Humans cannot multitask well (even if it feels otherwise). The brain can’t pay attention to multiple things at once, just switch back and forth really quickly. When you are used to driving a usual route the brain switches to an automated mode and we can more easily listen to the radio because we’re not as focused on driving. When we want to park or look for an address, we require more direct focus and lowering the music volume stops it from competing for your attention. This allows you to focus better on parking, etc.
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