London - Speaking on a hands-free cellphone in a car makes drivers take almost twice as long to react to hazards, scientists have found.
They said motorists took a tenth of a second longer to see potential dangers on the road.
Hands-free conversations behind the wheel are thought to be safer than using a hand-held phone; worryingly, though, reaction times are affected equally badly whether the driver is on a hands-free or a hand-held phone - or ev3n simply chatting to a passenger.
Such distractions increase reaction times to a quarter of a second, creating a delay equivalent to a car speeding along at 100km/h taking three metres further to stop.
The US study from the University of Iowa backs up previous research that speaking on a cellphone is almost as dangerous as driving under the influence.
Co-author Shaun Vecera, professor of psychological and brain sciences, said: "It slows your attention down - and we’re just not aware of it because it happens so fast. It is going to delay your ability to react by braking. Three metres at 100km/h can be the difference between stopping safely and rear-ending someone.’
It was known that drivers who speak on mobiles often veer into the wrong lane, fail to check their rear mirror and speedometer, and remember less of their journey. Motorists using a mobile are also four times more likely to crash.
Hands-free calls are safer because both hands are on the wheel - but they are just as distracting, causing what scientists call ‘attention disengagement’.
The US researchers used computerised experiments to track eye movements while asking 26 volunteers to choose whether a series of statements were true or false. The test was designed to mimic the ‘active listening’ required to take in information and simultaneously prepare a reply during a conversation - while driving a car.
Those who answered questions took almost twice as long to look at a new object on a screen as those not asked anything. The average delay was 40 milliseconds, but because a driver’s eyes scan their field of view 2.5 times a second, it takes a tenth of a second extra to see everything. The reactions of the worst participants were three times as long.
The study, in the journal Psychonomic Bulletin and Review, says: "It becomes so effortful, while holding a conversation, to move your attention and eyes around that you reduce the amount you do it. Drivers on the phone have a kind of tunnel vision where they are looking out of the windscreen but not moving their eyes around as much."