Memory of the road
During my slightly hungover drive to work this morning, I had a rather interesting realization. At several points along the way, I had to break sharply, to maintain distance in front of the car ahead. Why were they braking? Precisely the same reason I assume. In all likelihood the real danger was long gone.
So long as the quantity of traffic stays at a sufficient level, such as on a highway like SR520; these artifacts can potentially last for hours. The memory of the road.