One of the neat things about traffic is how it can be analyzed in terms of some physics parameters. You get density waves that can propagate, and end up getting slowdowns that can persist long after the original cause is gone. Or, as this video shows, there doesn’t have to be a real cause, like an accident or scantily-clad jogger distracting the drivers. Just statistical fluctuation can be enough to send a shockwave through the traffic.
There’s a short article as well
(via Cognitive Daily)
UPDATE: more on this at Backreaction. Actual analysis.
The plot shows nicely how the perturbation – the zone of zero velocity (aka the jam) – travels at constant speed in the direction opposite to the traffic flow.