The problem is that the costs of sprawl aren't effectively conveyed to the commuter. The increased costs of infrastructure, road maintenance, etc aren't remotely made up for in vehicle, property or fuel taxes.

In which case, yes, it's a risk if it continues. Hell, it's a risk if it doesn't decline as the US approaches a GDP-growth situation not-unlike Japan's.

(Where per-capita GDP may continue growing, but net GDP barely moves due an aging populace and declining birth-rate.)