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The secret of those phantom traffic jams on motorways which seem to have no apparent cause is revealed in a study by the Royal Automobile Club undertaken for the ITV current affairs series 3D to be screened tonight. The jams are usually caused by one bad driver who brakes, forcing others to hit the brakes and reduce speed. Other reasons include middle lane- hogging, sharp late turns by motorists leaving the motorway and constant lane switching by drivers who think the traffic is faster on the other side.

The RAC says that a 10-mile tailback can take half an hour to build up and two hours to clear and will contain an average of 8,000 vehicles. Jams can be caused by motorists "rubbernecking" at an accident, watching hot air balloons or even answering their mobile phones. Christian Wolmar