I think you're forgetting, Stifler, that terrorists tend not to give much notice.
"Pssst. I'm about to blow this vest up, get yourself to the under-pitch bunker quickly"
What about cases like yesterday where they find a device and have to evacuate? What happens if an attack in already underway?
If an attack is underway the last thing people want is to be sent to a designated area where, presumably, the people carrying our the attack will be aware of and possibly waiting there.
I get the sentiment man, really I do, but as far as ideas go it's probably not your best.
Step 1: Blow up a tiny bomb, shout: "Oh no, terrorists! Everybody to the bunker where we'll be like fish in a barrel! Ehrm, safe! Where we'll be safe!
Step 2: Blow up the people in the bunker.