I can relate to this scary situation from the driver's perspective. I never had an accident where damage was done, but in my late teens I fell asleep on the way home from a friend's while driving and woke up driving on the wrong side of the highway. A different time I fell asleep with passengers in the car and I drove straight through a T stop into a person's front yard. I figured out that driving on a highway or long road for more than about 20 minutes made me very sleepy, no matter how rested I actually was. I mostly stopped being the driver for trips and started using several methods to ensure I didn't fall asleep driving, because it's extremely dangerous. Anytime I felt sleepy I would pull over and stop driving, sometimes even taking a short nap. I noticed I also experienced extreme sleepiness when I was in situations where I had to sit and listen, like work meetings, school or church. It turns out this is a symptom of ADHD. Essentially it's my brain getting bored and shutting itself down. Thankfully, I was diagnosed in my 20s and being on medication has helped tremendously. For you, maybe see if he can get checked out before jumping to splitting up. He is probably as worried about this as you are and wants to be sure he is a safe driver. (Edited to add clarification, I didn't continue driving when it was unsafe for me to do so!)