Leigh,

As the common law applies in most US states, the defense is available here also. But our 50 states and many local jurisdictions make any results from the same set of facts a real gamble. It is going to be a hard thing to prove. Here in the US you never know when some local prosecutor is going to decide to "make an example" of someone. There really is not much, if any Federal Law in this area that I am aware of, as these types of "crimes" are usually in the arena of local law. The Federal Court would (usually) if necessary, look to the law of the local jurisdiction, say Anne Arundel County, Maryland, in my case.

A good quick review of the theory of necessity is available at:
http://en.wikipedia.org/wiki/Necessity

Off the thread question, though, if it is proper: Two of the current threads have revealed 1) A 20 foot lethal range for a knife attack and 2) some, uh, strong, anti-EDC knife laws in the UK. Given that your police are unarmed, are #1 and #2 related?

Thanks,

--bws48