This wasn’t submitted to to Ex Unum, Pluribus! But we can’t pass up a good proposal when it falls into our laps. See this excellent post from a paper by Nelson Et Al, An Economic Geography of the United States: From Commutes to Megaregions.
The emergence in the United States of large-scale “megaregions” centered on major metropolitan areas is a phenomenon often taken for granted in both scholarly studies and popular accounts of contemporary economic geography. This paper uses a data set of more than 4,000,000 commuter flows as the basis for an empirical approach to the identification of such megaregions.
They include this nice cluser diagram, as well as a good border outline view like so: