If a city had increased its road capacity by 10 percent between 1980 and 1990, then the amount of driving in that city went up by 10 percent. If the amount of roads in the same city then went up by 11 percent between 1990 and 2000, the total number of miles driven also went up by 11 percent. It’s like the two figures were moving in perfect lockstep, changing at the same exact rate.
Now, correlation doesn’t mean causation. Maybe traffic engineers in U.S. cities happen to know exactly the right amount of roads to build to satisfy driving demand. But Turner and Duranton think that’s unlikely. The modern interstate network mostly follows the plan originally conceived by the federal government in 1947, and it seems incredibly coincidental that road engineers at the time could have successfully predicted driving demand more than half a century in the future.
A more likely explanation, Turner and Duranton argue, is what they call the fundamental law of road congestion: New roads will create new drivers, resulting in the intensity of traffic staying the same.
The article goes on to point out that adding alternative means of transportation like trains, buses, and other public transit options has the same effect. It frees up more capacity on the roads and induces more demand.
The idea that this sort of induced demand means that building more or bigger roads will lead to more traffic and therefore to more congestion is not a new one to me. However, I am a bit embarrassed to admit that it was only while reading this article that I realized there is a pretty straight line from this notion to the kanban/lean principle of limiting work in progress.