The vestibular system has a fast and slow pathway. The fast pathway has nothing to do with motion sickness. It is the slow pathway that has integration property produces motion sickness. High frequency stimulus hardly can produce motion sickness and the low frequency that evokes the slow pathway can.
I met a woman who told me that she had a old VOLVO that was like a truck when driving, implying the high frequency vibration, and she had no problem at all. She now sometimes became sick after changing to a very soft Toyota Avalon.