The shooter has a few options to get out of there. If the sniper went north on Route 7 he would get to I-66, south on Route 7 gets you to I-395. West on Arlington Blvd, Rt 50, takes you to the beltway.
My guess is the sniper was in the strip mall parking lot north of Arlington Blvd and shot into the Home Depot parking lot on the south side of Arlington Blvd. He then could take Wilson Blvd to Sycamore Street north to I-66. It's the fastest way to get to an interstate from the Seven Corners area.