Traffic. We’ve all been in it. But why does it happen? Aside from construction and accidents causing it, doesn’t it seem like sometimes traffic happens for no good reason? Indeed, ‘phantom’ or ‘shockwave’ traffic jams, caused by a chain reaction of slowing down, do actually exist. They begin when a car in dense traffic slows down – even slightly – which causes the car behind that vehicle to slow down, and onwards, getting worse the further back it goes. The cars far behind the original slowdown are forced to stop completely and traffic seems to come to a stop over nothing.


A few years ago, Japanese physicists ran an experiment: they gathered drivers on a closed loop course, asking them to keep a certain speed and following distance. They couldn’t do it. After a while, the system broke down and a reverse shockwave rippled back through the whole line of cars. So, it’s not so much that you’re driving into a traffic jam. Rather, a traffic jam is basically driving into YOU.


Another source of traffic? When it comes to merging due to road work reducing two lanes of traffic down to one, drivers typically merge into the single lane as soon as possible and form one long line. The main reason for this is people think it’s bad behavior to stay in the other lane and merge late. But traffic would be much better off if cars stayed in both lanes, then merged at the very end, one-by-one, like a zipper. It’s safer (fewer lane changes) and it reduces back-ups (often by 40%).


Tailgating, or, following too closely behind another vehicle, significantly reduces the distance between you and the car ahead. So, when the car that is being tailgated slows down, it causes the car tailgating to forcefully apply its brakes to avoid a collision, which can lead to phantom jams. A way to mitigate these jams is by finding a middle point between the car in front of you and behind you and trying to position your car so it has an equal distance between the two. This way, if the driver in front hits their brakes you can slowly hit your brakes or take your foot off the accelerator which may prevent a rippling event from happening at all. Schools of fish and flocks of birds and bats use these types of spacing strategies to avoid hitting each other while swimming or flying in densely packed groups. Unlike bats, birds and fish, however, humans have difficulty judging distances behind them. While many cars these days have forward-facing sensors to judge distances to vehicles ahead, more cars with rear-facing sensors can aid with determining the distance to the vehicle following behind a car. Until that happens, drivers can help reduce phantom traffic jams simply by not following other cars so closely.


According to the National Highway Traffic Safety Administration, nearly a quarter of all motor vehicle accidents are rear-end collisions. Yearly, these collisions cause approximately 2,000 deaths and nearly 1 million injuries. If you’re moving on a highway at 60 mph, it’s like you’re covering the distance from home plate to 1st base on a baseball field in 1 second. Human reaction time is about 2 seconds. Therefore, if you’re too close to the car in front of you and something happens, by the time you realize it, you’ve already run into the car. Many drivers fail to appreciate that the stopping is directly proportional to the size and weight of the vehicle. When it’s wet or the road is icy, keep even greater distances to prevent collisions.


Thanks to Ivan Horodyskyj, retired traffic engineer who has 35 years of experience in the field, for offering his insights for this week’s Science Wednesday!


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photo: Ulyana Peña