[SCR1]

EXACTLY - as well as the air raid sirens use the Emergency Broadcast System to alert drivers to all STOP in the streets and OBSERVE and REPORT.

This would be the alternative to STOP, DROP and ROLL as we in when we are on fire

[rwt60]

Actually, there is a simpler way than the Emergency Broadcast System. Simply get a number of these mobile construction signs with programmed messages. Leave them at strategic locations with an innocuous message like "Have a nice day." As soon as the 911 call comes in on the next shooting, switch the signs to read: "Sniper Alert - Please Stop Immediately." That would halt traffic well ahead of police roadblocks and may ensnare the shooter.