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Whenever a road project gets announced, the first thing officials talk about is how it’s going to reduce traffic. Just last month, for instance, the Connecticut DOT reported that it would be widening Interstates 95 and 84, a project that would result in major economic benefits from “easing congestion”:

The analysis found that adding a lane in each direction border-to-border will save I-95 travelers well over 14 million hours of delays by the year 2040. Likewise, the widening of I-84 will save travelers over 4.7 million hours of delays during the same period.

Never mind that it’s unclear whether major highway projects actually provide an economic boost (many of the supposed new benefits are simply a relocation of existing business activity). Congestion relief itself is a dubious claim when it comes to road expansions. Transportation experts have repeatedly found that building new roads inevitably encourages more people to drive, which in turn negates any congestion savings—a phenomenon known as “induced demand.”

So it’s refreshing—and rare—to see the California DOT (aka Caltrans) link to a policy brief outlining key research findings from years of study into induced demand. [Editor’s note: The original link from Caltrans no longer works, but you can still find the paper here.] The brief, titled “Increasing Highway Capacity Unlikely to Relieve Traffic Congestion,” was compiled by UC-Davis scholar Susan Handy. Here are the highlights:

There’s high-quality evidence for induced demand. All the studies reviewed by Handy used time-series data, “sophisticated econometric techniques,” and controlled for outside variables such as population growth and transit service.

All the studies reviewed by Handy used time-series data, “sophisticated econometric techniques,” and controlled for outside variables such as population growth and transit service. More roads means more traffic in both the short- and long-term. Adding 10 percent more road capacity leads to 3-6 percent more vehicle miles in the near term and 6-10 percent more over many years.

Adding 10 percent more road capacity leads to 3-6 percent more vehicle miles in the near term and 6-10 percent more over many years. Much of the traffic is brand new. Some of the cars on a new highway lane have simply relocated from a slower alternative route. But many are entirely new. They reflect leisure trips that often go unmade in bad traffic, or drivers who once used transit or carpooled, or shifting development patterns, and so on.

What’s significant about the Caltrans acknowledgement is that induced demand creates something of a mission crisis for transportation agencies that spend most of their money on building new roads. (The same can be said for peak driving.) A 2014 assessment of Caltrans, conducted by the State Smart Transportation Initiative, specifically cited induced demand as a research finding that had yet to filter down “into the department’s thinking and decision making”:

For example, despite a rich literature on induced demand, internal interviewees frequently dismissed the phenomenon.

Ronald Milam of the transportation consultancy Fehr & Peers tells CityLab that Caltrans has recognized the shortcomings of traditional traffic models and tried to improve its analyses to better account for induced demand. In response to new state laws designed to reduce vehicle miles traveled and thus climate emissions—namely, Senate bills 375 and 743—the agency is updating and broadening that effort. New guidelines are currently in the works.

Eric Sundquist of SSTI said via email that it’s “notable” to see Caltrans link to the policy brief, but adds that some DOTs have started to address induced demand whether they call it that or not. The big question, he says, is what policies get put in place to deal with its effects. Pennsylvania’s DOT during the Ed Rendell administration, for instance, “basically stopped building new highways and diverted resources to upkeep and non-car modes,” Sundquist says.

That culture change isn’t so easy. Connecticut officials also seem to understand that expanding roads won’t resolve the state’s traffic problems. “You can’t build your way out of congestion,” Tom Maziarz, chief of planning at the state DOT, recently told the Connecticut Post. And yet the interstate widening project moves forward.