Well, since the people in one of those flows will likely cease to exist through utter annihilation, they're probably going to end up "missing-presumed-dead" and

Well, since the people in one of those flows will likely cease to exist through utter annihilation, they're probably going to end up "missing-presumed-dead" and not actually reported as dead, since there won't be a body. But you are missing the forest for the trees. The point is, was this implemented, every single one of those people driving away would be dead (due to the traffic jam created by just one or two of these cars)... so, it's actually really hard to tell what the impact will be because you prevent what was once easy (evacuation using both sides of the road in hurricanes/volcanoes/etc), into impossible or probably impossible. There simply isn't a way to predict how bad this will be.