If you have a cell phone, call LE and let them know what your location is, they can check to see if a 911 call has been made regarding a shooting in that area.
Good plan, Moose-Owie. Send out the emergency broadcast system warning and just ask people to STOP their vehicles. All it would take is 1 in 100 radio listeners to snarl traffic all over DC.
For example, they would break in and say: there has been a sniper shooting at x & x in Maryland. If you are in the vicinity, please stop your car. Whether something like this would work, we don't know but I agree it will take something like this to stop these people and we ALL have a car radio.
The only problem is, how far can the sniper travel on foot in the 5 minutes it takes to get the broadcast out and how many different people are driving this "clean" van or whatever, if it is part of this at all.
The Emergency sirens should be reserved as the highest warning.
Maybe an amber alert type system (which someone has suggested might work best) and it could be in place forever. The one good thing about the car radio is that it is its' own source of electricity.
Sac