Washington Flyer bus leaves from the arrivals curb every 30 minutes and takes you to the closest Metro station. Take the Metro train to Arlington, switch trains and it will take you right in front of DCA. The bus is $8. Metro will run you about $3.50 or so.
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Mileage Workshop --- "You can't be a real country unless you have a beer and an airline. It helps if you have some kind of a football team, or some nuclear weapons, but at the very least you need a beer." - Frank Zappa (1940-93)