Which car traits and/or models would make someone the least sick?

I think a lot of what you're noticing is the whether the vehicle has independent rear suspension or not. Sports cars generally do not have independent rear suspension. Many newer SUVs have it now. The Ford Expedition, which does have it, is one of the smoothest riding full-size SUVs on the market. If you want a full-size SUV, the Expedition should definitely be at the top of your list as it is a class leader in almost every other aspect as well. I am very familiar with motion sickness as I had it as a child, and now have daughter (8) who has it as well. I currently own a 2011 Dodge Durango Express, and it is the first vehicle I've owned that has independent rear suspension, and not incidentally, is the first vehicle I've owned that does not make my daughter sick. You should definitely buy a vehicle with independent rear suspension. There are over-the-counter drugs that can be taken prior to a road trip. My daughter has used them several times, but with limited success. Also try to take main roads when possible and take turns as slowly as possible. Good luck! (the good news...with many people motion sickness gradually diminishes with age)