It is not enough to be holding the wheel and looking at the road-we need to devote our full attention to the task of driving.
If we are not concentrating entirely on driving because we are trying to think about a secondary task – a cell phone conversation, texting, reading e-mails, etc -than we are cognitively distracted. There have been a number of scientific studies which have looked at cognitive distractions and also, at whether hands-free activities that require concentration are safe.
Those studies have consistently shown that just giving up the manual distraction, holding the cell phone for example, does nothing to reduce the cognitive distraction. If we are concentrating on something other than driving, there are less brain resources available for safe driving. Multitasking, or performing these secondary tasks while driving, impairs our ability to focus on driving and is unsafe.
Trying to multi-task while driving can easily overload our brains and result in up to 37% of brain resources needed for safe driving being diverted to performing the secondary task.
For more information on cognitive distractions see these studies: