Traffic science unravels jam secret

London: A professor claims to have worked out the science behind what causes a traffic jam - put simply, it is too many cars on the road.

Prof Yuki Sugiyama, of Nagoya University in Japan, has spent more than a decade puzzling the problem.

In the New Journal of Physics, a study by his research team explains why we occasionally get caught in jams for no obvious reason. He says it is simply the result of there being too many cars.

The team applied techniques to model the movements of particles to real-life moving traffic and found that even tiny fluctuations in the number of cars on the road cause a chain reaction which can lead to a jam.

To demonstrate this, the team studied cars driving around a circular track. They put 22 cars on the road and asked the drivers to go steadily at 30km/h. While the flow was initially free, if a driver altered his speed it reverberated around the track and led to brief standstills. Prof Sugiyama says: "Although the emerging jam in our experiment is small, its behaviour 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 it might be possible to estimate critical density of roads, enabling roads to be built to suit the number of drivers that need to use it.

Mathematicians led by Dr Gabor Orosz, of the University of Exeter, have conducted similar work. They found that the late reaction of drivers by even one second can have serious knock-on effects when driving at much higher speeds than in the Japanese study. "We are currently developing algorithms for radar-guided computer controlled devices that could keep traffic smooth."