Have you ever sat in traffic and then, suddenly, the congestion lessens.... but there was no crash or construction, so what happened? There is a theory about the "Shockwave Traffic Jam". Which at its most basic says that when a driver hits the brakes (from merging or distracted driving) the driver behind him hits the brakes too and so on, which grows into a traffic jam.
This theory has been recreated by having cars drive in a circle at the same speed and equal distance apart. When one hits the brakes though, the car behind him slows and congestion begins. In time, there is a traffic jam for no specific reason (like a crash or construction).
That is a a very simplistic explanation behind the theory of the "Shockwave Traffic Jam" and something for you to think on the next time you are in traffic!