About two years ago, New York Times reported on Mayor Bloomberg's decision to close Broadway to vehicles to ease traffic. The problem was it only eased traffic by about 4% which was much less than the 17% projected.
But why would closing the most effective route to cut diagonally across Manhattan ease traffic?
Counterintuitive as it may seem, closing the most efficient route is often a good way to ease traffic. This occurs when it is clearly the shortest route to take so all drivers wanting to take the shortest route (basically everyone) will end up taking it. This is caused by creating a disturbance to the equilibrium that exists when you to have two equidistant routes to get from point A to point B and the drivers split rather equally between the available routes. The principle is called the Braess Paradox.
To quote Wikipedia:
"Braess's paradox, credited to the mathematician Dietrich Braess, states that adding extra capacity to a network when the moving entities selfishly choose their route, can in some cases reduce overall performance."
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