King of Clubs often laments to me “Why the heck is there traffic here all of a sudden? There is no reason!”.
And he would be correct, in the traditional sense of what causes traffic (a police officer, an accident, etc). But as I have often stated to him, vehicles on a road behave remarkably like fluids (and are sometime modeled as such). You see, the reason you most often have traffic is when the density of vehicles on the road reaches a critical point, the tiny variations in driver speed become exaggerated as the distances between vehicles begins to vary in relation to those speed changes.
The net result is that at a certain point, the flow of vehicles can no longer absorb those fluctuations without changing the average speed of the vehicles. Much like a vehicle, a shock wave of traffic propagates back along the flow of traffic, slowing all the vehicles which are following. Hence, traffic forms not because of an incident, but most often because of a some idiot who is a little too nervous at speed.
Here is an interesting demo of the phenomenon.
