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Motorists who are too aggressive or too timid in their driving style

are the cause of major traffic jams, scientists have discovered.


Traffic jams are often caused by the stop-start driving typical in aggressive drivers Photo: ALAMY


While the underlying cause of a jam might be an accident, a bottleneck, or drivers simply changing lanes on busy roads, it is how the drivers react in the cars behind that cause traffic to slow to a halt.

Researchers say aggressive motorists, who drive too fast and too close to the vehicle in front, or timid motorists, who leave too big a gap, send a "wave of deceleration" backwards down the road until traffic grinds to a stop.

Such behavior leads to the stop-start traffic jams which infuriate motorists.


Too Many Cars Cause Traffic Jams

A Japanese team has found the underlying cause of traffic jams when there is no obvious reason for the delay.

Many traffic jams leave drivers baffled as they finally reach the end of a tail-back to find no visible cause. An accident?  Construction work? A bottleneck? No, just too much traffic, says a team led by Prof Yuki Sugiyama of Nagoya University, who has spent more than a decade puzzling over the problem.

In the New Journal of Physics a study by his group explains why we're occasionally caught in jams for no obvious reason.

The real origin of the snarl up often has nothing to do with obvious obstructions such as accidents or construction work but is simply the result of there being too many cars.

The team discovered the importance of traffic density by applying techniques to model the movements of lots of particles to real-life moving traffic. The research shows that even tiny fluctuations in car-road density cause a chain reaction which can lead to a jam.

The team also studied cars driving around a circular track with a circumference of 230m. They put 22 cars on the road and asked the drivers to go steadily at 30km/h around the track. While the flow was initially free, the effect of a driver altering his speed reverberated around the track and led to brief standstills.

Prof Sugiyama says, "Although the emerging jam in our experiment is small, its behavior is not different from large ones on highways. When a large number of vehicles, beyond the road capacity, are successively injected into the road, the density exceeds the critical value and the free flow state becomes unstable."

The research suggests that it might be possible to estimate the critical density of roads, making it possible to build a road fit for the number of drivers that need to use it.

Mathematicians led by Dr Gábor Orosz of the University of Exeter have done similar work and he comments: "Many researchers believe that the effect of spontaneous jam formation (caused by tiny fluctuations above a critical traffic density) is the main reason for traffic jams and this view is supported by Prof Sugiyama."

The Exeter work is different because the reaction time delay of drivers is included, revealing that the late reaction of drivers even one second can have big knock on effects when driving at much higher speeds than in the Japanese study.

"In a typical situation a vehicle dropping its speed from 128Km/h to 104Km/h may cause a ripple that later vanishes while dropping its speed from 128 km/h to 100Km/h may cause a ripple that is amplified and leads to traffic jams."

Heavy traffic on highways does not automatically lead to congestion but can be smooth-flowing, he says. "We are currently developing algorithms for radar-guided computer-controlled cruise-control devices that could cut down over-braking and keep traffic smooth."

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