Why traffic jams appear out of nowhere

A jam with no cause

In 2008 a team of Japanese physicists led by Yuki Sugiyama ran one of the simplest experiments in traffic science. They put 22 cars on a circular track about 230 metres around and asked the drivers to keep a steady speed of 30 km/h. There were no junctions, no lane changes, no merging and no crash. Everything a driver needed was in front of them.

The steady flow lasted less than a minute. Then one driver slowed slightly, the car behind slowed a bit more, and soon a knot of stopped cars appeared on the track. The knot didn't stay put. It crept backwards around the ring while the cars themselves kept moving forwards: each driver came to a halt at the back of the jam and pulled away again at the front.

This is a phantom traffic jam. Physicists call the travelling knot a jamiton. The jam in the Nagoya experiment moved backwards at roughly the same speed as stop-and-go waves on real motorways. That told the researchers they were looking at the same thing.

Why a small tap becomes a full stop

Nobody reacts instantly. When the car ahead brakes, you notice it a moment later, and by then the gap has already shrunk. To win back a safe distance you brake a little harder than they did. The driver behind you sees your brake lights late too, and brakes harder still.

So each car passes the disturbance on slightly bigger than it received it. Engineers call this string instability: a chain of followers that amplifies a wobble instead of damping it. After a dozen cars, someone who only lifted off the accelerator has become a line of people at a dead stop.

Getting going again is slow too. Drivers at the front of the jam leave one at a time, each waiting for a gap to open. Cars join the back faster than they leave the front, so the jam survives and drifts backwards against the flow. On a motorway it can keep going for many kilometres, long after the driver who caused it is gone.

Try it yourself

This is the Nagoya ring as a simulation. Each car follows a simple rule: it has a comfortable speed for the gap in front of it and steers towards that speed. Alertness is how quickly a driver closes the difference. Colour shows speed.

Things to try:

• Leave the defaults and wait. A wave forms by itself from a tiny initial wobble, just as it did in Nagoya.
• Raise alertness until the ring turns smooth, then press Tap the brakes. Alert drivers absorb the shock.
• Drop the number of cars. With more room each driver has slack to absorb a wobble, and the wave can't form. Add cars and it comes back.

One calm car can fix it

If the problem is overreaction, a single car that refuses to overreact can break the chain. In 2016 a team from the University of Arizona and partner universities repeated the ring-road experiment with more than 20 cars, one of which was a lightly automated vehicle. While people drove it, stop-and-go waves formed as usual. Then the automated car's controller switched on. It simply held a steady speed close to the traffic's average and let the gap ahead grow and shrink instead of copying every brake tap.

The waves died out. The researchers reported that fuel use across the whole group fell by up to 40%, and they estimated that similar control could work with only about 5% of vehicles automated. Turn on One calm driver in the simulation to see the effect: the spread of speeds shrinks and the average rises, because nobody wastes time at a standstill.

What you can do about it

You can be that car. Leave a bigger gap than feels necessary, look several cars ahead instead of only at the bumper in front, and ease off early instead of braking late. When the gap ahead opens, don't rush to close it; that space is what soaks up the next wave. You may feel you're losing ground, but the traffic behind you moves more smoothly, and on average everyone, including you, gets there sooner.