New research suggests that mobile phones, even when used hands-free, are more distracting to drivers than passengers or being drunk.

The study team, led by Dr Frank Drews of the University of Utah, put research subjects in a driving simulator and sent them on a 24-mile journey on a multi-lane highway.

The drivers were tested while using mobile phones, then with a talkative passenger and a silent passenger.

The results showed that drivers talking on a mobile phone were more likely to weave across lanes, kept a larger distance between themselves and the car in front and were four times as likely to miss turnings.

"Cell phone and passenger conversation differ in their impact on a driver's performance at the operational, tactical and strategic levels," the researchers wrote in a paper entitled Passenger and Cell Phone Conversations in Simulated Driving (PDF).

The study will be bad news for legislators who have introduced regulations insisting on mobiles in cars being operated hands-free only.

The study also found that drivers were less able to concentrate on calls while driving, and simplified their speech while attempting a manoeuvre in the car.

They also tended to speak more often, and the researchers theorise that this is an attempt to dominate the conversation and avoid having to concentrate on listening to the caller.