Addiction to the automobile. Ironically, most of US's growth came at time when we used a far more efficient mode of transport for long-distance transport-the train. Don't say I'm against auto's just our over dependence on them.
The train is only more efficient under very specific conditions.In normal use it's far more wasteful of resources and it's relatively inflexible.Under ideal conditions it does a brilliant job , in particular mass transit between high density areas of population but outside of this it's not a solution.