I do think we're in for a bit of a reality check on how human attention works.

I have a HUD in my car that shows me directions, speed etc and when I'm looking at that the rest of the view out the windscreen is hardly even there to my visual perception even though I'm looking right at it. This seems to be getting largely overlooked but I feel like over time statistics are going to emerge that HUD type displays are increasing accidents rather than preventing them.

You mean a HUD projected on the windshield itself? That's not my experience with it at all; I don't have to "look at" it, when my eyes are focused on the road in front of me, the HUD is sharp enough and positioned so that I always know my speed etc. without having to actively look for it.

Your car might have settings to adjust it somehow, have you tried those?

A HUD reduces the difference in focus distance between "looking at road" and "looking at speedometer." It matters more as you get older, because your eyes focus slower.

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.

Eye tracking and depth of field adjustments are the missing pieces. A HUD has to be able to stay in the same focal range as what is viewed beyond it so you don't lose concentration on the task at hand.

In fighter jets, they project onto the visor. Obviously not the most convenient method for an automobile. There have been attemtps to figure out depth of field but it's tough. https://www.sciencedirect.com/science/article/abs/pii/S00304...

"Change blindness" is how this is characterized in the research field. Basically any abrupt change in your visual field will prevent you from seeing another change on the road.

http://nivea.psycho.univ-paris5.fr/#CBdemos

Even less is needed to generate danger: I found myself checking my phone twice during a car trip because when listening to music through the USB-C to Jack dongle, it believes I am blasting music at full volume through my ears and decides to cut off the volume to 10% after 20 minutes.

Don't, and I mean DON'T decide things for the user.

I think there’s plenty of evidence that they’re better in jet fighters (where users are well-trained), possible also in automotive (where they have been sold for decades; see https://en.wikipedia.org/wiki/Automotive_head-up_display), but of course, it will depend on the design of the HUD and on what it displays.

Extreme example: showing random ads every ten minutes, even if the glasses c/should suspect you’re driving a car. I have my doubts as to whether Meta will make the right choices here.

Not sure HUDs in cars will cause same issues. There aren't a lot of things to read on car HUDs that require direct concentrated focus. Every HUD I've seen have displayed the most minimal information. Usually the speed, map directions and media currently playing. Even the map directions usually only indicate the next step in route. So a quick glance is all that is required. Regular dashboard gauges require far more concertation. Especially if you're new to that specific vehicle.

What's amusing is the original use case for HUD displays was to reduce attention problems. ;-)

There is a difference between status information like speed and directions and messages from other people.

Your attention reacts differently

Is this a physical or mental focus thing?

As in, are you just concentrating on the speedometer instead of the road, or do your eyeballs have to adjust because the optics aren't set correctly? I believe a HUD is supposed to focus at infinity, same as a road that's many times farther away than the size of your eyeballs.

"The World Beyond Your Head" from Matthew Crawford is exactly about this. Definitely recommend reading!

Meanwhile, I saw someone using their windshield mounted phone to watch videos while sitting in traffic yesterday (and driving erratically as a result which led me to notice). The self-driving cars can't come soon enough.

Is this an aftermarket device that doesn't project to infinity or a purpose designed feature from the manufacturer? The former is not a real HUD for the reasons you cite.