Now to be fair, cars are partly about transportation. Some cars are pretty much just functional ways from getting you from point A to point B. For example, I used to drive one of these:
It was actually a lot of fun.
Now if you're driving something like this to work,
That's about more than transportation--it's about one ton of transportation (a Festiva weighs 1800 lbs) and two and a half (a suburban weights 7000) of vanity, conspicuous consumption, and self-aggrandizement.
It's been pointed out before that America could be fuel-self-sufficient if everyone drove reasonable sized cars. Less discussed, but just as true, is the fact that larger cars exacerbate conjestion--it's not just that they take up more roadway themselves, but cause drivers leave more pavement unoccupied, because you just can't see around those things. And, obviously, a heavier vehicle wears out bridges and aslphalt that much faster.
So, if we're going to talk seriously about spending transportation funds purely on transportation, let's acknowledge that commuting in any car bigger than a compact is a matter of luxury, not transportation--and subsidizing it (by charging nothing for using as much road space as you feel like filling) is a ridiculous was of our taxes.