Changes

Take the orange line all the way out to Vienna, the last stop westbound. From here, catch the 642 or 632 Fairfax Connector bus to Centreville- all the buses numbered between 632 and 644 go there, but stop in different places close to the highway. Check with the bus driver in advance to make sure that the bus goes all the way. Sometimes the driver will let where he can drop you off right near the highway, other times walk from town ~1km. The on-ramp from Sully Rd onto I-66 is the connector for Dulles Airport, so it is popular with folks going a fair distance, but there's no official bus stop next to it. So sometimes the driver will let you off right near the highway, other times you'll need to walk. If you prefer not to walk, or not to wait and pay for the bus (the buses aren't frequent at all, maybe once an hour), you can try to hitch from outside Vienna metro station itself, but it's not the best spot. There are a pair of truck stops once you hit Interstate 81, just south of the town of Strasburg