Road Capacity, Traffic Problems, and Induced Demand

Posted on September 20, 2014. Filed under architecture, behavioral-economics

I’m reading Walkable City, by Jeff Speck (who recently spoke at the Plain Green Conference, part of which I attended).  In it, he talks of “induced demand,” which he says is the thing that everyone in city planning understands but doesn’t talk about.

Basically, if you build it, they will come. He mentions a meta-study called “Build More Highways, Get More Traffic” by Randy Salzman (I could not find it online):

[...] on average, a 10 percent increase in lane miles induces an immediate 4 percent increase in vehicle miles traveled, which climbs to 10 percent – the entire new capacity – in a few years.

Here’s a Wired article which says much the same thing:

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.

I remember reading an article in the local paper about traffic problems in Sioux Falls.  I was struck by one of the quotes from a city planner – something to the effect of, “if it gets bad enough, people will find another way.”

I think this was an end run around the fact that he knew if they built more capacity, people would just suck that up too, and perhaps the right answer was either (1) find a different way to get to work, (2) travel at a different time, or (3) work from home.

Later in the chapter, Speck makes a counter-intuitive claim: “road congestion saves fuel."  Basically, idling in a traffic jam ultimately saves gas because the frustration of it causes people to drive less.  Or, more clearly, not being stuck in traffic – so having lots of road capacity – is so pleasant that people drive more and use more gas in the process.

[...] the threat of being stuck in traffic often will [keep us home], at least in larger cities. Congestion saves fuel because people hate to waste their time being miserable.

The solution, it seems, isn’t to just throw more roads at traffic and environmental problems, because we just end up with the same problem later on, at greater magnitude.

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