Not necessarily disagreeing, but I'm not sure automotive HUD technically qualifies as one.

Airplane HUDs occupy center of the vision, literally showing where you're going. Car HUDs don't, and instead stay out of sight, as it's illegal to do in cars what they do in planes. That makes car HUDs just heads down display that happens to be transparent.

Well for cars, you do need to be on the active lookout for pedestrians, other cars, etc, but for planes generally you are mostly looking at instruments. You only really look outside if you expect something is coming, and the HUD isn't really a distraction there.