Have you thought about gas?
My little one, now almost 3, was a great rider. She didn't actually cry until she was about 7 months old. She would fuss a little, but never cry. We used to take 4 hour rides for medical reasons with another of my children (at 3 weeks old), and in the beginning she was fine. After about the 3rd trip, she started fussing more and more, until it was a full scream. We stopped the car and when I took her out of her seat, it was easy to calm her, and as soon as I put her up to my shoulder, she let out quite a burp.
After that, I bought Mylicon Drops for infants and gave her some before every ride we took.
She would usually burp fine after eating, and not spit up a bit, but she was one who needed to burp more than once after eating. It took some time, but when we were home, it didnt seem to bother her much, only when we were driving, maybe it was because she was confined in a seat? I'm not sure why it was worse in a car, but it was.
Try gas drops before you go anywhere in a car, see if that works for your little one.
Unless she's had a traumatizing experience in a vehicle, there shouldn't be a problem with being scared. and any doctor will probably tell you, kids recover from most anything and at 15 weeks, she shouldnt be able to remember something bad happening.
If the gas drops do not help, try putting up one of those window screens to keep the sun off her face, it will also block the view. I had a window stick with pooh and tigger on it, my son loved it.
good luck