"Using a cell phone while driving - whether it's hand-held or hands-free delays a driver's reactions as much as having a blood alcohol concentration at the legal limit of .08 percent. (University of Utah)"
People drinking to 0.08 drove slower, braked faster (!), had normal reaction times, and got in 0 accidents. Yet the study says they are more aggressive because they followed the car in front of them by 9.5 meters instead of 10.3.
This study doesn't make me think hands-free sets should be illegal, but that the blood alcohol limit level is too low.