“These projections have a fatal blind spot: They fail to consider how humans respond to changing conditions like new vehicle lanes. When people see cars traveling freely over a recently expanded highway, they will recalibrate their travel decisions. Some will choose to drive at rush hour when they would have otherwise driven at a non-peak time, taken public transit, or perhaps not traveled at all. When a roadway is widened, Marshall said, “You might have less congestion at first, but it quickly goes away.”
Such behavioral adjustments will continue until traffic is as thick as it was before, when the roadway was narrower. The only difference is now there will be more cars stuck in traffic, emitting even more pollution.
This phenomenon is known as induced demand. In his book Fighting Traffic, historian Peter Norton notes that as early as the 1920s, a New York City engineer warned that new roadways “would be filled immediately by traffic which is now repressed because of congestion.” In the 1960s, the economist Anthony Downs wrote a seminal economics paper that codified the concept, which has been called the Iron Law of Congestion. As one researcher put it, “If you build it, they will drive.””
https://www.vox.com/future-perfect/363013/wide-highways-climate-environment-pollution