When Michaela was TTing if we went over to my parents (45-60mins away) I would put her in thick toilet training underpants for the journey (we had several pairs of bright bots ones) so if she went only herself and her trousers got wet, it didn't end up a huge cleanup mission.
The one time we did a really long car ride (to a funeral up north about 4 hrs each way) I put her in nappies and then we had a couple of days of accidents while she got back into the routine of using the toilet when she needed to go.