I doubt it would be enforceble if tested, however, it is in fact a great statement by the city. a few years back a town in Louisiana did the same crime dropped drasticly after the law was passed. In all cases where this has happened crime rates fell a considerable amount.
Proveing what gun owners know, crooks will look for the weak unarmed persons to rob or rape. If the think there is a chance you are armed they will most certainly look for easier prey.