Here is a picture the intersection in front of my workplace which was closed-off for the drill:
To conceptualize the scene as I saw it, you'd have to first image the steady wail of a siren. I have no recollection ever hearing one in the USA. The closest thing was the "fire drill" in school. This was a society-wide fire drill, I suppose.
So, right, the siren. Imagine, further, that all traffic, including foot traffic, is stopped cold. No exceptions. Picture two policemen blowing whistles at anyone who flinches, and four to six retired-age volunteers in a bright uniform that resembles a jersey. The volunteers are holding flags that say "Civil Defense" in Korean.
Fifteen minutes float away thusly. Everyone is frozen in place. A captivating nothingness.
Well, actually, it was not all nothingness. There were was the whistling whenever someone broke ranks and tried to cross. And, the highlight of the nothingness: A few minutes in, a little convoy rolled through: an army jeep, a fire truck, an ambulance, and two or three other such vehicles. They drove fast and on the left (the wrong way in Korea). As all normal traffic was stopped, the path forward -- on the left or right as needed -- was clear. That must be the purpose of the drill, to practice allowing military and other vehicles to travel at high speeds unimpeded.