Talking on a mobile phone while driving, even if using a hands-free kit, has proved to be more distracting than having a chatty passenger in the car .

Lee Strayer of the University of Utah and colleagues have found in a series of experiments using driving simulators that hands-free mobile phones are just as distracting as hand-held models. They have demonstrated that chatting on a cell phone can slow the reaction times of young adult drivers to levels seen among senior citizens, and shown that drivers using mobile telephones are as impaired as drivers who are legally drunk.

In their defence, motor-mouth passengers are more likely to alert drivers to traffic hazards and approaching turns, at least offering some assistance while driving.