It's the witness reports. Witnesses ID'd the truck and - perhaps - the shooter.
If they shot from the opposite side of Rt. 50, it is simply too dark, and too far, for witnesses in the HD parking lot to have seen anything in that detail.
My presumption is based solely on that fact that there are witness reports, which means that the shooter must have been parked briefly on the access road, which is 10 feet from the HD parking lot, and directly adjacent to it.
If that's the case, they were on a one-way street, which intersects (dead ends) at Patrick Henry Drive, and that's a right-turn-only intersection. Trying to get directly onto Rt. 50 (westbound OR eastbound) from that intersection would be more than a traffic infraction, it would actually be dangerous and bizarre (this is a very busy intersection at day and night).
Yes, I suppose they could have made the right, made a U-turn, sat calmly at the (very very long) traffic light, and headed west on Rt. 50 when they got the green light, but that doesn't seem sensible. These shooters seem to be very adaptable, and they certainly must have taken a lesson from the Friday roadblocks on all the interstates. So why would they head toward 495, an obvious spot for a roadblock, and they still have 10-15 minutes to get there, again after waiting at a long traffic light within eyeball distance of the HD parking lot (maybe 100 yards?). The whole scenario doesn't wash.
Rt. 7 west pretty quickly leads into Falls Church City (after maybe 1/4 mile or so from HD). The speed limit - strictly enforced - is 25 MPH for about 3 to 4 miles. Stoplight after stoplight, even at 9:30PM. Not a good choice for an escape route.
You didn't ask about Rt. 50 east. That also doesn't wash. It's a big, open road, not highly trafficked once past Seven Corners, and doesn't lead anywhere except into DC. Not the kind of road where one can blend in at 9:30PM, which is what these shooters seem to do.
Rt. 7 eastbound has enough 9:30PM traffic to make that possible.
I agree with you completely. And, there are tons of side roads off Rt. 7.