The technology, called Soak, changes colour if perspiration is high in salt, suggesting dehydration.

Previous research by the European Hydration Institute and Loughborough University found that dehydrated drivers were as error-prone as those who had drunk alcohol.

There are currently no plans to bring Soak into production.

The sweat-sensitive coating, which was developed with Dutch design company Droog, is also applied to the steering wheel and changes it and the front seats from blue to yellow to signal dehydration.

Prof Peter Wells, an expert in business and sustainability from Cardiff University Business School, said that measuring additional factors which impede drivers' abilities made as much sense as monitoring factors which affect the vehicle itself.

"This is part of the overall idea that it's not just about monitoring the car but also the driver," he said.

"This particular application is obviously on the edge of usefulness but it shows a willingness to think more generally about these things and find a way forward.

"I'm not sure that way of showing you are dehydrated is going to appeal to many people - but I like the concept."

Prof Wells added that other potentially measurable factors which affect drivers could include their emotional state - perhaps by monitoring adrenaline or hormone levels.

"We are going to get more and more of this kind of thing. Many factors affect our ability to drive," he said.