Isn't it a general rule of driving (or operating any sufficiently dangerous machinery) to keep the eyes on the road, constantly reminding oneself to do so, so the attention is kept where it is needed? I mean, in theory. In practice, I see people deep in their damn phones all the time - and it's scary - but I think that's more of an attitude (social) issue than a display (technology) problem.

And, yes, surely, one needs to periodically switch attention to mirrors and instruments, and I must imagine that shorter gaze movement distance shouldn't hurt. It's the same as checking the speedometer - you don't see the road, only have a rough idea from the peripheral vision.

Although I can imagine that a HUD can be actively distracting, constantly intercepting attention, e.g., flickering.

I have one in my car and it's not distracting. It basically displays the current speed, the speed limit, the current cruise control state, the current gear (PRND), and the current navigation instruction (eg. turn left in 1.5 miles).

It doesn't display notifications or other distractions, nor is it possible to configure it to do so.

It's not flickering when viewed in person, but when filmed with a phone camera they do flicker due to how the display works.

It's a pretty good system, and allows one to keep their eyes on the road without having to look at other screens, and keeping ones eyes focused on far objects.

I think the point is that it's much easier to forget you're focusing on the speedometer instead of focusing on the road when the speedometer is physically displayed by a HUD right on the road. Especially if the speedometer keeps changing, since your eyes are naturally attracted to movement in your current field of view.

With a normal car dashboard, you're much more aware you're not seeing the road while checking your speed, and you don't actually see the speedometer moving while you're looking at the road, so it can't accidentally catch your attention.

Of course, none of this will matter in the vast majority of cases. But driving safety is all about the tail end, when you're slightly tired or when someone in front of you does something unexpected and maybe illegal, or someone jumps on the road - these are the times where accidents are avoided, and a HUD might well hurt rather than help for these cases.

My 2025 Corolla has a HUD and it doesn't flicker. it's also fairly minimal and very easy to keep in peripheral vision such that while looking at traffic and such, I can still grasp what it's saying without messing up my attention.