[spollo]

That would be cool for aftermarket HUDs. As far as making a useful HUD goes, some carmakers are already doing that. I have a 2019 Mazda CX-5 and it has a HUD that is projected on to the windshield. It tells me my current speed, the current speed limit of the road I am on, if there is a stop sign ahead, and if I run the GPS system it displays the next cross street, and arrow symbols for upcoming turns.

I personally find it super useful! I don't need to look down at my instrument panel as my current speed gets sort of projected on road ahead of me. It's small and a barebones design but it works.

[wizzwizz4]

The thing you describe works by passing an image through lenses, and reflecting it off the windscreen. That way, you can focus on it and the road at the same time. For transparent OLED-based systems, you'd have to stare at the windscreen to look at them – so you wouldn't see them on the road; they'd look much nearer.

[Auracle]

My 2011 Camaro also has a HUD, and I’d hate to go back to a car without it. I only use cruise control very infrequently, so being able to see what speed I’m going without taking my eyes off the road is great. I honestly think it should be a mandated feature.