The idea of 100% attention when thinking about the brain is just a simplistic notion to make discussions easier but inaccurate. Even in this simplistic approximation of the brain, the idea of 100% attention is pretty thin. If I'm driving somewhere I'm probably listening to the radio (deduct 5% attention points!) or thinking about wherever it is I am heading (deduct 5% attention points!) or if someone is actually in the car with me, I am probably interacting with them in some way (deduct 10% attention points!). And while I'm driving and keeping a constant vigil through the windshield for danger what if I see a glorious new Apple store that I didn't know existed?? That is going to hijack my brain for a bit (deduct 100% attention points!).
The radio , passengers and thoughts are much more passive and easy to tune out but even then I'm sure there have been plenty of accidents that are caused by daydreaming. A passenger can see out of the window and has awareness of the car's surroundings so they will usually know to shut up or at least not expect a quick answer to their question if you are doing a tricky manoeuvre. Electronic devices or people on the other end of a hands-free conversation both require interaction and do not have awareness of the situation that the driver is in.
