THAT is why the collar can be turned up to 30 feet. That is 60 feet total. It may not still stop a really, really, really determined dog, but I suspect that training to stay away from open doors would help a lot too.
I have two very different dogs. Both are hounds. The one that be gone over three counties after the neighborhood cat will not go out the open door. The one that would take two days to get down 4 houses trying to smell every square inch of what he can see will try to sneak out if the screen door closes just a bit too slow. I'm glad it isn't the other way around.