“When will we finally be there?” Just a few years ago, this question could really get on the nerves of parents driving a car. Today, most people have a navigation system in their car, and you can read the estimated time of arrival there – even from the back seat – and for longer journeys, the forecasts of the devices are often accurate to the minute.
Unless you’re suddenly stuck in a traffic jam. How can this happen? There’s a boring and an interesting answer to that. Boring is boring because everyone can imagine it: suddenly something is blocking the road. Only very rarely is it something exciting, usually a construction site is the reason or the road is closed after an accident, and the cars are crowded or have to stop until the obstacle is removed again. What is more interesting is that traffic jams can also occur without any sudden blockages or even lane narrowing, but just like that, out of nowhere, so to speak.
This usually happens when there are as many cars on the road as can fit. If the cars keep the recommended distance from each other (rule: half the speedometer reading in meters), then, for example, with four-meter-long cars all driving at a speed of 100 km/h, that’s just over 1800 cars per hour and lane , driving past a spectator standing next to the road.
But suppose a car slows down a little – let’s say because there’s a child in there who suddenly got sick and the father behind the wheel now has to look for a parking space from one moment to the next. The driver behind also brakes, but with a slight delay, because time elapses between the flashing of the brake lights in front of him and his step on the brakes. During this time, the distance between the two cars decreases, which is why the second driver has to brake a little harder than the first.
The same thing will happen to the driver behind that one, and he’ll slow down even more, and so on – until a few more cars behind a driver has to stop completely. He, and everyone behind him, are suddenly stuck in a traffic jam. It takes a while for such a “traffic jam out of nowhere” to clear up, because each car starts up again with a delay. And the more cars have stopped, the longer it takes to move forward again.