You can also use a shim to release the catch on the ties, or if you have replaced your shoelaces with paracord you can use them as a friction saw by bicycling your legs.

Both of these methods are easier if you have a little slack to play with, so don't tighten your own restraints unless you are certain the zip ties are thin enough for you to break successfully, or you don't have anything available to use as a shim or saw.

Edit: To clarify, I don't mean to suggest shimming or sawing are easier than snapping (Though with very thick ties it can be). I mean it is easier to shim or saw with a little slack, than to shim or saw without slack.