There are obviously not many cars and no accidents, so why is there a traffic jam?

Audit expert: Zhu Guangsi

Member of Beijing Science Writers Association

The May Day holiday, which was exchanged for a workday adjustment, has finally arrived. Have you already started your travel (traffic jam) and sightseeing (driving) trip?

The trip has begun, but some of you may be stuck on the highway . Every holiday, there will be traffic jams on the highway, and this year's "May Day" is no exception. Even before the holiday, the topic on the Internet has already begun to "warm up" in advance.

Source: Weibo screenshot

When it comes to the causes of highway traffic jams, traffic accidents, road construction, weather influences (heavy snow or heavy rain), etc. can all lead to traffic jams. However, there is such a situation: everything is normal ahead, there are no accidents or construction, and parking is not allowed on the highway under normal circumstances, but there is still a traffic jam. Why is this?

Source: Sina

In fact, we call this rather strange traffic jam phenomenon "ghost traffic jam", which sounds a little scary, so what is a "ghost traffic jam"? When there is a traffic jam, what is the car at the front doing?

What is the strange "ghost traffic jam" phenomenon?

The so-called "ghost traffic jam" means that when there are many cars on the highway, any slight disturbance, such as a driver changing lanes, braking suddenly, or even just slowing down a little because of distraction, will cause the cars behind to slow down, and then a series of pauses will be triggered, forming a virtual traffic jam. This traffic jam will cause more and more vehicles to pile up, gradually spreading to the far rear, causing serious traffic jams.

Studies have shown that on a highway with a lot of vehicles, a novice driver's sudden braking may affect a road section of up to 80 kilometers . Nagoya University in Japan once conducted an experiment to explain this phenomenon. Many cars were allowed to drive at a constant speed in a circle. According to this logic, traffic jams should not occur. However, as time went on, some cars inexplicably got closer to the car in front, and when they got closer, they had to brake. As the impact expanded, traffic jams occurred!

Let's take a deeper example: A driver is driving on the highway and suddenly finds a small obstacle ahead, so he subconsciously brakes and slows down. Later, he finds that the small obstacle is just a floating plastic bag, so he increases the horsepower and continues to drive. But when he brakes just now, the car behind him will also brake and stop subconsciously because of his sudden braking. So, the car behind this car, the car behind it, and the car behind it will also brake. Thousands of cars slow down and a traffic jam is formed. Unfortunately, if your car happens to be in the middle and back of the traffic jam at this time, you really can't move forward or backward. There is no other way except to wait.

So, the phantom traffic jam is not a real traffic jam, it just creates a time difference in the speed of cars. The cars behind are more affected, so the stop-and-go effect appears, causing a large-scale traffic slowdown. This is why you wait and wait, and finally wait for the road to be clear, but find that nothing happened ahead, and you can only blame the "ghost"...

What causes "ghost traffic jams"?

In fact, uncivilized driving is the main cause of phantom traffic jams.

> > > > Inner lane: "Should I speed up?"

For example, we often answer phone calls, check Weibo, or reply to WeChat while driving, or remember to slow down and drive when we reach an intersection, resulting in traffic interruption and "jam".

>>>>"Rampage" and emergency brake

Some people cut in, change lanes at will, or even brake at will when driving, causing the vehicles behind them to move or stop suddenly.

Look at the picture below. When all the cars were driving normally on a densely packed road, a red car suddenly changed lanes from the second lane to the fourth lane. As a result, the red car in the fourth lane, which was originally driving smoothly, caused the light blue car behind the red car to brake suddenly. The cars following it also had to brake suddenly and slow down, resulting in slow driving and even traffic jams. It may take only 2 seconds for the first car to start after braking suddenly, but it may take dozens of minutes for the last car to start.

Source: See watermark

>> Import and export "slow in, fast out" reversed

According to traffic rules, whether it is a side road entering the main road or a main road entering the side road, side road vehicles should give way to main road vehicles, so that they can "enter slowly and exit quickly" to ensure smooth traffic on the main road. However, some people do not care about this at all. They do not merge lanes in advance before exiting the main road, and start to move outwards only when they see the exit, resulting in the two outer lanes being affected. Or they enter the main road too quickly, forcing the cars on the main road to brake suddenly to avoid them, thus causing traffic jams.

>>>>Psychological problems

Psychological factors also accelerate the "ghost traffic jam". When you encounter the above-mentioned situations, some people will ignore the traffic rules and take advantage of the traffic jam to cut in line. This unhealthy mentality makes the already tense road conditions even worse.

How to solve ghost traffic jams?

To solve the phantom traffic jam phenomenon, it is inevitable to change bad driving habits. In addition, scientists are also using mathematical methods to come up with various tricks. For many years, Berthold Horn, a professor of computer science at the Massachusetts Institute of Technology, has been committed to studying the causes and solutions to the phantom traffic jam phenomenon. He once proposed a solution - vehicle spacing bilateral control .

The so-called "bilateral control of driving margins" means that when the driver encounters a situation where the vehicle in front changes lanes or brakes suddenly, and the speed slows down, the driver should quickly adjust the distance between himself, the vehicles in front, behind, and around him, and try to place his vehicle in the middle of the vehicles in front and behind. This will prevent the vehicle from slowing down following the driver who brakes suddenly or changes lanes in front, but keep the vehicle in a uniform and gradual slowing state and keep the distance from the vehicles in front and behind, which can alleviate the phantom traffic jam to the greatest extent.

Some scientists also said that slowing down is also one of the ways to solve the phantom traffic jam. Mathematical models show that if you slow down and drive at a fixed speed, you can not only save fuel, but also help eliminate the "ghost traffic jam" phenomenon. For example, on the highway, driving at a constant speed of 80 kilometers per hour is much better than driving at a speed of 110 kilometers per hour.

Now that you understand these, don’t blame the “ghost” next time you encounter a traffic jam on the highway. As long as everyone abides by traffic regulations, this strange phenomenon can be avoided!