Well, let's look at the four questions:
(1) What are the risks of leaving a dog unattended in the car?
The answer to this question (and this question itself) really applies to any vehicle; in this sense, cars is not appropriate, but neither is travel; safety is fine on its own.
(2) Long Car Trips with my Dog and (3) How can I keep my cats safe in my car?
travel is much more appropriate for these, with cars as a synonym.
(4) What's a suitable way to drive with a big dog in the car?
Both travel (cars as synonym) and safety for this, since you can have a big dog in a tight space in other vehicles.