Chatting on a mobile phone is at least as dangerous as driving while over the legal alcohol limit, research suggests.
Talking on a phone creates a form of inattention blindness
The University of Utah study, like others, found hands-free mobiles were just as distracting as handheld ones.
The work involved 40 volunteers using driving simulators, and the authors say there is now enough evidence to warrant banning all mobile use when driving.
Currently in the UK, using a hand-held phone while driving is banned but the use of hands-free phones is permitted.
Under existing legislation, they can, however, be prosecuted if it is proved that they are not in proper control of their vehicle.
Forty volunteers had their reactions tested in four separate circumstances when they drove without distractions, while using a handheld phone and a hands-free phone and while intoxicated.
Motorists who talked on mobiles, both handheld and hands-free, were as impaired as drunk drivers and more likely to be involved in traffic accidents.
Professor David Strayer and his team of researchers believe having a phone conversation alters how drivers perceive and react to information.
This means motorists who were on the phone were more likely to fail to stop at a cross junction and to be involved in rear-end collisions than drivers not using a mobile.
Even when the drivers using the phones were looking at objects they often failed to see them.
Professor Strayer said: "We suggest that talking on a cell phone creates a form of inattention blindness, muting driver's awareness of important information in the driving scene."
Roger Vincent, of the Royal Society for the Prevention of Accidents, said: "Previous studies have shown that you are four times more likely to crash if you are using a handheld or a hands-free phone.
"You get sucked more and more into the conversation and you pay less and less attention to the road.
"You start to tailgate, you wander about on the road and you vary your speed, and those things make you more likely to crash."
He called for a ban on all mobile phone use by motorists and welcomed proposed tougher penalties, including points on licences, for offenders.