I drove from Minnesota to Florida with my cat. He was in the carrier in the front passenger seat and within 5 miles I thought I was going to have to throw him out the window!!
I decided to open the door on the carrier. The litter box was on the floor in the front . He just got out, peed in the litterbox, then went in the back seat and laid down on top of my computer printer. He was fine the rest of the trip.
Your vet can give you anxiety type meds for a trip too. I got them, but never had to use them. Good luck!