I always feel my head flop forwards when I'm in a car and wake myself up. Because of that, I have a hard time sleeping in cars. I used to drive when I was way too tired, and luckily my head relaxing the instant I fell asleep was enough to make me realize I fell asleep and jerk myself awake. I always managed to stay on the road, and even in my lane Red lights were always fun. Once I woke up to someone behind me beeping and the light was green.