The simple answer is nothing. Last time I checked the 10th Amendment was still part of the Constitution giving states broad authority to manage and operate as they see fit. Sadly, two Congressmen don't seem to read the Constitution that way. Congressmen Oberstar and DeFazio have issued multiple warnings to states about entering into public-private partnerships for transportation projects.
Their most recent threat includes withholding highway funds from states if there are public-private partnerships that fail to meet their (I mean, federal) standards. DeFazio has gone so far as to say that he "will put an end to this [public-private partnerships].'' The proposed regulations include a guartee that states could upgrade parallel highways and a protection against price gouging (not sure what this second thing is -- I wonder if Oberstar and DeFazio consider tax hikes a form of price gouging? At least we have an option to use a toll road.)
Virginia Transportation Secretary Piece Homer (also a Democrat) noted, in recent comments, that Virginia's experience (with public-private partnerships) [have] been "a major force in helping Virginia deliver more projects on budget, in shorter timeframe, and with results that please the customer – those who drive on Virginia's roads."
Governor Rendell of Pennsylvania has also fought back against the Congressmen's charge. I suspect that other state officials will be voicing their opinion in the coming days, weeks, and months.
In addition, all of Reason's transportation work can be found here.