Evacuating approximately 25 million people from Seoul (the 4th largest metro area in the world) should be something. It would impossible to so without the north noticing...if it could be done at all.
I was stationed in Korea from ‘95-’98. Chusok is the Korean autumn holiday, kind of a mix of our Thanksgiving and Memorial Day when a lot of Koreans return to their native villages. At the beginning of Chusok, the highways out of Seoul essentially become jammed parking lots...and that’s for a holiday, not a panicked evacuation.
Seoul has huge network of underground malls interconnected by a massive subway system. Did they build anything else useful if under military assault? Additional ways out of the city while remaining under cover?