After looking at traffic routes in London, New York and Boston, three scientists have concluded that having more routing options can actually slow down the overall rate of travel. More access routes results in Braess’s Paradox:
“For each point of a road network, let there be given the number of cars starting from it, and the destination of the cars. Under these conditions one wishes to estimate the distribution of traffic flow. Whether one street is preferable to another depends not only on the quality of the road, but also on the density of the flow. If every driver takes the path that looks most favorable to him, the resultant running times need not be minimal. Furthermore, it is indicated by an example that an extension of the road network may cause a redistribution of the traffic that results in longer individual running times.”
Eoin O’Carroll explains:
Imagine two routes to a destination, a short but narrow bridge and a longer but wider highway. Lets 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 doesnt happen. At first, everyone will go for the bridge because its 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 whats called the Nash equilibrium (named for the beautifully minded mathematician), 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.
- The Price of Anarchy in Transportation Networks: Efficiency and Optimality Control – PDF – Physical Letters – Paper by Hyejin Youn and Hawoong Jeong, of the Korea Advanced Institute of Science and Technology, and Michael Gastner of the Santa Fe Institute.
- Does closing roads cut delays? – Christian Science MonitorClosing Roads to Help Cut Traffic