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Applying physics to traffic
Oct. 2nd, 2003 06:11 pmFrom Sharon Begley's science column in The WSJ: ( Read more... )
Many of the successes applying theoretical physics to traffic mysteries spring from the fact that cars behave much as molecules in a gas. The equations of gas dynamics, therefore, do a pretty decent job of modeling and predicting traffic.
In a flowing gas, a random fluctuation in molecular movement -- analogous to a car slowing down briefly, perhaps while the driver tunes the radio -- flips smooth flow into a clogged mess. A compression wave, which is simply a zone where the molecules/cars have gotten very packed, propagates backward from the bottleneck.
So far, so intuitive. But the compression wave can persist for hours. That leaves drivers who never saw the first car decelerate totally mystified about why they have slowed to a crawl. By one estimate, three-quarters of traffic jams have no visible culprit. The cause came and went hours ago, but its effects linger.
A defining trait of nonlinear phenomena, such as moving cars and moving gas molecules, is that minuscule fluctuations can have effects disproportionate to their size, like the proverbial butterfly flapping its wings in Beijing creating a hurricane across the ocean.
One such fluctuation is a brief burst of cars from an on-ramp. It makes sense that if you suddenly get, say, a 20-car funeral cortege entering a road, cars behind it will likely have to slow down. But even after the cortege is at the cemetery and the on-ramp traffic has dropped below normal, a bottleneck can persist at the merge. Not trusting the memory of drivers, German physicists confirmed this effect with data from sensors on European highways. Next time you have to slow down as you pass an on-ramp that's empty, blame the butterfly effect. ( Read more... )
Many of the successes applying theoretical physics to traffic mysteries spring from the fact that cars behave much as molecules in a gas. The equations of gas dynamics, therefore, do a pretty decent job of modeling and predicting traffic.
In a flowing gas, a random fluctuation in molecular movement -- analogous to a car slowing down briefly, perhaps while the driver tunes the radio -- flips smooth flow into a clogged mess. A compression wave, which is simply a zone where the molecules/cars have gotten very packed, propagates backward from the bottleneck.
So far, so intuitive. But the compression wave can persist for hours. That leaves drivers who never saw the first car decelerate totally mystified about why they have slowed to a crawl. By one estimate, three-quarters of traffic jams have no visible culprit. The cause came and went hours ago, but its effects linger.
A defining trait of nonlinear phenomena, such as moving cars and moving gas molecules, is that minuscule fluctuations can have effects disproportionate to their size, like the proverbial butterfly flapping its wings in Beijing creating a hurricane across the ocean.
One such fluctuation is a brief burst of cars from an on-ramp. It makes sense that if you suddenly get, say, a 20-car funeral cortege entering a road, cars behind it will likely have to slow down. But even after the cortege is at the cemetery and the on-ramp traffic has dropped below normal, a bottleneck can persist at the merge. Not trusting the memory of drivers, German physicists confirmed this effect with data from sensors on European highways. Next time you have to slow down as you pass an on-ramp that's empty, blame the butterfly effect. ( Read more... )
