Lord Whitty: Scheme has woked elsewhere

Traffic lights are being installed at these sites to control vehicle flow on to the main carriageway.

They are linked to sensors which measure the volume of traffic travelling along the motorway and identify when it is safe for motorists to join the traffic flow.

Known by its American name of ramp metering, the system was launched on Thursday by transport minister Lord Whitty.

He said: "The system has proved highly successful in reducing congestion and improving safety in other countries.

'No long-term solution'

"We look forward to sharing those benefits with motorists in England.

"I look forward to seeing the results of the ramp metering trial which has been introduced by the Highways Agency in line with proposals outlined in the government's 10-year plan for transport."

But an AA spokesman said ramp metering was "not the long-term solution to easing congestion in the south of England".

"While the scheme may reduce rush-hour jams, it will have a limited shelf life during which long-term, more robust solutions - such as motorway widening - should be developed," he said.