Not so, according to the authority running the Forth Road Bridge which begins a £40,000 trial next month of an infra-red camera that detects the number of humans in a vehicle from 160ft. The system, called Cyclops, is a world first and has been developed by engineers at Loughborough University.

The trial comes before the introduction next year of automatic electronic tolling on the 41-year-old bridge near Edinburgh and of Britain’s first differential tolls in 2007 when charges will rise to £4 compared with the current £1.

Under differential tolls cars with one or more passengers will pay 50 per cent less to cross the bridge than those carrying only a driver. Cyclops uses infra-red optical technology which identifies human skin by measuring the water content and instantly distinguishing people from dummies, pets and luggage.

It has attracted interest from America and Australia where high-occupancy vehicle lanes already operate. At around £100,000 a camera the system does not come cheap, and one camera is needed for each individual traffic lane. But its accuracy rate is 95 per cent compared with 65 per cent for manual surveillance.

John Tyrer, the managing director of Vehicle Occupancy Ltd, the company behind Cyclops, says that it has had successful trials on a toll-free HOV lane on the A647 between Leeds and Bradford.

He said: “The system is a world first. Based on multispectral imaging techniques, it measures the water content in human skin. It is safe and accurate. It works on all vehicle types and in all weather, including heavy rain, snow and fog.”

Alastair Andrew, the general manager of the Forth Estuary Transport Authority, said that finding ways to tackle congestion was a priority and electronic tolling was a start. Bridge users can opt to have electronic tags fitted to their cars which automatically lift toll barriers and charge the motorist’s prepaid account.

“If we decide to operate differential tolling in the future we would need to know how many people are in a car, thus Cyclops. We have investigated other camera systems and some picked up pets travelling in a vehicle,” Mr Andrew said.

“Drivers even put plastic models as decoys in the passenger seat to try to confuse the cameras and pay a lower toll. Because this new camera system identifies human skin, these problems are eliminated.”

The cost of the trial is being met jointly by the authority and the Scottish Executive. But the AA said that while Cyclops might stop the use of plastic decoys, it will not prevent penniless students, for example, hanging around the bridge and offering to pose as passengers for a small fee.

“If the temptation is there [to cheat], some people will be tempted,” Neil Greig, the AA spokesman in Scotland, said.

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