I watched some one fall asleep in front of me once. I was driving and my dad was in the passenger seat. After a long winding road of nearly 1.5 hours, it opened up and the speed increased to 60 MPH. After 5 minutes, the 2004 corolla in front of me swerved to the right and disappeared into the bush down a 12' embankment without even a puff of dust. It was lucky we were the only two vehicles on the road, because he did cross the oncoming lane before going off the road at full speed.
They were an elderly couple with the husband driving and his wife in the back. She was slumped over the backrest of the passenger seat and was probably sleeping in the back without her belt off. Looking at how the car landed, it simply could not have been any better to "catch" them. 12" to the right was a 20" hardwood tree, to the left would hit the dirt, instead it landed on a grove of saplings that snapped one by one and broke the impact for them. Even rolled the car over slightly so it was easier to open the driver's side door.
The both walked away but were still hurt pretty bad. We drove them to the ER since there was no cell phone service out there. A few weeks later we drove past the aftermath of a rollover at 70 MPH of a 2004 dodge 4 door pickup, the driver did not make it but his son did. Also a probable case of falling asleep at the wheel.