see here:
http://www.bbc.co.uk/news/uk-36475180
"The study involved 20 male and 40 female volunteers who took part in video tests while sitting in a car seat behind a steering wheel.
One group of volunteers were allowed to "drive" undistracted while another two heard a male voice from a loudspeaker 3ft (0.9m) away.
Those who were distracted by the voice engaging them in conversation took just under a second longer to respond to events, such as a pedestrian stepping off the pavement, an oncoming car on the wrong side of the road or an unexpected vehicle parked at a junction."