The arrival of the production-ready dtect camera, which has been in trials for a number of years, could see the spread of car sharing lanes throughout the UK.

Four local authorities are currently using such lanes in Leeds, Birmingham, Bristol and North Somerset, and the Government believes they can help cut congestion and lower emissions. About 80% of cars driving into urban areas at rush hour are carrying just a single occupant.

Drivers in California have used blow-up dolls and shop dummies to get away with using car-sharing fast lanes, but the dtect camera will put technology rather than traffic police in charge of auto-enforcement.

It works by projecting two low intensity beams of infrared (IR) light into the vehicle at different frequencies. The two pictures are processed to produce an enhanced image of the occupants' skin, with non-facial features discarded using software.

The system works with all skin types through all current types of automotive glass, and is not dependent on temperature inside or outside of the vehicle.

Professor John Tyrer, of Loughborough University, said: "It is a fact that we are putting more cars on the road. It is a fact that we have a fixed infrastructure. Therefore, it is a fact that congestion is going to go up. We do not have the ability to build more lanes. We need to figure out a way of moving people quicker, and a way of better utilising our road infrastructure. We have to do something about congestion and high occupancy vehicle (HOV) lanes provide a very practical way of changing behaviour and making better use of the infrastructure."

He said congestion charging had failed to get cars moving more quickly.

A prototype version of the camera has already been trialled in various locations, including the Forth Road Bridge in Scotland and in Leeds, where the finished model is due to be piloted.

And Professor Tyrer said the 1.5km car sharing lane along the A647 Stanningley Road in the city, which operates in peak periods from Monday to Friday, has decreased journey times by as much as 13 minutes each way. Only buses, coaches, motorcycles, bicycles and cars with two or more occupants are allowed in the lanes.