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The signs, implemented by authorities in rural St Hilary, in the Vale of Glamorgan, South Wales, to warn drivers about placing too much faith in the directional gadgets, could be brought in across the country if the trial is successful.

Problems were reported after foreign drivers found it difficult to understand phrases such as "unsuitable for heavy goods vehicles" but could understand pictorial notices.

The four signs have been introduced around one particular black spot in the village, where the electronic devices direct truck drivers to a shortcut between the main M4 motorway and Cardiff airport.

The road is far too narrow for many to travel down, causing them to get stuck and sparking major traffic problems.

More than a dozen large trucks have

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become stuck in the road in the past six months, according to traffic engineer Mark Simpson who came up with the idea for the signs.

"We have had a series of problems with drivers getting into trouble by trusting their satnavs and we needed to do something about it," Simpson told newspapers.

"They can send drivers on the most direct routes which turn out to be narrow roads completely unsuitable for heavy and long trucks and lorries. Satnav can be a wonderful tool for drivers but it does have its dangers."

If successful, officials plan a national roll-out of the signs to combat what is seen as a growing problem for frustrated motorists, with recent figures showing that more than four million of Britain's 32 million drivers rely on satnavs.

Some have reported that software glitches have sent drivers down one-way streets or up impassable mountain tracks.

One ambulance driver with a faulty satnav drove hundreds of miles in the wrong direction while transferring a patient from one hospital in Ilford, East London, to another just eight miles away.

A spokesman for the Welsh Assembly regional government says that officials will be "watching closely" before deciding whether to expand the programme.

He also says that officials from the Department of Transport had been consulted and were also monitoring the experiment.