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The Chaos Theory As Seen In Today's Traffic Jams

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On: Thu, Aug 25, 2011 at 4:07PM | By: Peter C Sessler

The Chaos Theory As Seen In Today's Traffic Jams

You’re driving along at a nice pace and all of a sudden, you see brake lights ahead of you, and traffic begins to slow. Probably an accident, you think. After an indeterminable time, traffic once again, begins to move. You keep your eyes open to see what caused the traffic to slow down, but you don’t see anything at all. It’s a bit frustrating not to have someone to vent your anger against, but as you probably have noticed, this is far from being uncommon. Traffic slows and speeds up with no apparent reason.

Don’t worry, though. There are people who make a study of this. Traffic engineers, who use mathematical and computer models, typically find that traffic jams are caused by accidents, bad curves, hills, merging lanes—all logical reasons. The way to alleviate this problem is to simply build bigger and better roads. Sounds logical, but is it?

A few years ago, several German theoretical physicists began publishing papers on traffic flow in Physical Review Letters, Journal of Physics, Nature, and other academic journals. Basically, what they found was that if one simulated the movement of traffic on a highway using similar mathematical equations that describe how gas molecules move, some very interesting events occur. Naturally, cars don’t move like gas molecules—for example, cars avoid hitting the cars in front of them by slowing, whereas gas molecules can’t.

Still, when taking such factors into account, traffic does move or flow like a gas. For example, when a flowing gas hits a bottleneck, it becomes compressed and the molecules crowd together. That compression travels back through the stream of oncoming gas as a shock wave; which is exactly what happens as cars slow. Cars behind them begin to slow and a wave of stop-and-go movement is sent down the highway.

Big deal, you may say. However, the strange thing to come out of these studies is that under certain conditions, traffic jams can come up completely spontaneously. Traffic can be moving along (at a pace below what the road can handle) and then, for no apparent reason, traffic will suddenly slow. You don’t need accidents or other causes. Under the right conditions, a small and brief fluctuation in the speed and/or the spacing of cars can cause a system-wide breakdown that can persist for hours. And the Germans' analysis suggested that such spontaneous breakdowns probably occur quite frequently on highways.

This sort of thing is exactly what happens, mathematically at least, with many physical and biological systems, and is known as the “chaos theory.” Simply put, seemingly minor changes can have disproportionately great consequences. Random occurrences can set off truly chaotic situations without any pattern, and, all of a sudden, calm again. This has been corroborated in long-term weather patterns, various chemical reactions, and now, with traffic jams!

The scientists have found that under certain circumstances, traffic undergoes a phase shift into what they call "synchronized traffic." Cars in all lanes abruptly slow and start moving at the same speed as the cars in adjacent lanes. This, as we know, makes passing difficult at best, and can cause the whole system to jam up for hours.

They also found that it is easier to start a traffic jam than to stop one. For example, a small increase in the number of cars entering the highway from a ramp can trigger a traffic jam even after on-ramp traffic returns to its normal level. They found examples of this on Dutch and German highways.

Obviously, “chaos theory,” and the like, doesn’t sit well with American engineers because things such as widening roads or metering on-ramp flow and other conventional remedies may not always work. Many traffic engineers insist that when traffic jams occur, it is only because no one has really found the reason, which could just be a stretch of bad pavement or maybe a deer running across the highway. As they say, “more study is needed.”

Still, even if traffic engineers can shoot down the German theoretical physicist’s theories, there’s something else they will have to consider eventually. It turns out that the behavior shown by large numbers of cars moving over a network of highways has many mathematical features in common with the behavior of other things that flow over networks, such as data carried by telephone lines and, you guessed it, the internet. This has been a well-studied topic in communications research, and a recent paper on traffic flow draws on this research with this paradoxical conclusion: adding a new road to an existing highway network can, under the right conditions, reduce the carrying capacity of the network as a whole!

Well, so now when you’re late to work, you can blame the chaos theory for your tardiness.


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