I've experienced this many times as I used to drive a lot at night. To me it was like half of my mind falling asleep while the other half followed the white line for hours. I used to drive close to 70k miles at night every year, all along the same routes, with no stimulation, just the white line. I know I wasn't asleep or tired, but it was like driving in a dream sometimes.
It is probably much, much more common than we know, and I think this state would completely fool the attention-tracking systems in self-driving cars.
What is worse, many people drive the same routes commercially every night, like sleeper coach drivers. I think if they even did so much as spoke about this, it could significantly harm their careers. So it's unlikely that this demographic that could be most affected will talk about it openly. But there is almost nothing you can do to prevent it. After a certain number of thousands of miles along the same route, seeing the same white line, for hours, in the dark, you zone out.
Another thing that's making it worse is that I could only ever catch myself in this state when I already did something irresponsible. Like I'd go past a speed limit sign without reacting, and in a couple of seconds my brain would wake up to say "hey, something is happening, let's pay attention again". Otherwise, it's almost impossible to catch your mind drifting gently away.
I'm happy to finally put a label to the experience.
Certainly a heavily inebriated driver might have made this mistake if the car were difficult to see at a distance, but otherwise... no, I don't see this happening at all.