Responsive Traffic Lights Can Clear Congestion

Traffic lights should respond to cars, and not the other way around, says a new study.

Content provided by Rachel Ehrenberg, Science News
To achieve this rare bliss, traffic lights usually are controlled from the top down, operating on an "optimal" cycle that maximizes the flow of traffic expected for particular times of day, such as rush hour. But even for a typical time on a typical day, there's so much variability in the number of cars at each light and the direction each car takes leaving an intersection that roads can fill up. Combine this condition with overzealous drivers, and intersections easily become gridlocked. Equally frustrating is the opposite extreme, where a driver sits at a red light for minutes even though there's no car in sight to take advantage of the intersecting green.

"It is actually not optimal control, because that average situation never occurs," says complex-systems scientist Dirk Helbing of the Swiss Federal Institute of Technology Zurich, a coauthor of the new study. "Because of the large variability in the number of cars behind each red light, it means that although we have an optimal scheme, it's optimal for a situation that does not occur."
"It's a paradoxical effect that occurs in complex systems," says Helbing. "Surprisingly, delay processes can improve the system altogether. It is a slower-is-faster effect. You can increase the throughput -- speed up the whole system -- if you delay single processes within the system at the right time, for the right amount of time."
The flexible self-control approach reduced time stuck waiting in traffic by 56 percent for trams and buses, 9 percent for cars and trucks, and 36 percent for pedestrians crossing intersections. Dresden is now close to implementing the new system, says Helbing, and Zurich is also considering the approach.

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