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There is now a CONTENT FREEZE for Mercury while we switch to a new platform. It began on Friday, March 10 at 6pm and will end on Wednesday, March 15 at noon. No new content can be created during this time, but all material in the system as of the beginning of the freeze will be migrated to the new platform, including users and groups. Functionally the new site is identical to the old one. webteam@gatech.edu
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Highway at NightWhenever a road project gets announced, the first thing officials talk about is how it’s going to reduce traffic.
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. 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.
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.
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.
Eric Sundquist, SCaRP PhD alumnus, 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.
