In 2002 the U.K.’s Transport Research Laboratory found that drivers talking on a hands-free set reacted more slowly than those who were just over the drunken-driving limit.
Three years later an Australian study found drivers using phones hands-free or hand-held were four times as likely to crash as those not on the phone.
In 2008 University of Utah psychologist David Strayer found that talking on a hands-free phone was more distracting than talking to a passenger.
Now Strayer has returned to the lab and he has more bad news. Sponsored by the AAA Foundation for Traffic Safety, he compared driver response in different situations. Radio or audiobooks were judged mildly distracting. Talking on a hand-held or hands-free phone or to a passenger were all the more distracting, with hand-held the worst of these. But voice-activated systems to send and receive texts and email were the worst kind of distraction.
More information is also available on the European Road Safety Observatory