Cell phones are not dangerous because the driver was looking at his phone or playing with the buttons when he had an accident. They are dangerous because they encourage your mind to wander. Most people's brains are really not good at multitasking. (Try patting your head while rubbing your belly counterclockwise)
Anything that takes your mind away from the road when you're driving even if you can see the road is dangerous. (They found that talking to someone in a passenger seat is not as dangerous because if they notice danger and you don't, they'll inform you of the danger automatically by tensing up or whatever.)
If your brain worked like a single process cpu, it would be like setting Twitter to high priority while relegating driving your 2-ton SUV to low priority. It will work well enough most of the time, but sooner or later, you'll crash the SUV.
(Source: BS in psychology)