I've long thought that a large congestion tax on cars entering DC would be great. Charge $10 to enter the city. Take most of that cash and spend it on expanding Metro and putting giant parking garages out at the end of the Metro lines. Make those garages FREE. It would make public transportation a heck of a lot more attractive.

They've tried it in London with mixed results:

At first, the new fees did seem to ease the traffic moving through the congestion zone. Now, studies are finding that the measure has actually managed to somehow slow down the pace of traffic through central London.

The problem here is the reason it hasn't helped - construction and new pedestrian walkways have caused more traffic jams than before. It got rid of 100,000 cars each day, so it sounds like it made a huge difference. I don't think it's fair to blame London's mismanagement of construction and pedestrians on the congestion tax.