The trial aims to cut congestion on the M4

The two-tone colour symbols and simple messages are being displayed on signs between junctions 12 and 14, in Oxfordshire and Berkshire, from Wednesday.

The �16m signs, launched by transport minister Kim Howells, are automatically triggered to detect slow-moving or stationary traffic.

They then pass on that information to drivers via the screens.

I live in Newbury and I've seen the signs - they are huge - you really couldn't miss them Rebecca Rees, AA Motoring Trust

Police will also be able to use the signs to inform drivers of any crashes or incidents as soon as they have happened.

Mr Howells, a regular user of the M4, said the signs could have stopped him speeding.

The minister, who admitted previously being stopped twice for breaking the limit, said: "On both occasions it was when there were roadworks going on and you get into a mindset doing 70 mph and suddenly it's a 50 mph limit.

"I think that is what these signs will give you a better idea about."

Mr Howells ordered the signs to be switched on by mobile phone and they immediately showed a white exclamation mark inside a red triangle, with the words Drive With Care.

Drivers warned

Rebecca Rees, spokeswoman for the AA Motoring Trust, welcomed the signs, saying the more information and advance warning for motorists the better.

She said: "These signs don't just warn about road incidents, but also about conditions such as fog and snow and use images to show that.

"And it's great that it is being piloted along the M4, one of the busiest motorways in terms of congestion - drivers in this area have put up with delays for too long."

The trial, due to last until December 2004, could be extended nationally if successful.

During his visit to the region, Mr Howells also visited a �38.5m scheme to tackle congestion between Newbury and Oxford on the A34/M4 Chieveley underpass.