Like TURF
Follow TURFTheory of induced travel debunked?
More Highways, Less Congestion
The theory of ‘induced-demand’ fails the road test.
Jonathan V. Last
Weekly Standard
March 7, 2011, Vol. 16, No. 24
In 2008 the Virginia Department of Transportation began work adding a fourth lane to the six-mile stretch of I-95 between the Springfield interchange and the exit for Virginia State Road 123. This is likely of very little consequence to you, but it was a life-changing moment for me: I live not far from State Road 123. And my daily commute along that stretch of I-95 had been slowly killing everything that was once good inside me.
As it stood then, that section of I-95 had only three lanes in either direction, carrying 212,000 vehicles per day. That’s an average of 1,472 cars per lane, per hour. During peak commute times, the flow was much, much greater. Under optimum conditions, a lane of freeway can handle 2,000 vehicles per hour. Any flow above that rate disrupts the network. The result, at least on my little stretch of Interstate, is that between 6:00 a.m. and 9:00 a.m., and then 3:00 p.m. and 7:00 p.m., traffic grinds to a halt, accelerates to 25 mph, and then crashes back to 0 mph. Over and over again. The civil-engineering term for this is congestion. The theological term is purgatory.
So I was in a state of spiritual bliss when the Commonwealth of Virginia decided to spend $123 million to add an extra lane in each direction. And after three years of construction, the new lanes are set to open soon. The only problem is that people who take traffic seriously—the pointy-heads who staff university engineering programs and government planning bureaucracies—keep telling me that the extra lanes aren’t going to help one bit. After a year or so, they say, I’ll be mired in congestion all over again.
Like many sects, traffic planners have their dogmata. One of their doctrines is that adding capacity to highways is futile because it merely creates more traffic. This phenomenon is called “induced demand,” or, more colloquially, “build it, and they will come.” The theory is simple: Traffic systems that suffer from -congestion have latent demand—that is, a universe of drivers who would use the freeway, but don’t, because of the traffic.
When you add extra capacity to the highway, there may be an initial decrease in congestion. But then the latent demand begins to flow into the system and it quickly fills the road back up to the previous traffic level. “Congestion” is not a problem, but rather an equilibrium point to which traffic systems inevitably tend. To use a science-ish metaphor, in this worldview traffic is not a liquid, which can be funneled; it is a gas, which expands to fill any constraint.
The idea of “induced demand” comes up a lot in modern transit discussions. Whenever a government entity wants to add highway capacity, the induced-demand crowd tut-tuts about it being wasteful, or even harmful. Induced demand doesn’t just fill up new highway lanes, they say; it pushes suburban growth further out—leading to even worse congestion as people move away from metropolitan cores. It leads to higher fuel costs and more accidents. Adding capacity to an existing highway can even make traffic worse on non-highway streets (so-called surface roads) because of an obscure game-theory principle known as Braess’s Paradox.
In short, they say, building highways is a terrible idea all around. Those extra lanes on I-95 won’t make my life better. They might even make it worse. So the idea of induced demand is exceedingly important. If true.
Induced-demand theories have been percolating for a long time. In 1947 Roy Jorgensen published a study contending that the creation of the Merritt and Wilbur Cross Parkways in Connecticut had generated an entirely new flow of traffic that wouldn’t have existed in their absence. In the decades that followed, dozens of reports were written on the subject, with the conclusion always being the same: If you build a road (or add lanes to an existing road), more cars will materialize to fill it.
At the most superficial level, this is not surprising. The entire point of building roads is so people will use them. Indeed, a sparsely used new road or highway would be held up as an instance of government waste, or pork. But over time, the research sharpened to suggest that greater highway capacity actually caused greater congestion. In 1985, for instance, the British transportation planner Martin Mogridge argued that extra highway lanes lure mass transit riders into automobiles, creating even more traffic.
Unfortunately, even though all of these engineers and planners just knew that building more roads created more traffic, they couldn’t quantify their findings and prove to the lay bureaucrats and politicians that highway expansion was folly. In 1995, that changed.
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.
One of the reasons the unenlightened masses tend to be so pro-highway is that there’s some opposing research that suggests, heretically, that the induced-demand effect is modest. The most intriguing of these counterarguments came from a group of engineers and statisticians at the University of California, Davis, in 2002. The team reexamined the results of the original study by Hansen and Huang, using the same data, but a different mathematical model. They concluded, “the size of the induced-traffic effect that can be attributed to capacity enhancements may be sufficiently small that its detection . . . would be difficult, if not impossible.”
And in the real world, adding highway capacity can prove quite helpful. The Texas Transportation Institute’s annual Mobility Report, for instance, demonstrates an uncanny correlation between capacity and traffic congestion: Areas that add capacity tend to have lower levels of congestion. And induced demand doesn’t always -materialize. Take, for example, the city of Phoenix, a town built with almost no freeway system.
As a result, the Phoenix metro area historically had some of the worst congestion in the nation. Between 1982 and 2007, Phoenix decided to build the highways it should have had in the first place. They added so much asphalt that, according to the research firm Demographia, the city’s highway-lane-miles per capita grew by 205 percent. During that period, highway-vehicle-miles-traveled per capita increased by only 12 percent. And, like magic, traffic congestion plummeted.
Now what is true for Phoenix may not be true for Philadelphia. And building highways almost certainly induces some demand. The real issue, lost in all the statistical analysis and wonky planning, is, Why do we get in our cars in the first place?
That’s a question that drives Wendell Cox a little nuts. The principal at Demographia, Cox has spent a lot of time pushing back against the induced-demanders. He maintains that the entire framing of the issue is faulty: “Latent demand” for a highway, he notes, isn’t actually a desire to drive on that stretch of road. People only want the road as a means to an end. “Transportation is not a primary activity,” Cox explains. “There is no ‘love affair’ with the automobile. Driving is not something we would choose to do.” What we choose to do, for the most part, is go to our jobs.
A metropolitan area typically has about half as many jobs as people. But, because of geographical constraints, not every job is accessible to every person. Highways are, far and away, the most efficient way of delivering people to a job. When we add capacity to a highway, we’re actually expanding the universe of available jobs to everyone in the area. If that means that more people take advantage of the road space in order to find a job, get to work, or take a better job, that’s not a bug. It’s a feature.
In other words, a metric like “vehicle-miles traveled” is only superficially important. What we really care about is maximizing the number of potential jobs that people can reach, which increases affluence. Traffic, then, is a symptom of an economy with a mobile workforce, where workers can seek out better jobs with ease. Modern, multi-lane highways, even when they are sometimes congested, are the road away from serfdom.
Jonathan V. Last is a senior writer at The Weekly Standard.