Thought it was a gorilla - bbc have a good podcast from last week about human inability to dedicate brain resources to two tasks,
epitomized by the plane crash where they failed to see they had run out of fuel whilst trying to fix an undrcarrage problem, but in motorist context people on hands free just looking at the road, without seeing potential hazards, they also had evidence that concentration is not immediately restored when call ends
Probably waiting for a prosecution where someone is prosecuted for being distracted irrespective to the fact they were on hands-free, at which point (like with cigarettes) would the car manufacturers be able to abdicate responsibility.