Sat-nav systems can not only sometimes send motorists the wrong way but also impair driving, a study suggests.
Researchers at University of London and Lancaster University tested drivers in a car simulator.
The study suggested motorists did not drive as safely when they were trying to concentrate on the directions given.
While they could cope with simple instructions, they started to swerve, speed up or fail to notice pedestrians if they had too much information.
Volunteers were set tasks by a computer which imitated the instructions given by a vehicle's satellite navigation system.
Lead researcher Polly Dalton, from the department of psychology at University of London's Royal Holloway college, told the BBC that the gadgets needed to be kept simple.
She said: “People are capable of following spoken instructions in a car and it's actually a really good way of presenting information to them.
“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.”
But she added that listening to instructions was still safer than looking at a map.
Lancaster University's Dr Pragya Agarwal said the research could help sat-nav design to be more effective and user-friendly in the future.
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.