been studied through different urban structure indicators. One can distinguish between two categories of such indicators: (i) indicators related to the internal composition of the urban area and (ii) urban area morphology indicators.2 Urban area morphology refers to the spatial disposition of the urban tissue (e.g. dispersed urban plots, linearity of urban areas) while internal composition refers to tissue composition (e.g. wetland, green spaces).