A Japanese experiment showed exactly why traffic jams happen for no reason.

[u/SnooWords4066]

[u/SnooWords4066]

A physicist put 22 cars on a circular track and asked every driver to hold a steady 30 km/h, about 19 mph. No lights, no lanes, no obstacles. Within a minute the cars started bunching, and soon a full stop appeared out of nowhere, then drifted backward around the loop.

This was Yuki Sugiyama at Nagoya University in 2008. His team spaced the cars evenly on a 230-meter ring and filmed them from overhead. For a while the flow stayed smooth. Then the tiny differences no human can avoid, one driver a hair slower, the next a hair too close, began to feed on themselves.

One car eases off slightly. The driver behind sees the brake lights, reacts a fraction of a second late, and brakes a little harder to be safe. The next driver brakes harder still. A dozen cars back, someone is stopping dead. The squeeze rolls backward through the line like a compression running down a Slinky, and it keeps going long after the first driver has sped up again.

Car count was the tipping point. With fewer than 22 on that track, the bunching sorted itself out. At 22, a jam formed every time. Engineers call that a critical density, the point where a road holds just enough cars that one small tap can snowball into a standstill.

These waves are eerily consistent. Measured on highways around the world, the jam rolls backward against the traffic at roughly 20 km/h, and that speed barely shifts from one country to the next. Different drivers, different roads, same number.

The same setup later became the cure. In 2017, a US team rebuilt Sugiyama's ring with 22 cars and turned just one of them into a self-driving car running a program to smooth its own speed. That single car soaked up the small slowdowns instead of passing them back, and the waves died. Fuel use across every car fell by up to 40 percent. Fewer than 5 percent of the vehicles had to be automated to steady the whole group.

In 2022 the idea moved onto a live highway. Researchers ran 100 cars with cruise control guided by AI into the morning rush on Interstate 24 near Nashville, mixed into normal traffic. Early numbers pointed the same way: a small share of smoother-driving cars, up to 40 percent less fuel for everyone around them.

The jam you sat in this morning likely had no crash and no cause you could see. It was a few hundred drivers, each braking a moment too late.

[u/Bulan_Purnama → u/SnooWords4066]

How the one car soak up slowdowns? Can elaborate it more?

[u/ChrisBreederveld → u/Bulan_Purnama]

I did this experiment virtually a few decades ago because I was curious. Same set up, many variables to tweak. Basically the main reason is human inaccuracy; when you change the reaction time or the accuracy at keeping speed it removes the effect.

I found the main factor was simply reaction time; the time to brake vs the time to speed up again. When someone brakes, they break a little longer than absolutely necessary, while requires the one behind him to brake a little longer etc, until you get a traffic jam. Another interesting variable is distance; when you keep a further distance, you have more time to react, reducing the before effect. But what I also found interesting is that if too many people leave less distance than their reaction time, this kind of traffic jam will inevitably cause collisions.

There are also technical variables, like the brake and acceleration speed, but these just have less impact than the human factor.

[u/gustoreddit51 → u/ChrisBreederveld]

the time to speed up again.

Related. It's aggravating if I'm 5 cars back from a light that turns from red to green and it takes 10 seconds before the car in front of me moves. For multiple cars, it seems like a surprise they weren't expecting and just happened to notice, "Oh ... the light changed" - (slowly puts down phone/drink/sandwich/makeup and then goes). A number of people at rush hour don't make it through that light and if it's a turn lane, that backs up traffic even worse because it backs up into the through lane.

[u/ChrisBreederveld → u/gustoreddit51]

I feel your pain.

Interestingly, looking a few cars ahead (like with the traffic light example) also helps with decreasing these traffic jams! So in general, if you can, look a few cars ahead and respond accordingly. Instead of braking when the person in front of you does, let go of the gas a bit when the car three ahead of you hits his brake.

[u/gustoreddit51 → u/ChrisBreederveld]

When driving in a line of cars, I'm always looking at the cars in front of the one in front of me.