Performing complex mental tasks whilst driving is dangerous, say Spanish researchers, who have carried out experiments on motorists.
By Dr David Whitehouse
BBC News Online science editor
The research shows that thinking too much can dramatically reduce the ability to spot potential problems and react to them safely.
Using a mobile phone whilst driving is illegal in Spain
The work also reveals that hands-free phone conversations are as safe - or as risky - as talking to a passenger, depending upon the demands of the conversation.
If the conversation becomes too involved, heated or argumentative, then driving safety can be compromised.
'Remarkable distraction effects'
Psychologists from Spain's Direccion General De Trafico and the Universidad Compultense in Madrid studied 12 drivers who drove for four hours on a highway north of Madrid.
The car they used was a standard Citroen equipped with an unobtrusive eye-tracking system that let the researchers see what the drivers were looking at. This permitted the scientists to detect signs of distraction.
Drivers were also tested by a device that flashed tiny lights into their field-of-view to which they had to respond by pressing buttons on the steering wheel.
To test their awareness, drivers were given a series of mental tasks to perform as they proceeded along the motorway.
They listened to audio messages containing information of varying complexity and were asked to repeat them. It was found that doing this had little or no effect on driving performance.
Other tasks were not so harmless, such as a mental calculation involving converting Euros to Spanish pesetas, and giving detailed information about where they were and what they were doing.
Both these tests produced what the researchers call "remarkable" distraction effects.
"When performing complex mental tasks," the authors say, "the percentages of detected targets and correct responses decreased significantly."
"Some tasks showed a reduction in detection probability of almost 30%, something that is practically meaningful as an estimate of distraction errors that can lead to traffic conflicts or accidents," they add.
Experiments were also carried out to evaluate the effect of using a hands-free phone in the car whilst driving.
What they called a "low-demand" phone conversation had a distractive effect about the same as talking to a passenger.
But the psychologists point out that, "complex conversations," whether by phone or with a passenger, are dangerous for road safety."
The results are important, they say, for the evaluation of the safety of various in-car devices, and the best way to present information to drivers with minimal impact on safe driving.
They also say that because it is not easy to enforce safe driving practice legislators may wish to consider restricting the use of certain in-vehicle devices.
The research is published in the Journal of Experimental Psychology.