We've all been there: Traffic grinds to a halt and you inch along expecting to see a crash or broken down car, but then everyone gets back up to speed without any apparent reason for the initial slow down.
It turns out that there's a good reason for this and it can all be explained through science.
It comes down to humans' inability to hold a consistent speed or distance from the car in front. Essentially what happens is people travel at different speeds, then slow down a bit too much when they catch another car.
The car behind them then slows down a bit more, and the car behind them a bit more, and so on. Eventually, thanks to the fact we can't just hold it steady and impatient drivers can't resist tailgating, we end up with pointless jams.
That's why smart motorways are being introduced that manage traffic flow, while some countries use blocks of police cars to keep everything moving smoothly.