Shoot To Kill

You may have seen this image floating on the net:

I could fully believe this is genuine (but it's not. See http://www.snopes.com/photos/signs/tubesign.asp ), especially in light of this article in the Guardian:

A police source has told the Guardian that there is no need for officers to verbally warn a suspect before opening fire.
The source said: "If the firearms team are reasonably certain the person is a suicide bomber then there is no need to issue any warning.

Hmm... I feel safe. What constitutes "reasonably certain"?