When the term "Megalopolis" was first introduced by Jean Gottmann in 1961, the Eastern seabord of the US was already seen as a new urban form: what he called "BosWash." Gottmann predicted that two other regions -- "SanSan" and "ChiPitts" -- would also form megalopolises as they continued to build shared transportation and communication infrastructure.

More than forty years later, it seems that his predictions were largely correct, though one could make a case for adding Portattleouver and Fort San Haustin as regions as well. But not all megalopoli are the same: Bosnywash is an almost uninterrupted 400-mile stretch of mid-density suburbs, while western conurbations tend to be connections of cities separated by state-protected or agricultural land.