Car sickness tends to happen when you're focusing on the interior of the car. That's what leads to the disconnect between your visual field and your mechanical sense of motion. I expect this is the main reason that drivers are rarely affected -- they're always looking out the window (more or less), so their visual sense of motion remains aligned with their other senses.
In my personal experience, car sickness is caused by high jerk (http://en.wikipedia.org/wiki/Jerk_%28physics%29), primarily during deceleration. (This is also commonly known as "bad driving".) I imagine this is something that self-driving cars won't have a problem with.
Seconded. I can take road trips like crazy, but hop in the Mountain View Caltrain shuttles with their crap drivers on a warm day, and I seriously feel like I could lose my lunch.
That doesn't necessarily account for people getting car sick on long, smooth, winding canyon roads driven at constant speed. The jerk induced by following the curves in the road should be lower than that caused by braking.
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.
