I have an odd psychological response:
If I'm in a vehicle for any length of time - plane, train, or car - and I'm not driving... I go to sleep. I seem to have no control over this reflex.
I don't have any issue if I'm behind the wheel, but if I'm not in charge then it's as if I'm hypnotised. I'd hate for that to happen on the driver's side of the car, with a robot telling me 'wake up and take control, meatsack... oh. Oh well.'