In practice, new cities, when that even happens, doesn't happen out of thin air either, not in the US at least. We don't build cities (too much infrastructure investment required), but even when we do, they don't go from 0-100 mph and go from being a small sleepy town into a city overnight. History of a place thus dictates what the future of it will look like. How we build cities, effectively, involves decreasing traffic capacity in some places. Strongtowns.org has a large number of other changes, not just limiting stroads.