Cars will always fill the amount of available road given to them, so I’m not sure that closing the rat run to cars would actually result in the traffic armageddon you suggest.
Maybe not. I'm just saying that I can see how it's ended up like it has. The closed section has a good road alternative (Kingston hill + A3), the park sections still open have the alternative route generally already being a traffic jam down streets that people live on.
Cars will always fill the amount of available road given to them, so I’m not sure that closing the rat run to cars would actually result in the traffic armageddon you suggest.