Absolutely definitely get them some harnesses (the kind that ties not only around the neck, but behind the back legs) and get them used to wearing them for very short, but lengthening times inside the house, minus the leash part, long before you leave. That way when you travel, they can be wearing them the whole time, and when you need to get them outside for any reason, first clip on the leashes inside the car, and you won't lose any of them to traffic, scary dogs, high trees, or whatever. Do ask your vet for suggestions also, there are various things he can give you with light sedative effects, so they don't all start yowling in the car from the start, hopefully! And do stop as often as you can. That all being said, I flew 4 cats over 2,000 mi. a few yrs ago with no ill effects, though a couple (can't actually how many or if all) had the sedatives before leaving.