Verified by 
Most of us think we're pretty good at paying attention. Lots of us think that's true even while multiple things at the same time. Like when we drive.
Each us has a story where we've been driving and found ourselves stopped at a light with an advanced green signal flashing. Those drivers who are paying attention realize that the advance green signal means they get to turn left. However, in these stories the driver ahead fails to notice the signal and is instead talking on a cell phone and not really paying attention to what is happening. S/he doesn't even respond to the semi-polite honking of the car horns.
Part of the answer to this has to do with the cognitive demands of speech and “inattention blindness.” Speaking and talking are motor acts, but they also involve listening and attention. Listening has lower cognitive demand than does speaking and getting ready to speak, but while listening to someone, preparing to speak to them, and then actually speaking we are constantly trying to figure out where the person is.
In the background we are trying to form a kind of mental image of where the person actually is and maintaining this takes up a lot of the processing power in our brains. It is almost like the ancient part of our brain is constantly searching for who we are talking to but we cannot “see” them.
Because of this, just listening to background music or talking to someone in the same vehicle does not cause the same distraction as trying to have a conversation with someone on a cell phone. This applies to hands-free phones too. Removing the motor act of holding the phone makes it marginally better, but it still doesn’t address the issue of attention and our abilities to concentrate on different kinds of tasks.
Recently Patrik Sorqvist and his colleagues in Sweden used functional magnetic resonance imaging (fMRI) to study brain activity and "distractibility". Their paper, published in Frontiers in Human Neuroscience, looked at whether cognitive load in a visual task suppresses task-irrelevant auditory processing in other brain areas. They wanted to know if making a visual task more difficult (and requiring more attention) would actually affect the ability to pay attention (and thus be distracted) by an auditory task.
The really interesting result of their study was that higher cognitive load while concentrating on a task decreased the ability to process information that wasn't related to that task. Linking back to our driver example above, this explains why the concentration involved in talking on a cell phone kept our annoying driver from having attention resources to notice the green light or to hear the car horn.
The important point is that our brains help us allocate attention so that we can concentrate on aspects of what we are doing that we consider relevant while ignoring stuff that is irrelevant. All of which underlines why it's so dangerous to combine cognitive task with a complicated motor act such as texting and driving.
Many jurisdictions now ban hand held cell phone use (and texting of course) while driving. Distraction during multitasking with technology even as “simple” as an automobile and a phone don’t mix. No matter what we may think of our prowess at multitasking, concentrating, and allocating our attention.
