I will start, we took our 2 cats each in their own carrier on our holiday trip with included 3 long drives....
Drive 1: 2.5 hours, IN to Chicago: Escher was good as usual, a little meowing for the first 20 min. then he went to sleep. Phoebe, as we predicted, was not as good, whining/yowling for the majority of the drive, she did finally fall asleep for a little while, but then I almost got side-swiped by a semi so I had to swerve and that woke her up, and scared her so much she had diarrhea in her carrier. YUCK!
While at FI's family's house, with 3 other cats, a dog, and 10 new people, our cats ran and hid in the basement, only emerging to eat, poop, and of course hiss and growl at anything and everything that moved (including eachother). But Phoebe did find her way out of the basement at night to crawl into bed with either me or FI and purr.
Drive 2: 4.5 hours, Chicago to St. Louis: Escher was good again. Phoebe continued her crying and after 2 hours, FI took her out of her carrier to hold her, which resulted in fewer, but louder cries. Plus, when she was out, Escher started to meow b/c he couldn't see her anymore. When FI tried to put her back in her carrier, she jumped and hid under the seat, we had to pull over to get her out from under the seat and back into her carrier, then they both whined for most of the drive.
In St. Louis, with no young children, but one allergic relative and two dogs that have been known to hunt and kill rabbits, birds, and squirrels, the cats were given their own room in the basement, with the dogs oblivious to their existence. They were much happier and enjoyed the quiet.
Drive 3: 5 hours, St. Louis to IN
Drive home, Escher slept the whole way. Phoebe was even louder, trying to convince FI to hold her again, but we weren't going to go through that again, so we dealt with it, until she fell asleep for about 45 minutes starting 3 hours into the trip.
What didn't work: holding Phoebe, covering the carriers with a blanket to help them sleep, putting on soothing music, being quiet. We're stumped, but it might just be that Phoebe isn't and never will be a good traveller.