The heterogeneous effects of motorways on urban sprawl: causal evidence from Portugal

As urban land increased in mainland Portugal by 55.9% between 1990 and 2012 and the country developed an extensive motorway network between the 1980s and the early 2010s, we set out to investigate the effect of motorways on urban sprawl across mainland municipalities. We document the evolution of urban sprawl for these 275 municipalities across several dimensions, including the population density of urban land, its degree of fragmentation and shape irregularity (which we combine in a summary “total interface” indicator), and the differences between the central urban unit and the remaining “peripheral” urban land. Given that the spatial distribution of motorways is likely to be endogenous, we use road itineraries from the 18th century as an instrumental variable. Our results suggest that motorways contributed to the fragmentation of urban land into numerous urban patches. Also, we identify important within-municipality heterogenous effects, in that motorways did not cause the contiguous growth of the central urban unit (typically the largest urban unit in each municipality) but, conversely, appeared to contribute in a significant manner to the development of peripheral urban land. There is also some evidence that motorways contributed to an increase in the shape irregularity of urban areas. Finally, we show that motorways caused a decrease in urban population density, but only in the relatively small group of more urbanised municipalities.