Traffic lights that respond to local conditions could ease congestion.
Have you ever wished, as you sat at a red light, that you had the power to switch it to green? A traffic researcher is proposing that giving motorists precisely this power could improve the efficiency of city roads.
The catch (there had to be a catch) is that this control wouldn't be handed over to individual drivers. Instead they would exercise it collectively. If a large enough convoy of cars approached a red light, this would force the light to turn green, while the other lights at the junction turned red.
Carlos Gershenson at the Free University of Brussels, Belgium, outlines his theory in a preprint published on Arxiv1. He says his system of traffic lights would be able to adapt to changing traffic conditions, allowing it to find a better switching sequence than one imposed rigidly on all situations.
Most city streets are currently controlled by lights that operate on fixed sequences, resulting in that frustrating middle-of-the-night situation where you wait at a red light on empty streets.
Down on centralization
There have been some attempts to make traffic signals more flexible, responding to the state of the traffic. These intelligent 'advanced traffic management systems' generally connect the lights to a centralized computer that is constantly seeking an optimal switching sequence.
Such approaches are costly to implement, however, and can be computationally very challenging. In Gershenson's method, by contrast, traffic lights at a junction act on their own, responding simply to the local conditions.
This would not be hard to arrange using existing, relatively low-cost traffic-flow sensors to control each set of lights. Indeed, Gershenson says that these systems already exist in the UK, where they are used for isolated junctions.
No one has dared to install them in the dense network of junctions on urban roads, however, because it seems like a recipe for chaos. But Gershenson says that it is not. He has shown that with just such local control, the system of lights directing flows on a road grid is able to self-organize so that it typically finds a good global solution to the problem.
Gershenson demonstrates this using computer simulations of traffic flow on a Manhattan-like square grid. He compares four different schemes for controlling the lights at junctions. The simplest method alternates between having all the north-south lights green at the same time, and all the east-west lights green. A somewhat smarter scheme aims to match the switching sequence to the typical length of time it takes a vehicle to move from one junction to the next, creating waves of green lights in step with the moving traffic.
In both of those cases, the switching sequence is fixed at the outset. Gershenson also tries two adaptive schemes. In the first, called 'request' control, a traffic light switches from red to green if the number of vehicles approaching it, or the time vehicles have spent waiting, exceeds a certain threshold. Under such conditions, a large convoy of cars can force a red light to go green as it approaches a junction, opening up a 'green corridor' as the convoy progresses across the grid.
Alternatively, in so-called 'phase' control, the same rules apply except that there is a minimum time for switching from stop to go or vice versa. Gershenson finds that, in his simulations, adaptive request control is the most efficient for low traffic densities, but works poorly for dense traffic. Adaptive phase control also works well at low densities, and doesn't clog up at high densities either; so on average, it is the best method overall. Both schemes are typically around 30% more efficient than the non-adaptive ones.
"In a real city the benefits won't be so great," Gershenson admits, because the situation will be complicated by such things as pedestrian crossings and junctions that are more complex than simple crossroads. But even a small improvement in traffic flow, if it comes at a modest cost, would surely be welcomed by motorists.
GershensonC.. Arxiv, Preprint http://www.arxiv.org/abs/nlin.AO/0411066 (2004).
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Ball, P. Beating the lights. Nature (2004). https://doi.org/10.1038/news041129-12