Traffic Jams are Caused by ... Too Many Cars on the Road!

This is a familiar experience for most of us: getting stuck in a traffic jam only to find out later that there is no obvious cause (like accident, construction work or bottleneck).

Well, Japanese scientist Yuki Sugiyama of Nagoya University has finally found the answer: the traffic jam is caused by ... too many cars!

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 (19mph) 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 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."

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