Originally Posted by JCFlack

The Mt. Vernon trail spur is there, but be warned that it has grade crossings through heavy traffic at several spots. The Arlington bike map shows that rut as being a paved trail. I've been in contact with the Arlington county organization responsible for the map, and they say that the rut is supposed to get paved, but the National Park Service owns the land, and Arlington has been in negotiations about who will pave it, and who will pay for it. They may fix the map in its next edition.
I've used the Rt.110 shoulder, which is quite wide, to go north from Pentagon to Rosslyn along that route, but I wouldn't advise it southbound.