I think if you expand a low-rise city you end up with Washington DC or LA, meaning debilitating traffic. The density is not high enough to support mass transit effectively, but it's too high for effective roads.
As I said in response to another comment, you actually can. Paris has a density of 50000/sqmi (to compare with Manhattan's 70000) without high rise buildings.
What you can't have though, is large avenues and parking lots everywhere. Every inch of space than can be build has to be built.
What you can't have though, is large avenues and parking lots everywhere. Every inch of space than can be build has to be built.