I believe the circle should be green, not red. Still, it's pretty good. :)

The City of Houston should have used that sign in their DHS funded Run/Hide/Fight video. It would sure beat using a folding chair against a shotgun.

Agreed, Green would be a better color.

How do we get businesses to hang them?

This is fairly extreme pedantry, but technically, it should be a green square with a white gun. That would follow ISO 3864-1 for a "safe condition" -- there is no red circle without a slash for "permitted". You can find a recent PDF of the standard if you google around a bit.

Having said that, I don't think anyone would recognize what the "correct" signage meant, nor is that anywhere near as important as the fact that this sign exists at all. I ask pardon for even bringing this up. :)