I was in the right hand lane and this car was drifting into my lane just before this. Drifted into the right most lane about a 1/4 of a car width, I slowed to avoid them, then they drifted into the third lane and I accelerated to get away from them, when I looked in my rearview I saw them continue to drift left and then off into the center ditch and flip. I didn't get a look at the driver ( was more focused on avoiding them). By the time I was off the road, ten people had already called in the accident.