[Kadin]

Boston, Philadelphia, and DC all have substantial car-free populations.

They are small in proportion to the overall populations of the respective cities' urban areas, but that's almost entirely because public transportation has lagged the construction of suburbs and sprawl. Boston's T, or Philly's SEPTA were mostly constructed in the early 20th century so it's only those older areas that are feasible for car-free living. (The DC Metro is an interesting exception in that it's a much newer system, but it too fell out of step with development.)

In fact, I'd say that there's a sort of "car free radius" that's basically the edge of residential areas just before World War II. The suburbs that got built after that are hugely car-centric, ones built before typically aren't.