J., my daughter also had this problem, but it started when I had turned her car seat face forward at 12 months. I guess it was just her being in a different position and being up higher and she could see everything now. If we drove for any longer than 10 minutes she would start to cry, scream and eventually vomit every time. As soon as we stopped the car and took her out, everything was back to normal, like it never happened. We made sure there was nothing else wrong with her, but that seemed to be it and her doctor assured us that she would get used to it. It took about 6 months or so before she was ok with riding in the car for more than 10 minutes. The only thing we could to do comfort her in the meantime was to try to distract her, with either her favorite toy or song, and it helped a little when I would place my hand on her chest and talk to her softly. We tried to keep our car rides under 10 minutes but sometimes that was just not possible so we always made sure we brought a towel and covered the front of her so that there would not be vomit all over her and the car seat, and we would stop the car when it was safe for us to do so and take her out for a few minutes so she could recoupe. I hope this helps you, just hang in there it will pass, sounds like you are doing a great job already. Good luck.