Blame Tailgaters for Your Traffic Woes

Keeping an equal distance between cars ahead and behind may eliminate “phantom” traffic jams

Traffic Jam
Wikimedia Commons

Sometimes traffic jams are inevitable—there’s an accident, road work or a messy police stop slowing down the flow. But other times there are the “phantom” traffic jams, where hundreds or even thousands of cars crawl along the highway for no obvious reason. As Tom Metcalfe at LiveScience reports, in recent years researchers have figured out why these jams appear out of thin air. And now, they may have a solution for preventing them: Be a more courteous driver.

Phantom traffic jams form similar to a wave, Joseph Stromberg wrote for Vox in 2016. If there is a critical mass of cars on the highway moving at a steady rate, even a minor disruption in the flow of traffic can affect cars far behind. For instance, if someone abruptly brakes, it causes others behind the car to brake, creating a “traffic wave” that propagates backwards, causing all the cars to slow. “It’s typically 100 to 1000 meters long, and it usually begins with vehicles running into a sudden increase in density at the start, and a drop in velocity," Benjamin Seibold of Temple University who studies the phenomenon told Stromberg.  “Then, after that, they slowly accelerate again.”

Combine the actions of dozens of drivers braking when they reach areas of higher traffic and these waves ripple back through the stream of cars, eventually forcing drivers in the rear to come to a complete stop.

But in a recent article in the journal IEEE Transactions on Intelligent Transportation SystemsMIT researchers offer a simple solution to phantom jams. If drivers stop tailgating, they brake less, which should eliminate these waves of slow downs.  

Berthold Horn, MIT researcher and co-author of the study, tells Metcalfe that he calls the anti-tailgating technique "vehicle spacing bilateral control." In essence, a driver tries to keep equal spacing between the car in front and car behind, reducing the need to brake.

While that sounds simple, keeping an equal distance between cars is tough for modern commuters who are often distracted scrolling through podcasts, putting on lipstick or unwrapping a cheeseburgers while behind the wheel. And we don't usually drive while keeping an eye on the road behind, Horn explains. “We humans tend to view the world in terms of what’s ahead of us, both literally and conceptually, so it might seem counter-intuitive to look backwards,” he says in a press release. “But driving like this could have a dramatic effect in reducing travel time and fuel consumption without having to build more roads or make other changes to infrastructure.”

Horn thinks that new technologies, like a modified adaptive cruise control, could help. This would automatically keep cars equally spaced so we wouldn't have to add another item to the driving checklist. He is currently working with automaker Toyota to create new sensors that will keep a cars centered between their neighbors.

Computer models of the technique show that bilateral control not only reduces traffic jams, it could drastically improve the efficiency of highways. “Under reasonable conditions today, you might get 1,800 cars per lane per hour throughput. With bilateral control, you could almost double that,” Horn tells Metcalfe. “If we can increase the throughput on major highways, even if it's only by 50 percent, that would be a big deal.”

While adaptive sensors may be one solution, other researchers believe there may be another trick to stopping jams. Last year researchers tested the idea of mixing autonomous cars into traffic to help brake traffic blockages. As Randy Rieland at reported at the time, computer simulations suggest that one driverless car that keeps a constant speed could cause a group of human operators to drive less erratically, leading to fewer traffic waves. Last summer they tested the hypothesis in Arizona. The technique seemed to work, and unexpectedly reduced fuel consumption by 40 percent.

We’re still a few years away from the autonomous car revolution or bilateral control sensors. But Horn tells Joe Palca at NPR that since doing this research he’s tried to keep his distance from other cars during his own commute, and it has led to small improvements. At least for him. We're not sure how the guy behind him who just spilled a macchiato in his lap is doing.

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