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.