I was once in a similar situation - though I saved the day.:p
We were in Scotland when I was ten or eleven, and I noticed the car heading towards the loch. "Dad!" I shouted. But no answer - he'd fallen asleep!
So I got the steering wheel, and put my foot on the accelerator so we wouldn't end up having a car behind crashing or something, and drove at the correct speed (think it was 30, but it was years ago so not 100% sure) for that road, looking for a safe place to pull over.
Suddenly my Dad woke up and said "What the bloody hell are you doing?"
"Saving your live, that's what!"
Anyway, realising what had happened, he started driving again and pulled over for a rest.