From The Wikipedia entry on mobile phones and driving: "The NHTSA considers distracted driving to include some of the following as distractions: other occupants in the car, eating, drinking, smoking, adjusting radio, adjusting environmental control, reaching for object in car, and cell phone use."

When I was a kid and my dad was driving and had to make a left turn or perform some other maneuver that required his full attention, he'd ask everyone in the car to be quiet. This was a man that raced automobiles as a hobby and worked in professional racing at the highest levels his entire life.