If You Build It, Cars Will Come: the Common-Sense Logic of 'Induced Demand'

For decades, conventional wisdom in transportation planning circles held that "when a highway is choked with traffic, the solution is to expand it or build another road nearby." Yet research from projects around the country, writes Alan Ehrenhalt in Governing, has disproven this theory. When Los Angeles widened its notorious I-405 freeway in 2014, "[t]ravel times actually increased once the project was finished, although rush hours shortened slightly." Meanwhile "the Katy Freeway in central Houston, expanded in 2011 to more than 20 lanes in some segments, making it one of the widest highways in the world," saw travel times out of downtown increase "by some estimates as much as 30 percent" after the project's completion.

The increase in traffic is due to a phenomenon known as "induced demand," a concept that dates back to the 1960s, when economist Anthony Downs argued that "peak-hour congestion rises to meet maximum capacity." As it turns out, "highways are vulnerable to latent demand — people who haven’t been using them start to fill them up once the capacity is expanded." But "traffic planners, especially those in state highway departments, refused to believe it," writes Ehrenhalt. "They went with what they considered common sense and kept expanding and widening."

Expanded highway capacity also leads to "more development: new trucking depots and large numbers of employees who commute to work; new housing developments and shopping centers at the exits," all of which add to the congestion.

"Most big cities seem to have caught on and are no longer seeking to expand the highways within their borders. But quite a few state transportation agencies have failed to get the message, or are simply ignoring it," with major projects continuing to move forward. "Urban transportation needs many things," says Ehrenhalt, "but it doesn’t need more asphalt."