I used to get terribly car sick as a boy but fortunately it went away by itself in my mid-teens. But I know adults who never get over it and this makes life quite difficult for them. But one thing that has been noted is that of you are the driver of the vehicle, you are less likely to feel sick and people have tried to understand why.
The thought that comes to mind is that somehow being in control of the motion mitigates the effects of the motion and a new study directly examines that hypothesis. They had two people whose head motions were constrained so that they had to move together but only one person could determine the motion while the other had to follow passively. Here’s the abstract of the paper.
The central hypothesis of the work is that the dimension of control-no control plays an important role in motion sickness. Although it is generally agreed that having control over a moving vehicle greatly reduces the likelihood of motion sickness, few studies have addressed this issue directly, and the theoretical explanation for this phenomenon is not completely clear. In this study, we equated groups differing in controllability for head movement, vision, activity, and predictability, which have often been suggested in the literature as explanations for the driver’s immunity to motion sickness. Twenty-two pairs of yoked subjects were exposed to nauseogenic rotation. One subject of each pair had control over the rotation and head movements, while the other was exposed passively to the same motion stimulus. Subjects who had control reported significantly fewer motion sickness symptoms and less of a decrement in their well-being, as compared to the yoked subject without control. The results are discussed in relation to Reason’s sensory rearrangement theory and the concept of feed-forward mechanisms in motion perception.
Unfortunately the paper is behind a paywall but you can read a little bit more about the study here.
It is important to realize that the authors are not claiming that this is the sole factor, just an important one. There are other aspects of control that also come into play.
One is the mismatch between what your eyes tell you (that you are not moving if you are looking only at objects inside the vehicle) and what your body tells you (that you are moving) can also play a role. The latter effect may explain why reading in a car is not recommended but looking steadfastly at the moving scenery may help. Of course, when one is on a plane or a big sea-going vessel, seeing outside may not be easy.