Researcher Arthur Kramer from University of Illinois said that they have done years of study on driver distraction, and previous studies suggested that passengers often aren't distracting, rather, passengers can be helpful, especially if they're adults who have had experience and also are active drivers themselves.
The researchers found that driving alone was the safest option, in line with previous research as there were significantly fewer collisions when drivers were alone in the simulated car than when they spoke to a passenger in the car with them.
Kramer said that passengers helped drivers find their exits and improved their memory of road signs, but they detracted from overall safety (avoidance of collisions), whereas speaking to someone on a cell phone while driving was the most dangerous of the conditions as talking to someone who had no awareness of what was going on inside or outside the car more than tripled the likelihood of a collision.
The most interesting results, however, involved the fourth driving scenario, when a driver spoke to someone who was not in the car but who could observe the driver's face and the view out the front windshield on a videophone.
Researcher John Gaspar added that drivers were less likely to be involved in a collision when their remote partner could see what they were seeing and this benefit seems to be driven by changes in the way partners talked to the driver.