"The Mirage of Free-Market Roads," Timothy B. Lee, The Atlantic, March 28, 2012

 "...as tolls go up, they begin to look less like a user fee and more like a tax."

This article written by an author at a libertarian think tank, Cato Institute, and published in a liberal online journal, The Atlantic, highlights the issues in applying ideologies to real-life problems. 

In this case, the author focuses on public vs. private roadways.  We face a narrower,  if similar, issue in the decision to complete the building of rail to Dulles and beyond:  Raising tolls for Dulles Toll Road users to pay for Metrorail.  Obviously, when tolls pay for something other than the entity that is charging the toll, they are no longer a "user fee."  When they are forecast to skyrocket 8-10-fold, they certainly become a tax.  When the tax base is limited to a limited number of people (toll road users) who do not benefit from the purpose for the tax, it becomes abusive.  All this is compounded when the taxing authority, MWAA, has no constraints or oversight of its taxing/toll-setting authority.

Beyond the quotation above, here is Mr. Lee's confusion on the concept of private roads:

While I'm generally sympathetic to the idea of privately-managed roads, I've become convinced that the broader vision of "free-market roads" is a conceptual confusion. In the abstract, the idea of competing, privately-owned roads has a lot of appeal. But the more I think about it, the less sense it makes. Roads are deeply intertwined with governments. They always have been and as far as I can see they always will be. This means that they'll never be truly private in the sense that other private companies like restaurants or shoe factors (sic) can be.

. . . and this doesn't even address the issue of a tax on roads to pay for rail.

Click here to read the rest of this excellent think piece.