The point of those making the existing tax argument is well taken, but currently 25-33% of useable roadway is accessible only to multi-occupant vehicles. And in the case of I-66, 100% inaccessible to single-occupant vehicles at peak times.
I-66 is the only road that I could see them tolling. Extend the Dulles Tollway from the Airport to I-66 and through to Downtown. And, charge EVERYBODY tolls, including those going to the airport (except delivery trucks, buses, and taxis- a special airport rate in the cab would make up for this).