Have you ever been zooming down the road and suddenly everyone comes to a stop and then just c r a w l s along. Sometimes we are slowing down for a construction zone that might have been better marked. Sometimes we are hitting the dead zone left behind by a wreck that has been cleared. But sometimes when we get through the delay there is no visible reason – no construction, no wreck, no attention grabbing scene along the road – just open road. It turns out the problem is people. When you let people drive cars and they have a chance to bunch up, they will. And when they bunch up they will slow down even to the point of stopping.
A group of inquisitive folks in the UK did an experiment – putting people in cars and having them drive around a small circuit. Sure enough – traffic waves.
Another group at MIT noodled on this a bit and wrote a computer program that simulated a group of people driving their cars around a similar circuit. It seems a few simple mathematical rules describe how we crowd the car in front and our delayed reaction in slowing and accelerating. These rules give us phantom traffic jams. There is no factor other than driver tendency.