From Traffic jams without bottlenecks—experimental evidence for the physical mechanism of the formation of a jam by Yuki Sugiyama et al., New J. Phys. 10 (2008) 033001 (including movies).
If ever you have been driving on a crowded highway, chances are high that you have taken part in a similar "experiment", just that no one has captured it on film and put it on YouTube. This happened to me last Monday on my way to work: First, I got stuck in a traffic jam at the merging of three lanes into two - no wonder in rush-hour traffic. But then there was a second full stop, a few kilometres down the road, and for no obvious reason at all - no construction site, no junction, no accident... it was the classical phantom traffic jam.
This kind of annoying phenomenon occurs on roads with a steady traffic flow if the distances between cars become too small: As soon as a car slows down a bit for whatever reason, the following car must break also, and so on. And because drivers are humans and have a reaction time, they break ever later, and ever stronger, and at one point, they come to a full stop. This stop then moves "upstream", in the opposite direction of the traffic flow - it's a shockwave-like phenomenon that has been intensively studied by German physicists since the 1990s.
But it seems that no one so far has checked the models that describe the phantom jams in a controlled fashion, and so, the Japanese guys have set up an experiment: Take 22 cars, put them on a road, and tell the drivers to go on at a constant speed. As so often in physics, periodic boundary conditions are a useful trick to simulate a much larger system - the cars are driving on a circular track. It doesn't take long before the shockwave develops.
Here is a chart, taken from the paper, that shows the evolution of the flow of the cars:
The horizontal axis shows distance along the circular track, the vertical axis indicates time. The lines trace the paths of each of the 22 cars. The flatter the line, the higher the speed, and a vertical segment of the line means halt. One can see how a perturbation of the steady flow set in after just 40 seconds around metre 150 of the track. At closer inpection, the culprit seems to be a car that was a bit slower than the others for a while. Speeding up (the kink in the orange circle) doesn't help - the following cars have to break, and the phantom traffic jam can't be avoided anymore. 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.
Too bad - phantom traffic jams just happen, it's all physics...
- The short paper about this experiment by the Japanese group (Traffic jams without bottlenecks—experimental evidence for the physical mechanism of the formation of a jam, New J. Phys. 10 (2008) 033001) is available as open access. The text is understandable also to non-physicists! And there is a second movie, showing a bird's eye perspective of the developing crisis.
- Modern understanding of phantom traffic jams started with the papers Cluster effect in initially homogeneous traffic flow by B. S. Kerner and P. Konhäuser, Phys. Rev. E 48 (1993) R2335 - R2338, and A cellular automaton model for freeway traffic by Kai Nagel and Michael Schreckenberg, J. Phys. I France 2 (1992) 2221-2229. A detailed review is Traffic and Related Self-Driven Many-Particle Systems by Dirk Helbing, Reviews of Modern Physics 73 (2001) 1067-1141, cond-mat/0012229.
- Here is a collection of great Java applets which show the emergence of phantom jams in simulations (by Martin Treiber, Technical University Dresden).
TAGS: physics, traffic jam, self-organization