Many people, myself included, can't talk and do things at the same time. Talking is far more distracting to me than pushing a physical button. It actually takes more brain power to talk than to navigate a screen interface, but the screen interface is still worse because I have to take my eyes off the road, but talking is a close second. If I'm in a high-interaction driving environment (ie not an express way with low traffic), I will literally, unconsciously slow down if you ask me a complex question. I think there is some gender asymmetry to this issue, as my wife, and all the women I regularly drive with, can talk and do things at the same time.