For the past few years, 90% of the time that I've had a close call, or seen someone in front of me driving erratically (weaving, going from 65 to 45, etc.) it's been because the other driver was doing something on their cell phone. That includes hands free, because I see people with headsets in their ear or else looking like they're talking to themselves also being erratic -- especially the speed up - slow down thing.
So, yeah they drive me absolutely nuts and shouldn't be on the road. As much as I always say, "There oughta be a law" whenever it happens, I have a problem, as stated above, about .gov micromanaging yet one more aspect of our lives.