Highway hypnosis usually creeps up on you when you have been driving on an expressway or long straight road for several hours. It can lead to you becoming a "spectator" instead of a driver and things just seem to 'float' by. Or it can send you to sleep.
To guard against highway hypnosis:
Don't eat a heavy meal before you set out.
Keep the car temperature cool with plenty of fresh air.
Stop every hour or two for refreshment and/or exercise.
Talk to your passengers.
Keep your eyes moving and check your rear view mirrors often (at night set your mirror for night driving).
Look at all the road signs and traffic around you.
Don't try to drive too far in one day.
Don't drive during your normal sleeping hours.
If you feel the onset of any symptoms, get off the road as soon as safely possible!