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Braess's paradox
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From Wikipedia, the free encyclopedia
Paradox related to increasing roadway capacity
Braess's paradox is the observation that adding one or more roads to a road network can slow down overall traffic flow through it. The paradox was first discovered by Arthur Pigou in 1920,[1] and later named after the German mathematician Dietrich Braess in 1968.[2]
The paradox may have analogies in electrical power grids and biological systems. It has been suggested that, in theory, the improvement of a malfunctioning network could be accomplished by removing certain parts of it. The paradox has been used to explain instances of improved traffic flow when existing major roads are closed.
Discovery and definition<br>[edit]
Dietrich Braess, a mathematician at Ruhr University, Germany, noticed the flow in a road network could be impeded by adding a new road, when he was working on traffic modelling. His idea was that if each driver is making the optimal self-interested decision as to which route is quickest, a shortcut could be chosen too often for drivers to have the shortest travel times possible. More formally, the idea behind Braess's discovery is that the Nash equilibrium may not equate with the best overall flow through a network.[3]
The paradox is stated as follows:<br>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 favourable to them, 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.
Adding extra capacity to a network when the moving entities selfishly choose their route can in some cases reduce overall performance. That is because the Nash equilibrium of such a system is not necessarily optimal. The network change induces a new game structure which leads to a (multiplayer) prisoner's dilemma. In a Nash equilibrium, drivers have no incentive to change their routes. While the system is not in a Nash equilibrium, individual drivers are able to improve their respective travel times by changing the routes they take. In the case of Braess's paradox, drivers will continue to switch until they reach Nash equilibrium despite the reduction in overall performance.
If the latency functions are linear, adding an edge can never make total travel time at equilibrium worse by a factor of more than 4/3.[4]
Examples<br>[edit]
See also: Induced demand
When an expressway in Seoul was removed so a creek could be restored, traffic flow in the area improved<br>Braess's paradox has a counterpart in case of a reduction of the road network, which may cause a reduction of individual commuting time.[5]
In Seoul, South Korea, traffic around the city sped up when the Cheonggye Expressway was removed as part of the Cheonggyecheon restoration project.[6] In Stuttgart, Germany, after investments into the road network in 1969, the traffic situation did not improve until a section of newly built road was closed for traffic again.[7] In 1990 the temporary closing of 42nd Street in Manhattan, New York City, for Earth Day reduced the amount of congestion in the area.[8] In 2008 Youn, Gastner and Jeong demonstrated specific routes in Boston, New York City and London where that might actually occur and pointed out roads that could be closed to reduce predicted travel times.[9] In 2009, New York experimented with closures of Broadway at Times Square and Herald Square, which resulted in improved traffic flow and permanent pedestrian plazas.[10]
In 2012, Paul Lecroart, of the institute of planning and development of the Île-de-France, wrote that "Despite initial fears, the removal of main roads does not cause deterioration of traffic conditions beyond the starting adjustments. The traffic transfer are limited and below expectations".[5] He also notes that some private vehicle trips (and related economic activity) are not transferred to public transport and simply disappear ("evaporate").[5]
The same phenomenon was also observed when road closing was not part of an urban project but the consequence of an accident. In 2012 in Rouen, a bridge was destroyed by fire. Over the next two years, other bridges were used more, but the total number of cars crossing bridges...