Here's DC:
Mentally, many people here often think of the Beltway as the dividing line between core and outer areas. TBH I think the Beltway is too far out (ie the real functional urban/suburban boundary is closer in to the core), but the Beltway is such a convenient and well-understood proxy that I'm willing to call it the boundary even though it includes some definitely suburban areas. If pressed, I'd cut out a lot of Prince George's and Fairfax counties. But the Beltway works as a good quick answer, and it's definitely better than using the District of Columbia's official boundaries, which leave out a lot of very urban places.
