Once long ago I was driving in LA (Pomona Fwy as I remember but could be wrong). It was 2:00 am on a Tuesday night and I was basically running through the desert. All of a sudden I ran into the worst traffic jam I have ever seen. Miles and miles of cars backed up. It was the dead of night on a week night! Several hours later when I reached the choke point of the jam, there was nothing there. Nothing at all. I am baffled to this day.
My cousin is a civil engineer with the NC DOT, he explained a few things about traffic flow to me, and one of the more memorable descriptions was the accordion effect. Traffic stalls, due to an accident, rubbernecking, debris in the road, just too many cars attempting to fill too few lanes or whatever, and the constant stop-start of traffic behind it gets magnified and radiates backwards, sometimes for many miles. It continues long after the obstruction of flow is removed. This is why you sometimes come to a dead stop in the middle of nowhere, and never even see the reason why. It was cleared hours before.