A team from Lancaster University and University of London tested drivers in a car simulator and found that motorists did not drive as safely when they were trying to concentrate on the directions given through a satellite navigation system.
While they could cope with simple instructions, they started to swerve, speed up or fail to notice pedestrians if they had too much information, the team found.
The findings suggest that the gadgets need to be kept simple, said lead researcher Polly Dalton of the department of psychology at University of London's Royal Holloway college.
"People are capable of following spoken instructions in a car and it's actually a really good way of presenting information to them," she was quoted as saying by the BBC.
"It's important not to make those directions too complicated because that runs the risk of asking them to keep too much information in mind when they are also trying to concentrate on the driving task."
However, she added that listening to instructions was still safer than looking at a map.
The study could help sat-nav design to be more effective and user-friendly in the future, the researchers added.
The Department for Transport acknowledged that sat-navs were very useful for motorists but warned drivers to use them responsibly and keep their eyes on the road.
