Only one hand available is irrelevant
Research suggests that handfree headsets are as bad as traditional cell phones. The distraction is the issue.
In particular distraction that pays no attention to what is happening in the car. Humans in the car naturally modulate their conversation in deference to driving conditions. Humans on the phone, don't.
Cheers,
Ben
I have come to believe that idealism without discipline is a quick road to disaster, while discipline without idealism is pointless. -- Aaron Ward (my brother)