Saturday, August 8, 2009

Traffic patterns and Braess paradox

An interesting study on traffic patterns in Boston, New York and London has found that individual drivers using navigation devices (including GPS devices that give real-time traffic updates) that guides them to the quickest routes, paradoxically ends up slowing everybody down. The study illustrates it with this example,

"Imagine two routes to a destination, a short but narrow bridge and a longer but wider highway. Let’s also imagine that the combined travel times of all the drivers is shortest if half take the bridge and half take the highway. But because each driver is selfishly trying to seek the shortest route for himself, this doesn’t happen. At first, everyone will go for the bridge because it’s shorter. But then, as the bridge becomes backed up, more drivers start taking the highway, until the congestion on the bridge starts to clear up. At that point more drivers go back to the bridge, which then becomes backed up again. Eventually, the traffic flow settles into what’s called the Nash equilibrium, in which each route takes the same amount of time. But in this equilibrium the travel time is actually longer than the average time it would take if half of the drivers took each route."


This finding is an excellent illustration of the Braess Paradox which states that adding capacity to a network in which all the moving entities rationally seek the most efficient route can sometimes reduce the network’s overall efficiency.

However, under such circumstances, it is possible to reduce travel times by closing a few roads, since it leaves individual drivers less able to selfishly optimize their routes. The simulation models in the study shows that un-coordinated drivers waste a considerable amount of their travel time and that simply blocking certain streets can partially improve the traffic conditions. They write,

"Un-coordinated individuals in human society pursuing their personally optimal strategies do not always achieve the social optimum, the most beneficial state to the society as a whole. Instead, strategies form Nash equilibria which are often socially sub-optimal."


Behavioural psychologist John Staddon of the Duke University has written about "shared streets" that involve removing all traffic controls – lights, signs, road markings, and even the distinction between streets and sidewalks – can actually make traffic move more smoothly, as well as cut down on the number of accidents and increase the area’s economic vitality. It is felt that the lack of traffic signs "makes vehicle drivers take personal responsibility for directly negotiating with the pedestrians, cyclists, and other cars around you, instead of, say, gunning it through an intersection just because you know you have the light".

An anecdotal example of the success of such anarchical traffic models is the "roundabouts" (whose only rule is to give way to traffic coming from the right) in Central Delhi, which despite the absence of guiding signages and traffic lights and the fear of collision that strikes a vistor, has been remarkably effective in minimizing traffic accidents.

Update 1
London is experimenting with doing away with traffic signals, so as to instil a bit of indecision in all road users’ minds to create a safe environment.

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