The Fundamental Law of Road Congestion

Most of us—including politicians—believe that building more and wider roads reduces congestion. However, research dating as far back as the 1960's proves these benefits are generally temporary as vehicles soon fill new lanes.

In “The Fundamental Law of Road Congestion: Evidence from U.S. Cities,” published in American Economic Review in 2011, researchers explore the relationship between road capacity and congestion. The researchers from the University of Toronto and the London School of Economics, tried to open up the relationship between infrastructure and congestion.

The study’s findings include:

1. The number of vehicle-kilometers traveled (VKT) increases in direct proportion to the available lane-kilometers of roadways. The additional VKT traveled come from increased driving by current residents and businesses, and migration.

2. Building new roads and widening existing ones only results in additional traffic that continues to rise until congestion returns to the previous level.

3. Increasing the lane kilometers for one type of road does not significantly reduce congestion on others — for example, widening highways does little to reduce local congestion.

4. Metropolitan areas appear to construct new lane-kilometers of roadway “with little or no regard for the prevailing level of traffic.”

5. Because roadways have “natural” levels of congestion to which they always return, mass transit projects will not reduce traffic.

Our results strongly support the hypothesis that roads cause traffic,” the researchers conclude. Once again common wisdom is wrong. The authors suggest that congestion pricing is the best approach.