My cat has the same EXACT problem. I moved from NYC to Maryland and I took my cat with me back and forth a few times before finally settling in our new home. I let me cat roam free in the car and I brought a small litter box with us and put it on the floor in the back of the car. Everytime we get on the highway my cat had to go! He goes in the litter box and once he is done I pull over and scoop out the mess. After that he is usually fine. I used this method over and over during my move and it really worked for me, I hope it works for you!