Okay, so everybody here is right and wrong. I love putting it like that. We need to look at the issue holistically, not just from the perspective of "a typical family" seeing as how there is no real typical family.

The basis for cost in the planes, trains, and buses are all based on personal usage, and thus, will always be more expensive by driving. The difference will only grow with the number of people you have in the car. This is called economies of scale.

Now, the other thing is that when you look at the difference between any alternatives, it is best to find the similarities and differences, and split these out into categories. For instance, you would compare the air fare, bus fare, and gas in one comparison to find out this difference. Once that is done, then you would look into the travel from airport, train station, and bus station to the final destination (hotel or whatnot). On top of this, you would consider how you would get around for the vacation, which was not considered in the NPR analysis, and could be a huge decider in which is cheaper. Has anybody tried parking around Washington DC lately? This method of looking at the situation is considered "incremental cash flow analysis", or kind of a derivation of it.

Also, I just have to mention that Amtrak stations are never anywhere near where you want them to be. Airports are much more likely to be close to where you are trying to go, at least they have been in my travels.

Lastly, when the mileage increases, so do the costs of food and shelter (hotels). These were not considered in the NPR analysis because they were only going to Boston. increase the mileage and you'll have a hotel stay, which will add quite a bit to the cost, along with meals. This could eat into the savings over any other alternative.

If you were to ask me, I think that the analysis was oversimplified and forced people to accept one single number for the cost when there are too many factors involved to make such a generalized statements. A case in point is my Grandfather. He is traveling across Texas by plane cheaper than driving because:
1) he is alone
2) gas costs a lot
3) he would get a hotel room on the way
4) meals cost a lot

Well, that's my two cents. Good night all!