"Runaway"? From a safety aspect, theoretically an alternator-equipped car can continue to run with the battery disconnected. The car will run as long as the electrical system has voltage; the alternator will continue to put out voltage as long as the field circuit is "excited"; the field circuit will remain "excited" as long as the alternator is putting out voltage. See the Catch-22 there?

I've never tried shutting of my car with the kill switch without the alternator/ignition properly wired, so I can't say that's what will happen for sure. But I certainly don't want to try it on my car. Maybe this is a good one for Mythbusters?
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The car keeps running. My car had a single pole switch on it when I got it that just disconnected the battery from the car. I didn't know any better at that point. I go to my first ever annual tech and the first thing the inspector does is throw the kill switch. The car keeps on running. My comment was "It's not supposed to do that is it." I have no idea how Bob passed annual. I replaced it with a three pole switch that disconnects the computer signal to the ignition coil. The car now stops. Moral of the story: You need something other than just disconnecting the battery from the car.

David