Self-organizing traffic lights

Currently, traffic jams and road congestion do a lot more than annoy millions of people every day. In the United States alone, delays linked to backed-up traffic cost nearly $100 billion each year, and waste more than 10 billion litres of fuel, not to mention countless human hours. And then there's all the extra CO₂ and other pollutants spewed into the atmosphere. As developing nations become more industrialized, these problems will only grow worse.

Unless there is some radical new solution. We can build more roads, of course, encourage more people to ride bikes or share their cars with others, and improve buses and other forms of public transport. But there may be another way.

As Stefan Lämmer at the Institute of Transport & Economics of TU Dresden and Dirk Helbing of ETH Zurich have recently shown, we could reduce traffic congestion markedly by re-thinking the way we try to control how traffic flows. We're fixed on the idea that lights should cycle on and off in a regular and predictable way, but this idea, they say, is unnecessarily restrictive. And less orderly patterns could be far more efficient, reducing travel times for all, and making traffic jams far less frequent.

Engineering without engineers...

At the moment, traffic engineers normally tailor the cyclic operation of lights to match known traffic patterns from the recent past. Lights on main roads stay green longer during peak hours, for example. But so far it requires supercomputers or engineers, who do the tuning.

Lämmer and Helbing wondered if traffic lights might devise better solutions on their own, if given some simple traffic-responsive operating rules and left to organise their own on-off schedules. To find out, they modelled the flow of traffic as if it were a fluid, and explored what happens at road intersections, where traffic leaving one road has to enter another, much like fluid moving through a network of pipes.

Jams can arise, obviously, if traffic entering a road overloads its capacity. To avoid this, Helbing and Lämmer gave each set of lights sensors that feed information about the traffic conditions at a given moment into a computer chip, which then calculates the flow of vehicles expected in the near future. It also works out how long the lights should stay green in order to clear the road and thereby relieve the pressure. In this way, each set of lights can estimate for itself how best to adapt to the conditions expected at the next moment.

Efficiency of self-organization

They found, however, that this simple rule isn't enough: the lights sometimes adapt too much. If they are only adapting to conditions locally, they might stay green for too long and cause trouble further away. To avoid this, Lämmer and Helbing modified things so that what happens at one set of traffic lights would affect how the others respond. By working together and monitoring the lengths of queues along a long stretch of road, the self-organised lights prevent long jams from forming.

Despite the simplicity of these rules, they seem to work remarkably well. Computer simulations demonstrate that lights operating this way would achieve a significant reduction in overall travel times and keep no one waiting at a light too long. One of the biggest surprises, however, is that all this improvement comes with the lights going on and off in a seemingly chaotic way, not following a regular pattern as one might expect.

Reducing delay time by 10%-30%

The key is that this kind of control does not fight the natural fluctuations in the traffic flow by trying to impose a certain flow rhythm. Rather, it uses randomly appearing gaps in the flow to serve other traffic streams. According to their simulations, this strategy can reduce average delay times by 10%-30%. Remarkably, the variation in travel times goes down as well, although the signal operation tends to be non-periodic and, therefore, less predictable. You can't say precisely how the lights will go on and off, but you can be sure your drive will be shorter.

What's more, Helbing points out, the scheme eliminates other irritating problems, such as drivers have to wait a long time at empty intersections because the lights' schedules are determined by the traffic flow at busier times, or lights cycling even in the middle of the night when there is no need. The self-organising traffic scheme eliminates these problems because the lights remain responsive to local demands, for instance sensing an approaching car and changing to green to let it through.

Town planners are beginning to look at self-organising lights as a practical solution to looming traffic congestion. Lämmer and Helbing are working with a German traffic agency to implement the idea, soon. In previous tests based on Dresden's road layout, they've had encouraging results.

Working paper: Self-Stabilizing Decentralized Signal Control of Realistic, Saturated Network Traffic