Asleep at the wheel...
When you yawn so much that you can't see straight...
When having the music turned up and windows rolled down isn't enough to keep your eyes open...
When you play a fun game which involves only one rule: See how long you can blink and keep your eyes shut without falling alseep while staying on the road...
When you realise you can't remember driving for the last 20 seconds, or 2 minutes, but there you are, swerving slightly and feeling more tired than before...
When you wake up to see a truck coming straight at you and you have no time to brake, and have no time left to get back on your side of the road...
When it's too late, and you should have had a nap before you left for your trip.