Well, there are a bunch of close-in places in Boston which most people wouldn't think of as "suburbs", but which are technically suburbs, "inside I-95", where you don't need a car. I mean, IIRC Cambridge and Somerville are denser than Boston. They lack circle routes which bypass downtown; the #1 bus is constantly delayed. They should have such routes.
Once you get outside the 95 beltway the dense urbanity ends in most directions (there's some more to the southeast in the Quincy area), but there's a lot inside that radius.
Something similar is going on in DC, where everything inside the Purple Line route is pretty dense. In Chicago, there's a similar pattern, though I couldn't pinpoint the boundary line.
There's such a boundary in Philadelphia too.
New York City is weirder, because as you head into New Jersey, Connecticut, and Long Island, there are areas which are just as dense as Queens but which have been so car-oriented that you can't walk from one place to the next; some urban fabric repair would make them more manageable.