Reminds of a trip I took to NY state. My friend's dad drove a group of us to the Ausable near Lake Placid for a day of fishing. Everybody fell asleep on the drive home--including the driver. It was a winding road that cut through small rolling hills. I woke up slightly, and groggily noticed that the car was starting to cross the center line.
I started to say something to the driver, but we went off the road where it curved and drove up a hill at high speed. Everyone woke up flying through the air with tree branches scraping the windows. The car landed on the other side of the hill and rammed into a tree trunk. Miraculously, nobody was hurt. We were lucky the car was airborne, because there was a narrow spot that would have probably flipped the car upside down onto the median.
It's a good idea for the passengers to stay awake. Don't hesitate to say something to the driver if you notice the car beginning to veer off course.