We can learn the secrets of smooth traffic flow by watching fire ants

The next time you’re stuck in traffic, consider taking a cue from the lowly ant. Fire ants may hold the secret to regulating traffic flow, whether it be dealing with cars packed on a freeway during rush hour, shepherding crowds through narrow passageways, or coordinating swarms of robots.

“Ants that live in complex subterranean environments have to develop sophisticated social rules to avoid the bad things that can happen when you have a lot of individuals in a crowded environment,” said Georgia Tech physicist Daniel Goldman, who has been studying fire ants for years and is co-author of a new paper in Science detailing how they optimize their tunnel-digging efforts.

In a jam

Physicists have long been fascinated by traffic jams, especially so-called “phantom” traffic jams (aka, “jamitons”), where there doesn’t seem to be any good reason for the slowdown. It all comes down to density and the physics of self-organization. Traffic moving freely “flows” like a liquid. Traffic jammed to a standstill is akin to a solid.

But there’s a special state in between that German physicist Boris Kerner dubbed “synchronized flow.” It’s where car density reaches a critical threshold and vehicles become highly correlated with other, moving in unison. Here, the slightest perturbation, even if it’s a single driver braking suddenly, sends little ripples through the chain of cars behind. It’s an example of emergent collective behavior, and it’s one reason why slowdowns typically occur near merge points.

Fire ants (and ants in general) provide another textbook example of collective behavior. A few ants spaced well apart behave like individual ants. But pack enough of them closely together, and they behave more like a single unit, exhibiting both solid and liquid properties. You can pour them from a teapot like ants, as Goldman’s lab demonstrated several years ago, or they can link together to build towers or floating rafts–a handy survival skill when, say, a hurricane floods Houston. So it’s not surprising that they also excel at regulating their own traffic flow. You almost never see an ant traffic jam.

Here’s why. In 2008, German scientists built a tiny ant motorway, complete with the equivalent of highway interchanges, so that the ants in their laboratory could navigate between their nest and a sugary food source. Then they monitored how the ants quickly found the shortest possible route between the two. You’d expect jams to form near interchanges, as they do on human highways during rush hour. Instead, whenever a route started to clog, the ants returning to the nest with sugar blocked ants traveling in the opposite direction toward the nest, forcing them to find an alternate route.

Uncooperative

It’s more difficult to get human drivers to alter their behavior for the collective good. Aided by apps like Waze and Google Maps, we generally take whatever is the quickest route for us, with nary a thought about how this affects traffic patterns at large or our fellow drivers. That’s one reason why just widening highways doesn’t really reduce congestion—there’s an inherent conflict of interest between what benefits us personally and what benefits us collectively, so the result is 30 percent longer commute times, per another 2008 study. (The authors dubbed it the “Price of Anarchy.”) But shutting down a few select streets—akin to blocking oncoming ants—forced drivers to act like the ants and find alternate optimal routes, even if they didn’t consciously do so.

Now there are some new insights into the issue, thanks to Goldman’s crew in Georgia. The group first collected ten nests of fire ants over three summers and set up colonies of 30 ants each in the laboratory, housed in transparent containers filled with glass particles to simulate soil. They painted individual ants different colors, the better to track them for the experiments. Then they let the ants go about their business digging vertical tunnels in their containers for 48 hours, all under the watchful eye of a video camera. Those tunnels are narrow, with barely enough room for two ants to pass, yet jams rarely happened.

Why? When an ant encounters a tunnel in which other ants are already working, it retreats to find another tunnel. It also helps that only a small fraction of the colony is digging at any given time: 30 percent of them do 70 percent of the work.

According to Goldman, this inequality among worker ants actually benefits the community, ensuring that the digging gets done efficiently with minimal delays while expending the least amount of energy. But don’t assume that the idle ants are lazy: when the scientists removed five of the hardest working ants, other members the colony stepped in to keep the work flowing smoothly.

To find out if this optimization strategy might work more broadly, one of Goldman’s graduate students built ant-like robots and programmed them to dig through 3D printed magnetic plastic balls designed to simulate moist soil. The robots traveled along narrow tracks, mimicking the narrow tunnels of the ants. The researchers found that up to three robots could efficiently dig together, but adding a fourth jammed up the process and work came to a halt.