as an example davis 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 urban area morphology refers to the spatial disposition of the urban tissue eg dispersed urban plots linearity of urban areas while internal composition refers to tissue composition eg wetland green spaces