Re: So many things to consider.
"Pilots have lots and lots of training. They don't have other aircraft jumping out in front of them or traffic lights turning red suddenly as they fly along."
They might. Wingmates might break away to track a bogie they spotted and so on. Plus there's always the danger of incoming fire. One of the things drilled into pilots through history is to maintain situational awareness. Target fixation is a killer.
As for the HUD itself, it needs to be as concise as possible: able to convey the most information with the littlest amount of clutter. Pilot HUDs cram quite a bit along the edges of the display, keeping the center cleared for all-important targeting. In the case of the car, a driver's HUD should be as unobtrusive as possible UNTIL it needs to draw your attention to something immediate, and these signals should be discernible from peripheral vision. This means the indicators have to be distinct enough to be detected from the corner of the eye. Color can be used in this case. For example, a speedometer's number can be ignored through familiarization, but perhaps if it changed to yellow to indicate you're now crossing over the speed limit, it can be caught in the peripheral vision and be a useful caution message. Similarly, if the turnoff is coming up, perhaps part of the map can blink briefly as a hint to start looking around.