You don't often hear people claiming that downing a bottle of gin has no effect on their driving skills. Unless they've already had the gin, they're motoring ferociously down the A24 and you're sitting in the passenger seat, in which case it would be advisable to get them pull over and take the wheel yourself. Unless you've had a similar amount of gin, in which case it might make sense to pull over and just quietly sit on the outskirts of Leatherhead for however many hours it takes for gin to stop messing with both your central nervous systems.

You do, however, hear people saying that using a mobile phone while they're at the wheel doesn't affect their ability to stop at red lights or avoid slaughtering innocent pedestrians. There are a few reasons for this. Firstly, everyone is incredibly defensive about their ability to operate a motor vehicle; dare to suggest to someone that they drive too fast, and most of them will indignantly speed up by about 3mph while muttering something about never having had an accident. Secondly, people judge their own driving skills against those of the most cretinous, bad-tempered, dangerous drivers on the roads. They might be in the fast lane of the M5, squinting over the dashboard while burrowing in the glove compartment for a 2-month old packet of Werther's Originals, but they'd still claim that they're not as bad as this bloke they saw on an episode of Police Camera Action.

Lastly, they'll say that the fact that they splashed out on a hands-free set means that using their mobile is no worse than any number of other in-car distractions, including scolding their children, trying to find Willesden Junction on a satnav, or screaming in fury at Libby Purves's moribund interviewing technique on Radio 4's Midweek. But researchers at the University of Utah don't agree. Amongst the many findings in their report which appears in the current Journal of Experimental Psychology, the most striking is that drivers are far more likely to become distracted while talking on a hands-free set than having a conversation with someone sitting in the passenger seat. One of the reasons they suggest is the benefit of having a second pair of eyes on the road; if you find yourself at Spaghetti Junction, looking desperately for a signpost for the A38(M) northbound, it's likely that your passenger will a) stop talking about their gallstones, and b) help you navigate. You're not likely to get such consideration from someone at the other end of a phone line.

Of course, these things are hard to measure. It could be that the psychologists tasked with providing scintillating in-car conversation during the experiment were actually deeply tedious, thus freeing up the mind of the driver to concentrate on keeping the vehicle from ploughing into the central reservation. But my own feeling is that there is something innately stressful about talking on a hands-free phone while driving. You do experience a heightened state of anxiety, and for some reason you feel far more invested in the conversation than you would be if it was with a passenger.

Britian's 5-year-old ban on using mobiles at the wheel unless they're hands-free is still widely-flouted – you see innumerable people doing it every day – so it's unlikely that motorists would take kindly to governments acting on the information in such studies and banning hands-free sets, too. And nor would mobile phone companies. But while you can't legislate against idiocy, you can legislate against people increasing their risk of having an accident. Unfortunately, we all have such a conceited and deeply-skewed idea of our own driving abilities, that we'd probably start whining about the "nanny state".