Induced Traffic

I've mentioned Robert Caro's book The Power Broker before. After covid set in and Zoom interviews became a thing on television, I noticed that this book seemed to be the most common one I spotted on shelves behind various people I saw interviewed. Not that it was there all the time, of course, since there were so many books overall, but I did notice it at least a half dozen times. 

I recently noticed a page from the book snapshotted on Twitter by Ray Delahanty, who goes by CityNerd @Nerd4Cities:

The fact that we’ve known about induced traffic for nearly 100 years is just a massive scandal. (From Caro’s The Power Broker.)

Here's what that says:

These planners had said...that the movement of people and goods in a great metropolitan region required a balanced transportation system, one in which the construction of mass rapid transit facilities kept pace with the construction of roads. During the last two or three years before the war, a few planners had even begun to understand that, without a balanced system, roads not only would not alleviate transportation congestion but would aggravate it. Watching Moses open the Triborough Bridge to ease congestion on the Queensborough Bridge, open the Bronx-Whitestone Bridge to ease congestion on the Triborough Bridge and then watching traffic counts on all three bridges mount until all three were as congested as one had been before, planners could hardly avoid the conclusion that "traffic generation" was no longer a theory but a proven fact: the more highways were built to alleviate congestion, the more automobiles would pour onto them and congest them and thus force the building of more highways — which would generate more traffic and become congested in their turn in an inexorably widening spiral that contained the most awesome implications for the future of New York and of all urban areas. (emphasis in the original)

Induced traffic (also called induced demand) has an inverse, called traffic evaporation. When highways are removed, after a relatively brief period of disruption, drivers make different choices and there is significantly less traffic. It doesn't all just go onto another nearby road or street. Traffic is not like water, which always finds a way through; traffic is created by people, and they make choices.