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More Highways, Less Congestion

The theory of ‘induced-demand’ fails the road test.

Mar 7, 2011, Vol. 16, No. 24 • By JONATHAN V. LAST
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That year two engineers from the Institute of Transportation Studies at—where else?—the University of California, Berkeley, published a paper in which they finally hung a number on induced demand. Surveying data from 1973 to 1990, from a wide cross-section of California counties, Mark Hansen and Yuanlin Huang created a sophisticated mathematical model. They measured the supply of “lane-miles” for California state highways (SHLM) and then measured the vehicle-miles traveled (VMT) on them. Accounting for all sorts of variables, Hansen and Huang determined that in densely urban settings, building an additional lane-mile would immediately induce an extra 10,000 to 12,000 cars per day on that stretch of road. Over time, they reported, the “elasticity” of demand meant that traffic would accumulate on the new highway lanes until at least 90 percent of the new capacity was eaten up. In other words, they concluded, “our results suggest that the urban SHLM added since 1970 have, on the whole, yielded little in the way of level of service improvements.”

This was groundbreaking stuff. But the study was not without its problems. For one thing, while the data for the lane-miles were fairly reliable, the data on the driver-usage side of the equation were not. Hansen and Huang had 17 years worth of numbers for the highway lane-miles, but they had only 5 years worth of data on vehicle-miles traveled. What’s more, these numbers weren’t that solid. Nobody counted the cars on the freeways. Instead, they derived them by working backward from gasoline sales and estimating how many miles, and on what kind of roads, people probably drove.

Understanding the limitations of their own data, Hansen and Huang were appropriately cautious about their findings. “It should not be assumed,” they wrote, “that the aggregate elasticities [meaning, the induced traffic] obtained in our analysis apply equally to every urban region, let alone to any particular project.” They even allowed that they could “envision situations where adding lane-miles, by removing some traffic bottleneck, [would result] in both better traffic conditions and a higher VMT/SHLM ratio.”

But their cautions were lost in the rush of research and analysis that followed, as study after study attempted to out-quantify the induced traffic effect. A 2001 report by the University of London’s Centre for Transport Studies claimed that half of a new highway’s capacity is eaten up within five years of it being built. And that 80 percent of it will eventually be consumed by induced traffic—cars and drivers that otherwise wouldn’t be there.

Some of the induced-demanders found themselves spinning into outer space. In 2008 a trio of physicists began examining traffic using game theory. Their study claimed that in some cases, it might be possible to decrease congestion by taking away certain roads. Their paper, “The Price of Anarchy in Transportation Networks,” was a theoretical, not a political, exercise centered around linked systems and Nash equilibriums. The same probably can’t be said of a report from Great Britain, where a team of researchers helpfully suggested that if planners were to reduce highway capacity, it probably wouldn’t create any more congestion. Certainly, they allowed, traffic would be murder if you vaporized a lane of freeway on a major artery. But eventually, they said, people would take mass transit, change jobs, or just move away. Problem solved!

Read enough of these studies and you get a sense that much of the “induced demand” hubbub is really a sub rosa extension of the war on the suburbs: Stop highway expansion and you can make life miserable enough for the minivan-driving masses that they’ll move out of their gauche “urban-fringe developments” and back to high-density metropolitan cores, where they belong.

One of the most recent induced-demand reports, from Canada’s Victoria Transport Policy Institute, laments that building more highways increases “automobile dependency,” creates “degraded walking and cycling conditions,” and “reduced respect for alternative modes.”

What do these critics want? The Victoria Transport people warn that if we don’t take the idea of induced demand seriously, we’ll end up building more highway capacity, when what we should really be doing is enacting “no build” policies, spending money on transit improvements, and—this last bit is catnip for the econometric set—instituting “road pricing,” i.e., some form of toll road, to force drivers to put a dollar value on their commute. Their report includes a section helpfully guiding civic planners on how to phrase questions about highway expansion so as to elicit negative responses from citizens.

Because it turns out that if you ask the average person (1) Do you think traffic congestion is a problem; and (2) Should roads be improved to relieve it, they’ll say yes, and yes.

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