The fact that the wave propagates explains why the source of the traffic jam is often not near where the traffic will clear up. By increasing the density of the incoming traffic, you can also see how intolerant of minor disturbances a heavy flow can be. Increase the traffic rate and note that halting as little as two cars can cause a jam that fills the entire screen! This implies that the hour you spent in traffic this morning may be little more than the remnants of a near collision when someone was messing with their stereo.
Those things are so frequent on the N1 between Pretoria and Johannesburg that I try to avoid travelling to Johannesburg unless it's absolutely necessary.
For more detailed explanations, see here http://trafficwaves.org/.