Traffic Jam Solution: The Jerk Who Cut You Off

Study: Aggressive drivers actually break up congestion.

Washington, July 25, 2009 — -- The next time someone cuts you off on your morning commute, don't be so quick to call the driver a jerk; you may have a reason to say thanks. According to the latest physics research, rule-breakers -- drivers passing you on the wrong side or changing lanes too close to the intersection -- actually help smooth the flow of traffic for the rest of us.

"The interesting finding is that if most of the people are law-abiding, and you have a certain amount of people who are breaking the rule, then you are actually getting the minimum chance of a [traffic] jam," said Petter Minnhagen, a physicist at Sweden's Umea University and an author of the paper published in the journal Physical Review E.

Physicists at the school uncovered this phenomenon while constructing a computer model of how a crowd of people move across a confined space, such as a pedestrian-only street. They divided the space into squares, like a chessboard, and randomly placed pedestrians in some of the squares. Like real people, the model pedestrians had a certain small probability of momentarily pausing, as if they had run into a friend or had bent down to tie a shoelace.

Spreading Out the Crowd

To make things more interesting, the researchers then tossed a few mavericks into the mix, who didn't follow the rules the other pedestrians used. The physicists ran the simulation over and over, each time boosting the percentage of rule-breakers. At first pedestrian deadlocks worsened. But as more and more rule-breakers joined the fray, something entirely unexpected occurred: traffic flowed best when onlyabout 60 percent of pedestrians were obeying the rules.

Simple interactions of individual cars, people, or molecules add up to large patterns in a system. The high concentration of pedestrians in a small area increases the chances of a jam, but rule-breakers made thecrowds spread out.

Morris Flynn, a University of Alberta professor who uses computational methods to study car traffic, agrees with the explanation. Because rule-breakers "carve out their own path," Flynn said, they dilute large concentrations of rule-abiders moving in the same way. He pointed out an example familiar to anyone who has driven on a two-lane road: breaking the speed limit to pass a slow vehicle prevents a long chain of cars from forming.

Congestion Solution: Bad Drivers

However, there is one rule you shouldn't break, according to a new analysis of how high-volume traffic flows along a highway. Cecile Appert-Rolland, a physicist at the University of Paris-Sud, looked at the tailing distances between cars traveling on a busy two-lane expressway in the suburbs of Paris. Most people have heard of the "three-second rule" for following distances; after the car ahead of you passes a point on the road, count to three. If you pass the same object before you get to three, you're following too closely. This time-based measure of the distance between cars is what Appert-Rolland terms the "time headway."

Her research showed that tailgating drivers were more likely than a non-tailgater to have a car in the lane next to them, so they weren't just speeding up in order to change lanes. She also found that theseshort time headways tended to extend across several vehicles, creating a platoon.

"We can identify at least seven or eight cars where they have time headways of half a second," she said. Considering that a driver's reaction time is about one second, these platoons are disastrous pileups waiting to happen. "If the first one brakes, the second one has to brake harder, the third one even harder, and the last wouldn't be able to brake hard enough."

So while unexpected behavior may help with congestion, always follow the three-second rule -- because if you're tailgating, chances are you won't be the only one.