We took a long car trip after potty training our DS (2.25). He pretty adamantly opposed the diaper when I told him it was a long car trip and that he should wear one just because he was new to using the toilet. He ended up having an accident in the carseat (even though it hadn't been that long) and then puking because he got so upset. What a mess. After that, he begged me for a diaper
I recommend a diaper while explaining to them it's just in case you can't pull off the road in time and he needs to go. Then immediately change him out of it and back into underwear as soon as the driving is over. This is what we did and had no regression.