It seems like the national consensus is that the threshold for "urban" is low enough to allow for a few more classifications of density which would probably be related to things like yards being squeezed out, single-family being replaced by high-rise, and presence of effective mass transit in spite of the American spite for such things. The term "downtown" would often fill such a role, though with even more variability than "urban". It's worth keeping in mind that our national capital doesn't even have any skyscrapers, and the term "urban sprawl" is widespread and widely understood to be something that actually happens with some frequency.