The development of mega-regions is mostly the outcome of three processes that reinforce the spatial extent and the coherence of an urban system. The first is the growth, intensification, and diffusion of economic activities, which require additional urban land. The second is the growing interconnectivity of urban centers, mostly through the setting of multimodal transport corridors. The third is the specialization and interdependency of urban centers and the intense passenger and freight interactions this process creates. It is possible to summarize the development of a mega-region into four stages similar to corridor development:
- Single Cities (A). Up to the late 19th century, cities were relatively autonomous and regional transport systems were of limited extent, particularly road systems. Economic activities were mostly localized, but specialized regional and long-distance trade existed.
- Interconnected Cities (B). By the early 20th century, national urban systems were established, particularly with railways connecting resource-producing, manufacturing, and major markets. This favored the development of regional economies and the functional specialization of urban centers through the principle of regional comparative advantages. This regional mobility was more concerned with freight than passengers, particularly since mobility was time-consuming and costly (relatively high friction of distance).
- Metropolitan regions (C). In the second half of the 20th century, the massive diffusion of the automobile and the setting of highway systems supported a growing mobility of passengers and freight as well as a regionalization of urbanization. Suburbanization resulted in the setting of a new urban space as well as the integration of subcenters into entities that came to be known as metropolitan areas (or metropolitan regions). Global trade was also becoming more significant, implying a growing role of external forces in shaping urbanization, an interaction taking place through gateways. The metropolitan region was becoming a competitive unit within an emerging global economy.
- Mega Region (D). By the 1990s, the growing integration and interconnection of metropolitan areas led to the formation of mega-regions, some being nodal (centered around a single large metropolitan area) while others were oriented along a corridor including several large metropolitan areas. Globalization and intermodal integration (through containerization) further reinforced the importance of gateways as articulation nodes regulating passengers and freight flows with transport terminals and logistics zones. Global and regional influences became embedded in the structure of mega regions. Corridor-specific infrastructure, such as high-speed trains and air shuttles, was further developed to support increasing regional interactions. Many mega-regions developed a functional specialization centered around specific manufacturing and service activities.
The mega-region is a spatial and structural outcome of urban expansion, exploiting comparative advantages and increasing regional and global interactions. The above representation is synthetic and assumes different time frames in the emergence of mega-regions across the world. For most developed economies, the setting of mega-regions was well underway during the 1960s. For the developing world, the process took place later but at a much faster rate.