Physics of 'green waves' could make city traffic flow more smoothly

[Jim__]

Timing traffic lights so traffic flows optimally in 4 directions has interested me since I was a little kid. I'd love to see the results of an empirical study on this.

From phys.org:

(Phys.org) —If you've been lucky enough to catch all the green lights as you drive down a busy street, you may have been benefiting from intentional synchronization called a "green wave." The green wave concept has been around in the US since the 1920s, but it doesn't always work as it should. When traffic gets backed up for some reason, "green wave breakdown" occurs. In a new paper, physicist Boris Kerner at the University of Duisburg-Essen in Essen, Germany, has modeled and analyzed the causes of green wave breakdown, and the results may lead to better coordinated green waves and more efficient traffic flow.

Kerner's paper, "The physics of green-wave breakdown in a city," is published in a recent issue of EPL.

Many large cities around the world, especially in Europe and the US, synchronize traffic lights on the busiest streets to create green waves. When a green wave works as intended, all vehicles within the wave can drive through a sequence of green traffic lights at a certain speed without having to stop at the signals. The timing of the lights can be controlled either by sensors or timers, and can be set up for traffic in one direction or both directions. Green waves have several benefits, such as allowing for higher traffic loads, reducing traffic jams, controlling traffic speed, reducing fuel consumption and emissions, and facilitating bicycle and pedestrian traffic.

The biggest disadvantage of green waves is that, when the wave is disturbed, the disturbance can cause traffic problems that can be exacerbated by the synchronization. In such cases, the queue of vehicles in a green wave grows in size until it becomes too large and some of the vehicles cannot reach the green lights in time and must stop. This is called over-saturation. As more and more vehicles stop, the traffic can cause a gridlock where vehicles can't move forward even when the light turns green because vehicles are backed up at the light ahead, which may still be red or turning green at the same time.

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[Scuba]

1. More and more traffic circles are being built here. Love them.

Safer and faster.

[Javaman]

2. There are a couple of instances of "green waves" here in Austin.

If you hit it just right, you could go north to south in midday traffic in about 15 minutes.

it's hell if you are going south to north, though.

[longship]

3. The green wave really works on 8 Mile Rd in Detroit.

I have driven all the way across Detroit without a single red light. I have done it many, many times. All the lights are timed for 45 mph. Once you get into the groove you can stay in the groove. From Grand River (in the far west) to Grosse Point (along Lake St. Clair) without stopping once.

On edit: 8 Mile Rd. forms the north border of Detroit. It is a divided boulevard with three or four lanes in each direction. There are many traffic lights along the way. There are two places where 8 Mile avoids a traffic light with overpasses, once at a Greenfield, once at Woodward, Detroit's Main Street. Otherwise it is just like any surface street, albeit a large one.

[phantom power]

4. I'd bet a donut that traffic optimization is NP-hard....

or maybe even more to the point, obtaining an approximate optimization is also "UGC hard:"

http://simonsfoundation.org/features/science-news/mathematics-and-physical-science/approximately-hard-the-unique-games-conjecture/

[tridim]

5. The green wave on the street I drive to work, works about half the time.

Or in other words, it doesn't work at all.

I think they need to hire a new programmer.