Conclusion: this is Islamic terrorism.
Apparently several witnesses saw the truck, and perhaps even the shooter. If that's so, he/they must have been on eastbound Rt. 50 itself or on the access road directly adjacent to the HD parking lot (being inside the lot would be a difficult exit at best, it's kind of tricky getting in and out).
It's unlikely they would have been on Rt. 50, as you say it's very crowded at the Patrick Henry Drive intersection, cars are moving at 40 to 60 MPH, and there's really no place to stop.
So they were probably in the eastbound access road, which means they would have driven another 100 yards to the end, and then forced to turn right onto Patrick Henry (not only is a left turn illegal, it also would be a difficult and extremely bizarre maneuver at that intersection).
So let's say they turned right onto Patrick Henry Drive. From there its another 200 yards to the intersection with Rt. 7 (it seems unlikely that they would have hidden in one of the nearby apartment complex parking lots, but who knows). In any case, a right turn brings them almost directly back to the scene of the crime (actually to the other side of the shopping center - the upper level - where the infamous Michael's is located). A left turn puts them on Rt. 7 east toward Bailey's Crossroads.
By all news reports, the shooting occurred at 9:15 and the police roadblocks started going up at 9:30. The route I've described above would have put them on Rt. 7 eastbound within 5 minutes tops, and that includes waiting out the normal red light at Rt. 7 and Patrick Henry.
They're not using the interstates anymore, they've seen the roadblocks (and these guys have proven themselves to be adaptable). So weren't headed for I-395, they only interstate in that direction. They know that they have a few minutes of an escape window. They know that they had only local roads.
They are obviously heading someplace knowing that (a) all the interstates will be blocked and (b) they have just a few minutes to get off the streets. They are on Rt. 7 east.
I hate to say it, but they would then be only 5 to 10 minutes from one of the largest Muslim enclaves in all of DC Metro. In fact they will pass one of the area's largest mosques only one minute after leaving the Patrick Henry/Rt. 7 intersection.
Of course this could be entirely wrong, the shooter was maybe on the westbound Rt. 50 access road (in the dark parking lot behind the school), but then (a) there would be no witnesses, it's simply too dark on the other side or Rt. 50, (b) its at least 10 minutes drive to the Beltway, if you're lucky (c) there are lots of traffic lights and busy intersections on the way there and (d) the shooters must have known the roadblocks were going up.
Whether they were forced or had chosen to get on Rt. 7 eastbound isn't that important. They knew they had only 5 to 10 minutes to get off the streets. They're headed toward Bailey's Crossroads. The can't get on major highways.
What was their final destination?
IMHO, they had a pre-planed escape and route mapped-out before they killed her. With an escape plan in place, they just waited in the parking lot or wherever for a target of opportunity. I think the killer is not a lone nut. They are hunter killer teams that do this with methodical planning. Shooting people at a distance with no apparent rhyme or reason is difficult things to solve. We can hope they make mistakes and somebody gets a good description that leads to there capture.
I grew up in Bailey's Crossroads and there are tons of escape routes from that Seven Corners area. If it were me, I would have gotten back into the sleepy hollow area and laid low and worked my way over to the beltway.
This perp is a cool customer and I think he isn't rushing onto the major roads to flee the crime scenes.