I remember we travelled heaps with DS1. First time (6wks) I was worried about him spending too long in the capsule etc, but quickly got over that. Our policy was - if he's asleep, keep driving
I agree with newme, we'd plan to start a trip before his best nap &/or leaving really early in the morning also helped. Especailly on a hot day, I'd want to get to the destination before lunch even if it meant starting at 4am. Crazy you ask? Maybe but DS slept better in the am