This has been known for a long time. Cellphone distraction has little to do with holding an object in one's hand. It is the brain-taxing effect of conversation,

This has been known for a long time. Cellphone distraction has little to do with holding an object in one's hand. It is the brain-taxing effect of conversation, and the fact that the other participant is not in the car to sense danger and therefore know when to stop talking. From the article: it is easy to equate talking to a friend on a cellphone with talking to a friend in the passenger seat. But a December report in The Journal of Experimental Psychology: Applied debunked that notion. Utah researchers put 96 drivers in a simulator, instructing them to drive several miles down the road and pull off at a rest stop. Sometimes the drivers were talking on a hands-free cell phone, and sometimes they were chatting with a friend in the next seat. Nearly every driver with a passenger found the rest stop, in part because the passenger often acted as an extra set of eyes, alerting the driver to the approaching exit. But among those talking on the cellphone, half missed the exit. “The paradox is that if the friend is sitting next to you, you drive safer,” Dr. Strayer said. “When you talk to that person on a cellphone, you’re much more likely to be involved in an accident.”