FAREBOX RECOVERY. The Texas Department of Transportation concedes that motor-vehicle-specific taxes do not cover the cost of roads.

Applying this methodology, revealed that no road pays for itself in gas taxes and fees. For example, in Houston, the 15 miles of SH 99 from I-10 to US 290 will cost $1 billion to build and maintain over its lifetime, while only generating $162 million in gas taxes. That gives a tax gap ratio of .16, which means that the real gas tax rate people would need to pay on this segment of road to completely pay for it would be $2.22 per gallon. This is just one example, but there is not one road in Texas that pays for itself based on the tax system of today. Some roads pay for about half their true cost, but most roads we have analyzed pay for considerably less. To conclude, in the SH 99 example, since the traffic volume for that road doesn't generate enough fuel tax revenue to pay for it, revenues from other parts of the state must be used to build and maintain this corridor segment. The same is true across the state, meaning that, as revealed by the tax gap analysis, overall revenues are not sufficient to meet the state’s transportation needs.

Note: this is Texas, not the sort of state where substitution to hybrid cars reduces the tire and gasoline tax revenues allocated to road building. The Overhead Wire notes,

And this isn't from some liberal think tank or a transit fanatic saying it, it's the organization that builds and collects taxes for them. And this is just the subsidy for the road, not the oil itself! So when simplistic folks from the Reason foundation propose building roads to relieve all congestion, you ask them who is going to be paying for that ...

Road construction cannot relieve all congestion, as the Law of Peak Hour Congestion is a binding constraint. On the other hand, road construction as a regressive transfer (possibly more regressive than the construction of rapid transit or commuter rail?) has potential commonalities for liberals and libertarians.