Why do traffic jams happen for no reason?

In dense traffic, small changes in speed grow as they pass from car to car. One driver braking a little makes the next brake a little more, until someone stops. By the time you reach the jam there's nothing to see, because the driver who braked is long gone.

What is a phantom traffic jam?

A jam with no crash, merge or bottleneck behind it: just dense traffic and a small disturbance. Sugiyama's experiment, published in 2008, produced one on a circular track with 22 cars, drivers asked to cruise at about 30 km/h, and nothing in the way.

Why do traffic jams move backward?

Cars at the front pull away as the road clears, and new cars arrive at the back, so the jam moves upstream even though every car in it moves forward. Stop-and-go waves on highways travel backward at about 15 to 20 km/h.

How can drivers prevent phantom jams?

Leave a bigger gap and don't match every brake of the car ahead, so a slowdown reaches your gap and stops there. In the model, one such driver out of 22 stops the wave. In that field test, one automated car cut fuel use by up to 40% and hard braking from about 9 events per vehicle per kilometer to as few as 0.12.

Live model · Phantom traffic jams

How Phantom Traffic Jams Actually Work

Many traffic jams start with no crash and no bottleneck. In dense traffic, one driver braking makes the driver behind brake a little harder, and the next harder still, until cars stop. The jam then travels backward while every car in it moves forward. In a 2008 study, one formed among 22 cars driving in a circle.

One tap of the brakes, 22 cars

22 cars on a 230 m ring road, evenly spaced, all at 28 km/h. Each driver speeds up toward the open-road speed and brakes to keep a safe gap to the car ahead, which grows with speed.

StopIntelligent Driver Model

Car 1 brakesSmoothing driverCars

CH1 5 km/h or more under cruising

Watch the Python behind this model run, line by line

How to read the stage

The ring holds 22 cars on 230 m of road, the track from Sugiyama's experiment, all at 28 km/h. At 5 s car 1 brakes for one second. The grid is the road over time: each row is 10 m of ring, each column 5 s, and magenta marks cars slower than cruising. The diagonal stripes are the slowdown moving backward. At 95 s cars start to stop; by 115 s, 12 of the 22 stand still.

Why the slowdown grows

Each driver keeps a gap that grows with speed and with how fast it's closing. When the car ahead slows, the driver behind brakes, and at this density ends up slower than the car ahead was. Each dip is deeper than the last, so car 1's drop of 7 km/h becomes a full stop. Give the same tap more room, with 18 cars, and it fades instead: try Four fewer cars.

Why the jam moves backward

Cars leave the front of the jam as the road ahead clears and join its back as they arrive, so the jam drifts backward while every car moves forward. In the model its back settles at about 13 km/h. Sugiyama's team measured about 20 km/h on their track and called it the common value on real highways.

One driver can stop it

Make one driver across the ring a smoother: instead of chasing the car ahead, it eases toward the cruising speed and lets its gap stretch and shrink. The wave reaches that gap and dies. No car stops, and by 150 s the slowest car is back to 27 km/h. In a field test published in 2018, on a ring with 21 and 22 cars, one automated car cut the spread of speeds by 49.5% to 80.8%.

The same tap, with one smoothing driver

22 cars on a 230 m ring road, evenly spaced, all at 28 km/h. Each driver speeds up toward the open-road speed and brakes to keep a safe gap to the car ahead, which grows with speed. Car 12, across the ring, smooths: it eases toward 28 km/h instead of chasing the car ahead.

StopOne driver smooths

Car 1 brakesSmoothing driverCars

CH1 5 km/h or more under cruising
CH2 The smoothing driver

Watch the Python behind this model run, line by line

What the model leaves out

Real drivers react after a delay, differ from one another and change lanes. These drivers are identical, react at once, and stay on a single-lane loop. Their half-second time gap is what 22 cars on 230 m allow at about 30 km/h; the model's published parameter sets use 1 to 1.6 seconds. The smoother is a simple rule, not the controllers used in that field test.

Questions

Why do traffic jams happen for no reason?: In dense traffic, small changes in speed grow as they pass from car to car. One driver braking a little makes the next brake a little more, until someone stops. By the time you reach the jam there's nothing to see, because the driver who braked is long gone.
What is a phantom traffic jam?: A jam with no crash, merge or bottleneck behind it: just dense traffic and a small disturbance. Sugiyama's experiment, published in 2008, produced one on a circular track with 22 cars, drivers asked to cruise at about 30 km/h, and nothing in the way.
Why do traffic jams move backward?: Cars at the front pull away as the road clears, and new cars arrive at the back, so the jam moves upstream even though every car in it moves forward. Stop-and-go waves on highways travel backward at about 15 to 20 km/h.
How can drivers prevent phantom jams?: Leave a bigger gap and don't match every brake of the car ahead, so a slowdown reaches your gap and stops there. In the model, one such driver out of 22 stops the wave. In that field test, one automated car cut fuel use by up to 40% and hard braking from about 9 events per vehicle per kilometer to as few as 0.12.