I dunno about studies, but for me it's definitely the eyes-not-on-the-road. I'm so thankful I haven't gotten into a crash despite the few times I've been stupid enough to text while driving.
"While many people know texting while driving increases crash risk, the lack of understanding about the risks of phone conversation while driving remains a challenge. Talking on hands-free or handheld cell phones requires the brain to multitask – a process it cannot do safely while driving."
Part of the problem with continuous partial attention, like the problem with alcohol, is that the practitioner/victim loses the ability to judge his/her own performance accurately.
