We always use(d) animal safety and animal sense of safety as the ultimate guide. For example:
1) All areas of vehicle must have fully functional air conditioning & other climate control that is tested/confirmed working before every trip and checked throughout the trip;
2) Animals are never to be left alone in a vehicle;
3) If a restroom or other brief stop is needed, at least one person would remain with the animals and ensure proper ventilation and other needs are being met.
4) One animal per kennel or if for some reason no kennel is used, then a crash tested/passed drive harness (for dogs anyway) like the SleepyPod ClickIt is to be used.
5) Water and comfort/well-being checked regularly throughout the trip.
6) For long trips especially, avoid extreme weather/temps travel.
7) For long trips where elimination is needed, we used only spook harnesses and leash to protect against loss due to startle etc. (Actually we prefer these exclusively.)
8) Ideally, we like to avoid change of transporter which helps avoid additional stress/fear on the animals due to multiple sudden changes of environment and other stimuli such as strangers, smells, etc.
9) For similar reasons we would want to avoid transporting animals with an animal likely to scare the others (e.g., a non-dog friendly dog with other dogs, or a high prey drive dog known to be not cat friendly, or a known hunting type, with cats, rabbits, or other small animals).
10) And last but not least we ideally want at least one person who is pet CPR certified and also a qualified, anti-aversive behavior professional to help ensure that the animals' needs are recognized and met throughout the trip, and we want all people with access/handling animals to be committed to and qualified to use only R+ / anti-aversive methods.
Of course there are exceptions for emergencies; e.g., if an animal darts out to the street, of course it's reasonable to grab the leash in panic rather than risk animal being hit by a car, even though that likely would result in aversive sensations. Et cetera.
Any other actions/decisions would all be expected to honor these ten ground rules.
Best,
Rain