We (USFK) actually practice evacuating non-essentials twice a year. It works pretty well on paper. I’m no expert but I also know the real world scenario would likely mean nothing going as planned.
I would definitely be more inclined to “shelter in place” than attempt to flee. I’m guessing my designated evacuation point is already on the list on North Korean artillery targets. And as the linked article notes, all roads south would be gridlocked anyway (hell, they get that way on major holidays). And I seriously doubt the Korean bus drivers will be showing up to transport miguks when their own family members are at risk.
So, there ain’t much point in worrying about it, is there?