Critical Mass

What We Know / by Jonah Lehrer /

For particle physicists who study phase transitions, a traffic jam is simply a solid made up of idling cars.

A few years ago, Swiss economists Bruno Fey and Alois Stutzer announced the discovery of a new human foible, which they called “the commuters paradox.” They found that when people are choosing where to live, they consistently underestimate the pain of a long commute. This leads people to mistakenly believe that a mansion in the suburbs, with its extra bedroom and sprawling lawn, will make them happier, even if living there might force them to drive an additional 45 minutes to work. It turns out, however, that traffic is torture, and the big house isn’t worth it. According to the calculations of Fey and Stutzer, a person with a one-hour commute has to earn 40 percent more money to be as satisfied with life as someone who walks to the office.

Long commutes make us unhappy because the flow of traffic is inherently unpredictable. As a result, we never adapt to the suffering of rush hour. (Ironically, if traffic were always bad, and not just usually bad, it would be easier to deal with.) As the Harvard University psychologist Daniel Gilbert notes, “Driving in traffic is a different kind of hell every day.”

But why is traffic so unpredictable? After all, the number of cars on a highway during a typical weekday rush hour is fairly constant. And yet, even when there are no accidents—and most traffic isn’t caused by collisions—the speed of traffic can undergo dramatic and seemingly inexplicable shifts.

The key to understanding traffic jams is something known as “critical density,” or the number of vehicles that any road can efficiently accommodate. Past this threshold, when too many cars are trying to cram onto the same six lanes of asphalt, the flow of traffic starts to break down. At this point, congestion becomes all but inevitable, as even seemingly insignificant events, such as a single driver tapping on the brakes, can trigger a cascade of brake lights. That’s when the highway becomes a parking lot.

While the concept of critical density has been repeatedly demonstrated using computer simulations—drivers are surprisingly easy to model as a system of interacting particles—it wasn’t until last year that this theory of traffic was experimentally confirmed. A team of physicists at Nagoya University wanted to see how many cars could maintain a constant speed of 19 mph around a short circular track. It turned out that the critical number was 22: Once that density was reached, tiny fluctuations started to reverberate around the track, which caused the occasional spontaneous standstill. As the scientists note, this is actually a pretty familiar scenario for particle physicists, who are used to studying phase transitions, such as the transformation of liquid water into solid ice. In this case, the critical threshold is temperature, which triggers clusters of molecules to slow down and form a crystal lattice, which then spreads to nearby molecules. A traffic jam is simply a solid made up of idling cars.

The hope, of course, is that by understanding traffic jams we can learn to prevent them. Tom Vanderbilt, in his authoritative book Traffic, describes a simple experiment performed by the Washington Department of Transportation that involved a liter of rice, a plastic funnel, and a glass beaker. When the rice was poured into the beaker all at once, it took 40 seconds for the funnel to empty; the density of jostling grains impeded the flow. However, when the grains were poured in a gradual stream, it took only 27 seconds for the rice to pass through. What seemed slower actually turned out to be 30 percent faster. This helps explain why traffic engineers are so eager to install red lights on highway onramps: By slowing traffic before it enters the concrete funnel, they hope to prevent the road from exceeding its critical density.

And then there’s the insect solution. Dirk Helbing, a “congestion expert” at the Dresden University of Technology, constructed a network of “carriageways” between an ant nest and a source of sugar. Within a few hours, the ants located the most direct route to the sugar, which became dense with hungry insects.

If ants were people, this density would eventually lead to gridlock. However, Helbing discovered that just as the carriageway approached its breaking point, ant “traffic cops” physically blocked the road. This forced the ants to find another route to the sugar, and thus prevented a traffic jam.

Obviously, human cops can’t shut down the interstate just because it’s busy. But the hope is that humans will be able to build smarter GPS networks. By taking real-time traffic data into account, devices linked to this system will direct drivers away from saturated roads. Is the highway approaching critical density? If so, take surface streets. Traffic will always be caused by a miserable sort of randomness—the stochasticity of too many automotive particles in too small a space—but there’s nothing inevitable about gridlock. We can learn to drive like ants, even if we still insist on driving in from the suburbs.

Front image courtesy of Walid Hassanein.

Originally published June 22, 2009

Tags complexity decision making efficiency systems

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