It’s very difficult to measure networks’ performance, in part because they are so complex, but also because people use them differently at different times, and because those choices affect others’ experiences. For example, one person choosing to drive to work instead of taking the bus puts one more car on the road, which might get involved in a crash or otherwise contribute to a traffic jam.
In 1968, German mathematician Dietrich Braess observed the possibility that adding a road to an area with congested traffic could actually make things worse, not better. This paradox can occur when travel times depend on the amount of traffic. If too many drivers decide their own optimal route involves one particular road, that road can become congested, slowing everyone’s travel time. In effect, the drivers would have been better off if the road hadn’t been built.
We shouldn’t waste time and money building or repairing network links the community would be better without. But how can we tell which elements help and which make things worse?