Just one clarifying point -- the road from Dulles to Udvar Hazy isn't the Beltway, it's Virginia Route 28. It's quite solidly west of the Beltway, which is a Federal Interstate (#495) and circumnavigates DC, passing through Virginia and Maryland at various points.
http://g.co/maps/5hws6
This is why the locals find getting to Dulles from DC to be something of a nuisance; it's a long way, and there are no good public transit options, so you generally have to arrange a taxi or private transport.
twj