Honestly I think it is a combination of where your awareness is focused, and jerk. Long windy roads put varying lateral forces on passengers which probably become an issue if their focus is in the car.
Sometimes I get "carsick" while working with very small components (such as eyeglass-sized screws) that aren't behaving as they should, so I would have to attribute greater weight to the idea that carsickness is caused by a disagreement between the senses.
