ignoring the political/philisophical prism many see this through, it is my direct and constant experience on US interstates that the majority of odd or erratic driving is by people who, when I pass them, either have a phone on their ear or are looking at something in their lap/hand.
The number of people who believe they drive well talking on a cell is much higher than the number that actually can allocate proper attention and response-time.
I was driving on I-95 and saw a car drifting between lanes ahead of me. I was thinking "drunk driver" - no such luck. When the driver manuevered the car to the right lane and I passed, I saw he had his shiny new iPad propped on the steering wheel and was engrossed in reading something.