Although some people can multi task while driving, alot of people can't. These people who can't, make it extremely dangerous to be sharing the same highway, freeway, parking lot, and street with. What I have seen on the highway and freeway are people doing 20 plus miles below the speed limit (while texting or talking with their hands, as if the person they are talking to is right there with them) and you can't get around them, it causes others to come up on your bumper at very fast speeds, while you pray that person doesn't slam into you. Others are texting or talking in parking lots totally oblivious that there are other cars travelling in the same lot with them. It seems like their eyes are fixed to their cell phone and not the road.
I am not sure if there is a safe speed for calls using a handset and certainly texting should be banned while driving.