This topic is far from being simple. Technically, you can carry a knife that was not designed to function as a weapon (like a dirk or dagger). Some LEO's will apply the rule "blade bigger than the hand's width is forbiden"... which is an urban legend but many people beleive it's the rule. Some will consider a locking folder as a weapon... even though it's not stated in any law.
To be on the safe side, go for a non locking folder with a 3" blade or smaller. Something cheap so you can easily replace it.