Video: Infuriating traffic jams that appear for no reason can be replicated on the test track (Footage courtesy Mathematical Society of traffic flow in Japan)

Traffic that grinds to a halt and then restarts for no apparent reason is one of the biggest causes of frustration for drivers. Now a team of Japanese researchers has recreated the phenomenon on a test-track for the first time.

The mathematical theory behind these so-called “shockwave” jams was developed more than 15 years ago using models that show jams appear from nowhere on roads carrying their maximum capacity of free-flowing traffic – typically triggered by a single driver slowing down.

After that first vehicle brakes, the driver behind must also slow, and a shockwave jam of bunching cars appears, travelling backwards through the traffic.


The theory has frequently been modelled in computer simulations, and seems to fit with observations of real traffic, but has never been recreated experimentally until now.

Creating congestion

Researchers from several Japanese universities managed the feat by putting 22 vehicles on a 230-metre single-lane circuit (see video).

They asked drivers to cruise steadily at 30 kilometres per hour, and at first the traffic moved freely. But small fluctuations soon appeared in distances between cars, breaking down the free flow, until finally a cluster of several vehicles was forced to stop completely for a moment.

That cluster spread backwards through the traffic like a shockwave. Every time a vehicle at the front of the cluster was able to escape at up to 40 km/h, another vehicle joined the back of the jam.

The shockwave jam travelled backwards through the ring of vehicles at roughly 20 km/h, which is the same as the speed of the shockwave jams observed on roads in real life, says lead researcher Yuki Sugiyama, a physicist in the department of complex systems at Nagoya University.

“Although the emerging jam in our experiment is small, its behaviour is not different from large ones on highways,” he told New Scientist.

Showing it is possible to recreate shockwave jams is important if researchers are to find ways to prevent or control the phenomena, says Sugiyama.

Human error

“It would be interesting to know whether the Japanese experiment had a consistent trigger for the flow breakdown,” says Tim Rees of TRL, a UK transport research firm.

“I suspect that the trigger would either be a particular driver who was more nervous than the rest, or a particular location on the circle where the capacity was slightly lower,” he says.

Pinpointing the causes of shockwave jams is an exercise in psychology more than anything else. “If they had set up an experiment with robots driving in a perfect circle, flow breakdown would not have occurred. Human error is needed to cause the fluctuations in behaviour,” says Rees.

Rees’s team at TRL is calibrating detailed models of traffic flow through different road designs to minimise the probability of shockwave jams. One strategy already in use to reduce shockwaves is imposing temporary speed limits, a method TRL introduced on London’s M25 orbital motorway.

Journal reference: New Journal of Physics (DOI: 10.1088/1367-2630/10/3/033001)

Cars and Motoring – Learn more about the latest technologies in our comprehensive special report.