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A megapolitan area as defined here has the following characteristics:

Combines at least two existing metropolitan areas, but may include dozens of them

Totals more than 10 million projected residents by 2040

Derives from contiguous metropolitan and micropolitan areas

Constitutes an organic cultural region with a distinct history and identity

Occupies a roughly similar physical environment

Links large centers through major transportation infrastructure

Forms a functional urban network via goods and service flows

Creates a usable geography that is suitable for large-scale regional planning

Lies within the U.S.

Consists of counties as the most basic unit

The "lies within the US" appears to be an artifact of Lang's academic work; there's no reason that the megapolitan concept should be limited to the United States. In fact, there are good arguments that some of the identified American megapolitan areas actually encompass parts of Canada (e.g, Vancouver in Cascadia) or Mexico (e.g, Matamoros in the "Gulf Coast" megalopolis).

Thinking in terms of megapolitan areas leads to somewhat different conclusions about some key planning issues -- sprawl, transit -- than does the traditional core-periphery metropolitan model. Washington Post columnist Neal Peirce summarized the various US approaches to megalopolis planning in an essay this week; he notes that the handful of groups in the US looking at megapolitan models see "immense" economic improvement from regional cooperation and co-planning, especially around transportation.

One of the differences between megapolitan and core-periphery metropolitan planning is the interpretation of sprawl. Over time, suburban areas have the potential to become micropolitan areas as they grow more populous and dense; in a megalopolis, micropolitan infill between core metropolises adds to the "polycentric" character of the megalopolis. This evolution suggests that planning models that assume suburbs remain static in nature (even as they grow in size) are incorrect.. Groups like the UK's Young Foundation are starting to construct new models for sustainable management of these polycentric regions.

Our hypothesis is that falling costs of transportation and (more particularly) communication, combined with new informational agglomeration economies, lead to the emergence of a highly complex “space of flows” (Castells 1989) at the level of the “Global Mega-City-Region”, reconfiguring previous geographical relationships. This follows a “Christaller rule”: lower-level service functions may be performed at lower levels of the urban hierarchy (Christaller 1966 (1933)). These new urban regions achieve major agglomeration economies through clustering of activities, not in any one centre, but in a complex of centres with some degree of functional differentiation between them.

At this scale “sustainable concentrated deconcentration” suggests that growth should be guided on to selected development corridors along strong public transport links, including high-speed "regional metros" such as proposed for London, or even along true high-speed lines such as London-Ashford, Amsterdam-Antwerp or Frankfurt-Limburg (Ipenburg et al 2002). These would not represent continuous urbanisation, but clusters of urban developments around train stations and key motorway interchanges up to 150 kilometres, from the central city, thus reducing the probability of long-distance commuting and assisting remoter areas.

(With terms like "concentrated deconcentration," it's clear that the Young Foundation is primarily academic in character.)

There's an interesting dichotomy at work: study of planning based on cross-national regions is more advanced in Europe, but even the Europeans argue (PDF) that such agglomerations (which they term Global Integration Zones) are more common in the US than in the EU. The Global Integration Zone (GIZ) concept is somewhat broader than Lang's "megapolitan areas" -- the single West Coast GIZ encompasses Lang's Southland, NorCal and Cascadia megalopolises, for example -- but the underlying concept is the same. The big difference is that Europe has a transnational agency devoted to analysis of megapolitan/GIZ/polycentric metropolitan issues. ESPON -- the European Spatial Planning Observation Network -- seeks to better understand the planning implications of the expanded European Union, with a focus on these cross-border zones. Their March 2005 report (PDF) covers the finest details of how polycentric planning is accomplished. It's nearly 350 pages, and does not appear to be beach reading.

Megapolitan planning is still in its infancy compared to traditional urban planning, and it's likely that much of what holds true for individual cities will apply to trans-metropolitan groupings. But it's already clear that reexamining economic and transportation scenarios in a megapolitan context can lead to different choices. What is still in question is how to integrate the strategies developed at the megapolitan level with the day-to-day decisions of city leaders.