A "traditional" serial killer spaces out his crimes, if for nothing more than to prolong the "pleasure" he gets from it; a mass killer is more concerned with maximizing kills in a single location. This is much more calculating; it appears (at least to me) to be geared toward placing the general public in and around DC in genuine fear of going out in public; to "regular places" that people visit daily - school, the supermarket, the post office, department (or craft) stores, gas stations - I wouldn't be surprsied to see a church/synagogue or hospital next, and God forbid, perhaps even a daycare center before this thing ends.
Disagree. He only needs to be able to identify the right kind of place, and he needs only to stop at the ones he spots as he drives around. If you've got an idea of what would work, it would be easy to spot likely targets.
OTOH, there are many fewer victims now than when this first started. Perhaps he did carefully plan the first spate, and now he's doing them as soon as he can put together another shot. Either that, or he's being more careful to avoid getting caught, which doesn't necesarily fit a nutjob profile.
My theory: they have a car retrofited for this purpose or some other such arrangement to hide the shooter, and the driver just waits for the report and then flees the scene after the one (usually on-target) single shot.....