While conventional American urbanism is known to drive excessive sprawl, it is worth remembering that, ultimately, the true extent of a city's horizontal expansion lies far beyond the reach of urban planning controls.
Take Houston, for example. Whenever something positive is said about its relatively flexible approach to land use, someone invariably replies: “Yes, that is all well and good in theory. But in practice, heavily regulated cities like Boston are much more walkable, so perhaps looser land-use regulations aren't such a good thing after all.”
