They may be annoying but a back seat driver is apparently far less of a distraction than a phone call - hands free or not.

According to research carried out by the University of Utah, and published in the Journal of Experimental Psychology: Applied, phone calls cause drivers to miss exits and follow other cars too closely.

While phones distract, passengers can act as a second set of eyes on the road so giving the driver warnings and also pointing out directions.

Said Dave Strayer of the University of Utah ot Reuters: "...chatting on a mobile 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".

And the test results from the driving simulator used by the team also were quite damning of hands-free kits.

Drivers using a hands-free device drifted out of their lanes and missed exits more frequently than drivers talking to a passenger.

"The difference between a cell phone conversation and passenger conversation is due to the fact that the passenger is in the vehicle and knows what the traffic conditions are like, and they help the driver by reminding them of where to take an exit and pointing out hazards", Strayer concluded.