i'll pipe in my .02. i am a cat person....so naturally i have had to travel
 with them..  usually when i have gone on trips with the cats (two cats,
domestic shorthair)  i have a harness on them that they wear when they are
in the car, and they stay in a large 2'x4' folding wire cage with plastic
tray bottom that fits in the back of the vanagon or the cargo area with  their
beds in the cage and litter pan and water/bowl. .  i dont let them  have
free roam of the car, this isnt safe, and dont want them to jump  out at pit
stops fuelups etc.   if they are to be let out of the  cage when the car is
in motion i make sure the passenger is holding them and  they have their
leash on attached to their harness. .