Traffic Jams
I have heard that when there is no apparent reason for a traffic jam it probably started from someone tapping their brakes. It gets worse because drivers move from lane to lane. The next lane is moving faster so a bunch of people move into it. This of course causes the ones already in that lane to slow down and hit the brake lights and eventually that lane slows down and all these idiots go back to the lane they just left. Of course an accident or something that causes people to rubber neck will cause jams but I was in one recently that just did not seem to have any reason for it to exist.