Urbanization is a well understood process that has led to a global urban system including several mega-cities. A further step concerns the emergence of urban regions, which are entities that transcend the conventional perspective of a city into a wider scale including rural areas and interconnected systems of cities. Even the concept of rural has been transformed through closer integration with urban activities including leisure, specialized agriculture (e.g. “truck farming” or market gardens), and punctual exurban developments (e.g. distribution centers, manufacturing). Although an urban region is rarely a jurisdictional entity, it is a functional spatial unit that can take three main forms:
- Urban corridor. A linear accumulation of transport infrastructure, mainly highways and rail networks (including high-speed rail), that supports a linear system of cities. Many corridors are the anchor structure of mega urban regions. However, many urban corridors are not mega urban regions as they do not have agglomerations large enough (commonly more than 2 million inhabitants) to provide a strong urban imprint or do not have extensive enough interactions (e.g. for less developed economies). As they further develop, many urban corridors can become mega urban regions.
- Extended Metropolitan Region (EMR). A large urban agglomeration, usually of more than 5 million inhabitants, exercising a substantial influence on its surrounding landscape and is complemented by a system of smaller satellite cities. An EMR is commonly a dominant gateway and cluster of economic activity within a national economy (sometimes more than 25% of the national GDP as for Bangkok, Paris, Madrid, Gauteng, Lima, and the Mexico City area).
- Mega Urban Region (MUR). A complex system of economically integrated and interconnected cities spanning a large territory (several hundreds of kilometers in diameter). They usually have more than 10 million inhabitants spread over several large metropolitan areas, but many MURs have populations above 25 million. All MURs are the economic, political, and cultural core of their respective countries.
Mega urban regions do not have formal names since they are not official jurisdictional entities. Therefore, they are usually labeled either by a significant common physical feature (e.g. a river delta) or by the name of their two most important cities (or the cities at their respective ends). Still, this naming can be subject to contention and different alternatives have often been advocated.
The world has about 40 mega urban regions and extended metropolitan regions with the urban regions on the above map accounting for a population of more than 1 billion people. The world’s most important urban regions are found in Asia. For instance, the MURs of the Yangtze River Delta, the Tokaido corridor and the Pearl River Delta have respectively populations of 88, 80, and 70 million inhabitants. They represent the largest accumulation of urban infrastructure on earth. The only true transnational MUR is the Rhine/Scheldt Delta which spans Belgium, the Netherlands, and Germany with a population of 26 million. Singapore – Kuala Lumpur is also a transnational MUR, but to a lesser extent. The United States, mostly because of its high level of economic development and its integrated national economic system, has several large MURs, with BostWash (Boston-Washington; 44 million) and ChiPitts (Chicago-Pittsburg; 54 million but the urban region is more diffuse and less integrated) being the most significant.