Cruising in our cities
If you drive a car in many European cities, I bet you're not always happy to see the traffic lights stopping you. And you're not alone: environmentalists agree with you -- but for other reasons. As traffic flows account for about one-third of global energy consumption, better control systems for traffic lights could reduce harmful CO2 emissions. Now, German researchers have developed a self-organized control system for traffic lights that could improve vehicular traffic flow by up to 95 percent. They even patented their combination of two strategies leading to this better control system for traffic lights. But read more...
Of course, an even better solution should be to suppress all traffic lights. "The most well-known design of intersection-free nodes are roundabouts. It is, however, also possible to construct other intersection-free designs based on subsequent merges and diverges of flows with different destinations." You can see two examples above. But what is possible in an ideal new city is not in old European ones, such as London, Paris or Rome.
This is why Dirk Helbing, Professor of Sociology at the ETH Zurich, and Stefan Lämmer, of the Institute for Transport and Economics at Dresden University of Technology, are proposing a self-organized control system for traffic lights.
The main reason to develop such a system is that "heavy investments in traffic light systems were made in the 1960s and 70s rendering most systems today, due to use, age and technological advancement, antiquated." Even today, most of the control systems are "programmed offline, regardless of the realities of the road."
So how Helbing and Lämmer designed a decentralized system that would make travel time more predictable? "First, the researchers tried to optimize the flow of traffic at one light of an intersection. This localized approach worked well as long as traffic flow through the intersection was not too high. Once volume rose, however, locally programmed lights did not clear traffic off of side roads fast enough and led to back-ups at other intersections. Professor Helbing concluded 'On its own, this optimizing strategy was worse than traffic light controls already in place.'"
So they tried another approach. "Another component, a stabilizing strategy, was then studied. This strategy cleared traffic when it reached a critical threshold, but it was inconsistent with travel time minimization. Unlike the optimization strategy, the stabilizing strategy performed poorly at all volumes. On its own, it too could not compete with today’s traffic light control systems."
Finally, they decided to combine the two strategies. "'It turns out that the two strategies properly combined perform better than today’s traffic light controls at all traffic volumes. So the combination of two inferior strategies can perform much better -- if we do it right,' Professor Helbing says. Simulation tests show the combined strategies work well. With non-periodic -- not cyclically repeated -- traffic lights releasing long traffic queues, travel time even becomes more predictable. Flow is kept stable, fuel consumption and emissions are reduced.
Now it remains to be seen if this system can be deployed successfully. In the mean time, you can read one of the latest research works by these scientists, published in Networks and Heterogeneous Media (NHM) under the name "Self-organized network flows" (Volume 2, Issue 2, Pages 193-210, June 2007). Here are two links to the abstract, one from NHM and another one from arXiv.
Here is the text from the arXiv abstract. "A model for traffic flow in street networks or material flows in supply networks is presented, that takes into account the conservation of cars or materials and other significant features of traffic flows such as jam formation, spillovers, and load-dependent transportation times. Furthermore, conflicts or coordination problems of intersecting or merging flows are considered as well. Making assumptions regarding the permeability of the intersection as a function of the conflicting flows and the queue lengths, we find self-organized oscillations in the flows similar to the operation of traffic lights."
You also might want to read the full paper (PDF format, 18 pages, 556 KB). The above image has been extracted from this article.
Finally, the two researchers have patented their self-organized control system for traffic lights. The World Intellectual Property Organization (WIPO) published their patent under the name "Method for coordination of concurrent processes for control of the transport of mobile units within a network" (Number WO/2006/122528, November 23, 2006). Here is the beginning of the abstract. "The invention relates to a method for coordination of concurrent processes or for control of the transport of mobile units within a network, characterised in that the control of the network occurs in a decentralized and self-organizing manner in the controllers of node points or local defined sub-networks."
Sources: Renata Cosby, ETH Zurich, November 15, 2007; and various websites
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