Betsy Mason:

An ever increasing share of the world’s population is living in what are known as megaregions—clusters of interconnected cities. The concept of the megaregion is decades old and fairly easy to grasp, but geographically defining them has turned out to be rather tricky.
 Now, researchers have attempted to map the megaregions of the contiguous United States by studying the commutes of American workers.
 As megaregions grow in size and importance, economists, lawmakers, and urban planners need to work on coordinating policy at this new scale. But when it comes to defining the extent of a megaregion, they find themselves running into the same problems geographers and cartographers have always had when trying to delineate conceptual areas. Because megaregions are defined by connections—things like interlocking economies, transportation links, shared topography, or a common culture—it’s tough to know where their boundaries lie.