Can the Physics of “Green Waves” Help Los Angeles Perfect Its Synchronized Traffic Lights?
If you drive in a major city, you probably experience it every once in a while: Traffic isn’t too heavy, you’re driving at a comfortable speed, and you just happen to hit a series of green lights. You blissfully cruise through several intersections without stopping. It’s a nice feeling when it happens—but it’s also makes for a more efficient and cleaner city. Smoothly flowing traffic means less pollution, less congestion, and less time wasted.
As you might imagine, cities try to engineer this phenomenon by synchronizing traffic lights. Synchronization involves calculating the time it takes to get from one intersection to another, based on the speed limit or likely speed of traffic, and then timing the lights at those intersections so that as a driver approaches each intersection, the light will turn green in time for traffic to proceed without stopping.
New York, San Francisco, and many other cities have synchronized traffic lights in limited areas. But this year, Los Angeles completed the largest traffic light synchronization project in the world. The city synchronized 4,398 traffic lights across 469 square miles. The project was first launched in advance of the 1984 Olympic Games. It was mothballed for a little while, but revived by Mayor Villaraigosa in 2005, and finally completed in February.
The new system “increases travel speed by 16% and reduces travel time by 12%,” says Jaime de la Vega, General Manager of the Los Angeles Department of Transportation. That’s not bad, but it’s lower than the city predicted. And a new report from the Associated Press suggests that the results have not matched the scale of the project. One veteran taxi driver’s take: "To be honest with you, I haven't felt it yet."
So the project has been good, but maybe not great.
Science to the rescue. Physicist Boris Kerner at the University of Duisburg-Essen has been studying the dynamics of “green waves” of smoothly flowing traffic. Using various models of traffic flow, he explored how different conditions can cause a “green wave” to break down. These breakdowns are generally due to an over-saturation of traffic or to a disturbance (as when a car turns into a green wave and disrupts its flow). He is, apparently, the first to rigorously study this phenomenon.
And this is especially promising for Los Angeles precisely because the city has linked a large grid of streets together in a system that can be studied and adjusted in real time. If discoveries like Kerner’s can provide solutions to green wave breakdown, it’s possible that Los Angeles could actually implement them and experiment with them.
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