> Especially out West, for vast areas of the US, there really are no good alternatives to driving ...
> And most of the cities are too far apart to reasonably take trains between them.
Heh, that's a very USian response ;). That's largely because the US doesn't have proper long-distance trains. In a lot of other countries you'll be about as fast or faster with a train rather than a car, and can comfortably work for most of that time. Even if a 6h car-trip becomes a 7h train-trip, I'd almost always take the train.
Compare driving Cologne-Frankfurt Airport by car (2h, if the traffic gods smile), and train (45min). That's a faster track, so not quite representative, but even Berlin-Freiburg on mostly slower tracks - I've done that over a hundred times - is ~6.40h train vs. ~8h by car.
I don't see the US getting comparatively nice long-distance trains anytime soon.
> For example, there's very limited ability to visit most National Parks without a car.
Yea, that's true. I've so far done that with rental cars, works well enough unless you're going in winter... Hm, I should plan a trip soon.