I don't know if the per-distance-traveled basis is really that meaningful. If air travel didn't exist, people would simply not undertake these trips under the s

I don't know if the per-distance-traveled basis is really that meaningful. If air travel didn't exist, people would simply not undertake these trips under the same "cheap" circumstances they do now, and our attitude toward long distance travel would be very different as a result. It's not like, if I want to go to Europe for a week, I'm choosing between flying and driving. I'm flying or I'm not going at all.