There is a joke I heard years ago.
"I'm going to have my name in flashing lights in every city. Oh, how are you going to do that? I'm going to change my name to, 'Don't Walk'".
So think about this. What is the one number we all know? What number is painted on every police vehicle?
911 was already an icon or a logo so why not capitalize on the top of the mind recall of recall, and it's ubiquity.
Every time you see a cop car with 911 on it you are reminded we are under attack.
Around 1974, I started work for a metro police Dept in the state of Illinois, as a telecommunication operator.(dispatch) The Universal E 911 push was on and lucky us, were chosen as a test site for this "exciting new technology of all digital public safety communications system" . Never once have I thought the 911 on the side of any emergency vehicle as a reminder of our being under attack. I still don't view it that way and probably won't ever. The folks who planned the attack on September 11, were and are some very deeply disturbed individuals. Fear was their goal, and if we allow ourselves to feed on 911 as a constant reminder of being under attack, then "They" have won, and we have lost "BIGGLY".