I dunno, I thought maybe post some guardsmen at intersections, near schools, etc. A random smattering to give the killer some pause as he drives around - maybe slow down the kill rate.
And to be on the watch as vehicles speed by: maybe if they get notice something has happened nearby, they can keep their eyes peeled and spot a suspicicious drive-away.
And maybe, just maybe get lucky and catch the perp as he tries to set up another hit and save a life or two.
The perp doesn't seem to be bothered by the fact that a State Trooper is only 50 yards from the last victim. The perp doesn't seem to be bothered by the fact that the whole population of the greater DC area is on alert to his actions and possible detection. I really doubt that having a bunch of weekend warriors standing on a couple of street corners will give him pause.
However, the thought of martial law, which is what this would lead to, sure as hell bothers me!