London, March 4 : A team of Japanese researchers has recreated the phenomenon of "shockwave" traffic jams on a test-track for the first time.
According to a report in New Scientist, shockwave traffic jams happen when traffic grinds to a halt and then restarts for no apparent reason.
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.
Researchers at Nagoya University, Japan, managed the feat by putting 22 vehicles on a 230-metre single-lane circuit.
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," said 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.
According to Sugiyama, showing it is possible to recreate shockwave jams is important if researchers are to find ways to prevent or control the phenomena.
Tim Rees of TRL, a UK transport research firm, and his team 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.