One interesting thing about US politics is that the concept of subsidies is well camouflaged. Taxpayers subsidise Wal-Mart because the employees need to resort to social services to live on the low wages. Likewise, the US subsidises the automobile by giving free parking, not fully funding highway maintenance, Land use laws that favour sprawl, and so on.
The major one is the cost of petrol since that would be the one place where adding a road tax would make sense: make the users pay for maintaining the roads they use. The other alternative would be to have more toll roads.
But the issue of making sure that transportation is somehow set up to handle varied modes of transportation so that the country doesn't shut down if the main forms are put out of commission due to poor maintenance or weather.
Also, transportation and the issues related to it need to be addressed, since those are yet another thing which is ignored due to distractions in the US political process.
Resources:
Streets Blog: How Driving is Encouraged and Subsidized — By Law
Stong Towns media articles
And a song!