Where do spontaneous traffic jams come from?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

13 November 2016

In 2008, some mad scientists at the University of Nagoya did a crazy experiment. They marked out a 230-metre circular path on an open and level field. Then they placed 22 cars in this circle and asked 22 drivers to drive them around the circular path at 30km/h.

Since there is no obstacle along the path, one would expect the cars to move steadily around the circle until they run out of fuel. But that’s not what happened. After several minutes, the scientists observed that a traffic jam had formed spontaneously out of nowhere!

Out of the blue, cars started clustering at one section of the circle and coming to a complete stop. Furthermore, the “clustered zone” was observed to move steadily backwards against the traffic flow at about 20km/h. The full details of this experiment were published in March 2008 in the New Journal of Physics – a publication of The Institute of Physics. A video of the actual experiment is available HERE

How did the spontaneous traffic jam come from? As the cars went round the field, one of the drivers must have gotten distracted momentarily and eased off the accelerator thus slowing down. The driver behind noticed this, and tried to avoid a crash by stepping lightly on the brakes. The third one in the circuit saw this and also stepped on his brakes, but harder than the second one.

This successive braking propagated along the queue and eventually, one drive had to press his brakes hard enough to stop his car. The traffic jam formed at that moment.

This happened because the cars were moving too fast for the distance between them. 22 cars in a 230m circuit means that each has a space of about 10m long. Since a car is about 5m in length, it means that only 5m was left between the cars.

Now, at 30km/h, each car was taking only 0.6 of a second to cover the 5m distance. Thus, any small disturbance in the flow will trigger a traffic jam. A physical obstacle is not necessary.

Once the jam has formed, the car at the front of the cluster waits until the drives feels it is safe to move again. As he is waiting, others have to stop behind him. When he drives out, the car behind him also waits until there is safe distance to move.

Eventually, we get a situation where one car leaves the jam and another one joins the back of the cluster. Consequently, the clustered zone drifts backwards. This is similar to the shockwave phenomenon observed when the flow of liquids or gases is obstructed for a short moment.

The traffic shockwave is also observed quite often on highways when the density of vehicles exceeds a certain critical value. Have you ever found yourself in a traffic jam and then found no obstacle when you reached the front of the queue? Now you know what caused it – nothing!