Monday, September 20, 2010

Adaptive traffic lights and the "Green Wave"

Dirk Helbing (click here to see many of his amazing simulations) has a new paper in which he investigates the potential of adaptive traffic lights. Wired has a nice recap. From the story:

Traffic lights that act locally can improve traffic globally, new research suggests. By minimizing congestion, the approach could save money, reduce emissions and perhaps even quash the road rage of frustrated drivers.

The new approach makes traffic lights go with the flow, rather than enslaving drivers to the tyranny of timed signals. By measuring vehicle inflow and outflow through each intersection as it occurs and coordinating lights with only their nearest neighbors, a system-wide smoothness emerges, scientists report in a September Santa Fe Institute working paper.

Here is the logic:

Helbing and his colleague Stefan Lämmer from the Dresden University of Technology in Germany decided to scrap the top-down approach and start at the bottom. They noted that when crowds of people are trying to move through a narrow space, such as through a door connecting two hallways, there’s a natural oscillation: A mass of people from one side will move through the door while the other people wait, then suddenly the flow switches direction.

“It looks like maybe there’s a traffic light, but there’s not. It’s actually the buildup of pressure on the side where people have to wait that eventually turns the flow direction,” says Helbing. “We thought we could maybe apply the same principle to intersections, that is, the traffic flow controls the traffic light rather than the other way around.”

Their arrangement puts two sensors at each intersection: One measures incoming flow and one measures outgoing flow. Lights are coordinated with every neighboring light, such that one light alerts the next, “Hey, heavy load coming through.”

That short-term anticipation gives lights at the next intersection enough time to prepare for the incoming platoon of vehicles, says Helbing. The whole point is to avoid stopping an incoming platoon. “It works surprisingly well,” he says. Gaps between platoons are opportunities to serve flows in other directions, and this local coordination naturally spreads throughout the system.

“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.”


Rethinking the way we manage the infrastructure we already have is a critical and under-appreciated aspect of environmental policy and congestion management. Just because the rules of the road (and traffic engineering) have been in place for as long as anyone can remember doesn't mean that they are right. There may be better ways to do things.

BONUS: Click here to simulate the "Mexican Wave."

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