Traffic
Nobody crashed. So why did we stop?
Some jams have no cause you can see: no crash, no roadworks, just a queue that stops, starts and clears. Here is where they come from, and how to stop feeding them.
You’re on a busy road, moving nicely, and then everything stops. You crawl for a minute, you stop again, and then the traffic simply clears. No crash on the hard shoulder. No roadworks. No broken-down van. Nothing.
That’s a phantom traffic jam, and it isn’t your imagination. Nobody crashed, so why did we stop? Because a few minutes earlier, somebody further up the road touched their brakes, and the queue behind them did the rest.
Why traffic stops for no reason
The chart below follows a queue of cars on one lane. It isn’t a recording: it’s worked out with the Intelligent Driver Model, a standard way traffic researchers describe how drivers follow the car in front. Every driver here is identical and sensible. Watch what one brief brake does.
Everyone flowing
A busy single lane: 80 cars at 52 km/h, each about 22 metres behind the next. Every line on the chart is one car. Time runs left to right; distance along the road runs up.
Neat, parallel lines mean everybody is doing the same speed. This could carry on all day.
One driver brakes
Somebody touches the brakes for 2.5 seconds. A phone, a pigeon, a lorry that looked as if it might pull out. They drop from 52 to 16 km/h, then pick up again.
Their line bends, briefly. That is the whole of the cause. Nobody crashed, and nothing is in the road.
The next one reacts later, and harder
The driver behind only finds out when the gap starts to shrink, so they react a beat late and need a little more braking to get their gap back. The driver behind them is later again.
The car right behind dips to 20 km/h. Eight cars back, it is 15. Sixteen back, 7. Each driver passes on a bit more slowing than they were given.
The wave goes backwards
The red is everyone under 12 km/h. Each car drives into it, crawls, and comes out of the far side, while the red band itself slides backwards down the road at about 21 km/h.
That is a stop-and-go wave, sometimes called a traffic shockwave. The cars move forwards; the jam moves the other way.
Stopped, for nothing
The car 23 places back comes to a dead stop 40 seconds after the brake, by which time the driver who braked is well up the road and back at full speed.
From inside that car there is nothing to see: no crash, no roadworks, no lights. That is a phantom traffic jam.
The experiment: 22 cars, one circle, no reason
In 2008 a team of Japanese researchers led by Yuki Sugiyama tried it for real. They put 22 cars on a circular track about 230 metres round, asked the drivers to drive steadily, and watched. There were no junctions, no lights and nothing to merge with: no bottleneck of any kind.
For a while, the cars flowed evenly. Then small wobbles in speed grew, and a cluster of cars began to stop and start. The jam drifted backwards round the circle against the traffic, the same stop-and-go wave you meet on the motorway. They published it in the New Journal of Physics under a title that says it all: traffic jams without bottlenecks.
Our version, worked out the same way as the chart above: 30 cars on a 300 metre ring, after 100 seconds of nobody doing anything wrong. Red cars are stopped.
The lesson isn’t that the drivers were bad. It’s that when cars are close together, a steady flow is fragile. Any small slowdown gets passed back down the line a little bigger than it arrived, and if the line is long enough, somebody ends up stopped.
Make your own phantom jam
Here is a ring road you can play with. Tap the brakes on the ringed car and watch the wave go round; the strip beside it records where every car was, so a jam shows up as a red stripe sliding down it. Then add cars until a jam turns up by itself, or ask everyone to leave a bigger gap.
Try it
300 metres of road. No junctions. Nobody in the way.
- Average speed
- – km/h
- Cars stopped
- –
Your browser can’t run the live version, so here is one we made earlier: 30 cars, left to themselves.
- Flowing
- Slowing
- Stopped
Each car follows the one ahead with the Intelligent Driver Model, plus a little random wobble in its speed. A bigger gap means a 1.6 second following time instead of 0.8.
What actually helps
You can’t stop other people braking. You can stop passing it on. Most of what helps comes down to giving yourself time, so the slowdown arrives at you small and leaves you smaller.
- Leave a bigger gap. Two seconds or more to the car in front. In the playground, the roomy ring averages about the same speed as the jammed one. The difference is that nobody has to stop.
- Look further ahead. Watch the brake lights three or four cars up, not just the bumper in front. You can ease off early and gently instead of late and hard.
- Drive smoothly. Lift off rather than brake when you can, and don’t race up to the back of a queue that’s about to stop again.
- Don’t tailgate. A short gap leaves no choice but to brake as hard as the car in front, or harder. That’s exactly how the wave grows.
Jam-absorbing driving
Some drivers go further: when they see a stop-and-go wave ahead, they slow early and roll towards it at a steady crawl, letting a big gap open in front, so that by the time they reach it the queue is moving again. They never fully stop, and nor do the cars behind them. Researchers have tried the same idea with a single automated car in a ring like Sugiyama’s, and found that one smooth driver can damp the waves for everyone.
How the game does it
In I Know a Shortcut, every car on the road is a physical body with pedals and a steering wheel. Nothing gets moved by hand. When a car is following another, the same Intelligent Driver Model you’ve just been playing with chooses how hard to press the pedals.
The drivers aren’t all the same, though. Each has a personality with its own following gap: a nervous driver hangs back a couple of seconds, a courier sits right on your bumper, and a driver who’s getting wound up starts to tailgate. It’s the same model, so the same rule holds: the closer the following, the more a little braking up ahead grows on its way back to you.
You’re the passenger, so you can’t do anything about it. Except, of course, find a shortcut.
Questions
Quick answers
What is a phantom traffic jam?
A jam with no obvious cause: no crash, roadworks or junction. It starts when one driver slows briefly in busy traffic. Each driver behind reacts a moment later and brakes a little harder, until somebody stops, and the stop-and-go wave carries on long after the first driver has moved on.
Why does traffic stop for no reason and then speed up again?
You have driven through a stop-and-go wave. The cars move forwards, but the slow patch moves backwards along the road, usually at around 15 to 20 km/h. You catch up with it, crawl through it and come out of the front, so it seems to clear for no reason.
What is a traffic shockwave?
The boundary where fast traffic meets slow or stopped traffic. In heavy traffic it travels backwards, against the flow, which is why a brake on a motorway can stop cars several kilometres behind minutes later.
How can drivers prevent phantom traffic jams?
Leave a gap of at least two seconds, look several cars ahead so you can ease off early, and drive smoothly instead of braking hard and accelerating hard. When you see a wave coming, rolling slowly towards it rather than stopping in it helps the cars behind you too.
In the game
Now try it with someone who doesn’t know the way.
In I Know a Shortcut you’re the passenger. You can’t steer or brake; you can only call it, and your driver does their best. Square Mile Sprint runs 2.3 km to Fleet Street offices, with 5 turns to call and a gold medal at 6:35.