When you reach for the cell phone while driving, watch out: you may behave decades older than your biological age. A new study has shown that when young motorists chat on mobile phones, they drive like elderly people, moving and reacting slower than usual and increasing their risk of accidents.

'If you put 20-year-old drivers behind the wheel with cell phones, their reaction times are the same as 70-year-old drivers who are not using cell phones,' says Dr David Strayer, a professor of psychology at the University of Utah and author of the study published in the current issue of Human Factors.

The study found that drivers speaking on cell phones ' whether young or old ' were about 18 per cent slower in hitting brakes than drivers who didn't use cell phones. The drivers chatting on cell phones also had a 12 per cent greater following distance and took 17 per cent longer to regain the speed that they lost when they braked.

Strayer and his colleagues had earlier shown that hands-free cell phones were as distracting as the hand-held ones. The reason is 'inattention blindness' ' a condition in which motorists gaze at the conditions on the road ahead, but do not really see them because they are distracted by the phone conversation.

The study found that when young drivers used cell phones, the reaction time in hitting brakes matched that of the elderly drivers who did not talk on cell phones ' an average of 912 milliseconds. When not talking on cell phones, young motorists hit the brakes at an average of about 780 milliseconds.

The study used driving simulators to show how talking on cell phones affected reaction time for drivers. It found that elderly drivers become even slower to react to brake lights from a vehicle in front when they spoke on cell phones.