Detours by Design; February 2009; Scientific American Magazine; by Linda Baker; 3 Page(s)

Conventional traffic engineering assumes that given no increase in vehicles, more roads mean less congestion. So when planners in Seoul tore down a six-lane highway a few years ago and replaced it with a five-mile-long park, many transportation professionals were surprised to learn that the city¿s traffic flow had actually improved, instead of worsening. ¿People were freaking out,¿ recalls Anna Nagurney, a researcher at the University of Massachusetts Amherst, who studies computer and transportation networks. "It was like an inverse of Braess¿s paradox."

The brainchild of mathematician Dietrich Braess of Ruhr University Bochum in Germany, the eponymous paradox unfolds as an abstraction: it states that in a network in which all the moving entities rationally seek the most efficient route, adding extra capacity can actually reduce the network¿s overall efficiency. The Seoul project inverts this dynamic: closing a highway¿that is, reducing network capacity¿improves the system¿s effectiveness.