Large metropolitan areas are the most complex stage of the urban phenomenon. When these cities are also the political and/or economic capitals of their countries, their functions become even more complex.
The overlay of highway infrastructures is the most present layer of the future configuration of public spaces and the visibility of the urban landscape, regardless of its qualities. Public transportation using often tunnels, the urban freeways create the true face of the metropolis.
The urban insertion of these road systems can be done with different degrees of success. Cutting traffic on the riverside embankments of Paris seems a simple solution; reclaiming the Manzanares embankment in Madrid by burying the M30 traffic means a huge cost, but brings back a quality public space that has become a clear public success. The Mumbai sea link proposed roads seem similar in concept to what was the M30 beltway in Madrid five decades ago: relocating an infrastructure problem in a public domain, solving the mobility flux with a strong impact on the environment and the landscape.
The urban quality of these spaces comes also from their ability to integrate open spaces and landscape features : rivers, large parks, sea shores, beaches… the presence of several uses on the elements that by themselves are transit ways is one of the main issues.
The hierarchic structure of the city is also relevant in metropolitan areas. The territory is never isotropic, and even if the urban theory is always devising polycentric structures that can often work, the most usual is to see a strong central core. The experience of many American cities, where this core has been depleted over time by the translation of functions to the suburbs, shows that urban quality can suffer when the center cannot hold. The historical cores that where almost the whole city a century ago are today just a small part of the population and a shrinking proportion of economic activity, but they still have a strong symbolic role.
The dynamics of the urban core, even if it keeps a relevant strength, can have a negative impact on population. The role of the core as a symbol can increase the presence of large public facilities or corporate headquarters, usually reducing the local scale public facilities and services for the area inhabitants, that can feel they have better chances in the suburbs.