This has to be one of the truest statements there could ever be. Any time I am listening to any form of metal in the car my attention span lessening and focusing only on the music. Maybe its like the subconscious takes over a bit giving way to music. For me, a lot of the time, I see it as the subconscious and consciousness of the mind and brain. Your mind takes over and just focuses on the brain while driving becomes like the involuntary things your body does like breathing.
When listening to metal, while driving, the part of your brain that controls your involuntary bodily functions takes over and starts to control the weight of your foot and the things your see. Basically at this point the only control you have over your body is what you hear. Thats why when you hear the sireness as the PoPo are coming up after you, you tend to slow down. Your mind is still in control of that portion of you.