Stationary wave on the M6

One of the joys of motorway travel is getting stuck in queues. You can be there for half an hour making very little progress. You get to the front of the queue thinking there must have been accident, but there's nothing there and you just drive away. So why were we queuing? There must have been a cause earlier on, like someone braking hard, and then because everyone else braked, a queue develops. It becomes an enduring pattern - a stationary wave. The pattern stays in place on the road even as each individual car moves through it. Eventually it disappears when the traffic density is low enough for the stopping distances to become much bigger.