Page last updated at 10:09 GMT, Friday, 2 April 2010 11:09 UK

Single vehicles slowing 'causes phantom traffic jams'

Motorway traffic jam
One driver braking can cause a "traffic tsunami"

"Phantom" traffic jams - ones with no obvious cause - could be the result of a single driver braking slightly, according to research.

Researchers at Bristol University say most changes in vehicle speed and road position are absorbed by traffic flow - but some are not, causing delays.

Vehicles were tracked along a busy one-mile stretch of the M42 near Birmingham in different conditions.

The research project could lead to more accurate forecasting of traffic flow.

Dr Eddie Wilson told BBC Radio 4's Today programme a "stop-and-go wave" typically begins at a busy motorway junction, with small fluctuations in traffic caused by a sequence of bad lane changes.

"Then there is something about traffic that magnifies those fluctuations into a full-scale traffic jam, which then rolls backwards down the motorway," he said.

"The reason you see nothing when you get to the front of the queue is that the jam itself was too far in front of you - maybe half an hour, an hour in front of you - and it's rolled back down the motorway to where you've met it."

Earlier research predicted road traffic would rise by 30% between 2000 and 2015.

This prediction pre-dated the recession, so it was possible traffic growth would be less than thought - but "sooner or later it will grow and we will run out of road capacity," Dr Wilson said on Friday.



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