Unlike traveling by plane, with takeoffs and landings, kinetosis or motion sickness occurs even on a flat highway. But what we aptly call the expression "seasick" has little to do with real rolling. Today, scientists believe that the reason is in the self-deception of our brain.
When riding a horse or even a bicycle, the human body makes a lot of movements and our nervous system clearly perceives this as a "movement". But if we are sitting in a car, our body remains motionless in relation to the passenger compartment. And at the same time, fluid is poured in the inner ear, which indicates the fact of movement in space. The part of the brain called the thalamus receives conflicting information and concludes that the body appears to have been poisoned.
This is a natural defense mechanism that did not have time to change, because from the point of view of evolution, people started driving cars quite recently. And when walking on foot, the brain accepts various abnormalities as a symptom of illness or poisoning, which is familiar to it. A reflex way to get rid of toxins is to clear the stomach, which causes nausea in people who are "nauseous". The nervous system confuses the causes and effects of the phenomenon, creating discomfort.
The best thing in such a situation is for the driver - his brain receives and processes many times more information than passengers, and therefore there are no paradoxes in assessing the situation. Everyone else can be advised to have a hearty meal before the trip in order to keep the stomach busy, listen to light music, look at moving objects outside the window and maintain a conversation. It's better than reading or playing on a tablet - the need to pay attention to the other person distracts the brain from the stressful reaction to conflicting signals.