Michael Byrne:

New roads don’t fix traffic. In fact, they often make it worse. This has not only been observed in many real-life cases, but, as the mathematician Josefina Alvarez pointed out recently in Plus magazine, it’s also a mathematical reality. In 1968, maths thinker Dietrich Braess even proved this: “an extension of a road network by an additional road can cause a redistribution of the flow in such a way that the travel time increases.” It’s called the Braess paradox, and has implications well beyond roads.
 First off, what we’re talking about isn’t really a paradox. It’s just counterintuitive. Here is the not-really-a-paradox stated more completely:
 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.