I don't actually know if this would be cheaper, but it might be worth looking into: using Metro to get into the city and then signing up for Zipcar and Capital Bikeshare to get around once she's downtown. If she's paying tolls she's out toward Reston, right? So she could potentially take the silver line into DC, then use Zipcar when she needs to drive from place to place within the District. For short distances she could use CaBi. Zipcars are all over the place; you have to return the car to the same place you got it, but you can park in any legal street parking space in DC for free. CaBi is also all over the place, and you can return the bikes anywhere -- I use them a lot for short trips. It sounds a little "Planes Trains and Automobiles," I know, but it's easier than it sounds, and I'd do virtually anything to avoid the headache of driving into town from the Dulles Toll Road!