Oddly, it was a bigger issue when I was younger. It hasn't happened to me in quite a few years, but the awareness of it has lingered, thus my desire to take extra precautions.

There's almost nothing a person in another seat can do if you are asleep, even if they realize that in time (and that's a huge assumption). Speaking as someone who has unhealthy interest at failures, disasters, and their root causes, this type of attitude is super common, and always leads to disasters. Please seek medical attention before you killed somebody and yourself. You can't rely on self-control.

When I used to pull 18 hour days, I would also notice myself getting tired behind the wheel, but never fully falling asleep. My low-tech solution to this was to set an alarm for 10 minutes after I started driving and setting the alarm volume to max. Then, the alarm would go off in the middle of my drive, usually surprising me and jolting me awake since I'm obviously not paying attention to the time. Then, snoozing the alarm would make it go away for another 10 minutes, and so on until I made it home. This is probably not the perfect solution but it worked for me. Obviously the better long term solution is to a) not do 18 hour days and b) address what is making you so sleepy and stop it from happening.

Avoiding highways was what saved me I think; a mistake at 20mph is far less expensive than a mistake at 80mph. But certainly you're right; the obvious answer is to avoid driving or as another commenter said, carpool!

This is fairly similar to what would trigger it with me. My solution was to ensure I have passengers or let someone else drive.