Well, back-seat drivers tend not to improve people's driving any too much, so perhaps this isn't so surprising after all - but it seems that satnavs, too, can make it worse.
British researchers carried out a series of experiments where a computer mimicked the instructions given by a satnav - and found that when people were following complicated satnav instructions they tended to drive faster, with more steering variations.
They were also less likely to notice pedestrians who might be stepping out.
"What is interesting is that people were able to follow one simple instruction without any significant impact on their driving but as soon as they had to remember a compound instruction, consisting of two sequential directions, we began to notice a difference in their driving ability," says Dr Polly Dalton from Royal Holloway.
"A lot of effort has gone into designing visually friendly satnav devices, but our research highlights the importance of the way in which the auditory instructions are given."
Indeed, three-quarters of those participating in the experiments said that they only used the satnav’s visual display for clarification, more detail, or as a reminder of the verbal instructions.
The scientists say that while spoken instructions alone are one of the safest ways to present navigational information while driving, they can still represent quite a distraction.
"The results from our research have implications for the way these systems can be designed to be more effective and user-friendly in the future," says Dr Pragya Agarwal from Lancaster University.