What do three American Megalopolises have in common?

The concept of megalopolis or "great city" has been popularized by Jean Gottmann, Megalopolis, with a capital "M," started from the urban region of southern New Hampshire to northern Virginia which sprung up to eight other regions around the world. What the three American Megalopolises have in common - a regi on needs to have an extensive network of metropolitan areas with a minimum population of 25 million with intense economic interaction.