The accelerated process of urbanization that our civilization has undergone since the middle of the last century, has managed to cross all borders and continents of our planet. Never before in history we have had cities so populous and dense, with so many people sharing increasingly smaller spaces, where the problems of congestion, pollution and overcrowding contrast with the buoyant economic and political power of these urban centres. Today there are more than 20 metropolitan areas in the world with more than 15 million inhabitants. Among them, we can count world-renowned cities like Tokyo, New York or Shanghai, as well as other less famous ones like Karachi, Chongqing or Dhaka. This has even led to the creation of a new category of urban agglomeration, in which beyond metropolitan areas or conurbations, we speak about urban regions, clusters or megalopolises, where several metropolitan areas generate shared dynamics in a complex urban network. Traffic in Beijing, China. Source: Pexe