In all my life I have known only one dog I regard as safe to be alone and loose in his owner's vehicle. He's a very, very weird dog--a laid-back, gentle, mellow
Jack Russell terrier. Wouldn't have believed it before I met this dog--although his chilled-out temperament is what got him neutered since it does not match people's expectations of the breed, nor is he a good example of the desired drive, energy, and temperament of what started out as a fierce hunting breed. This one has offed a couple of ground squirrels over the years, but by and large is the only dog no one reports for being off-lead around here because he has fantastic recall skills and loves attention.
Most of the time, from what I've seen personally, dogs + vehicles = there go your insurance rates (and credit rating, these days).
SUPER GOOD advice, btw! It's
never a good plan to approach or try to handle a vehicle with an unfamiliar dog inside, especially a medium-or-larger breed if your own safety is concerned--although the little ones, in addition to potentially biting, may also have a much easier time escaping. I also do not approach trucks with dogs in the bed whether or not they're tied the way they should be for their own and others' safety. No dog owner wants to think about their companion lost in a world full of predators, maniacs, and highways.
Personally, when I meet a new dog and know its owner is nearby, I always ask if the dog is friendly
before extending a hand (relaxed, fingers dangling down, and with the palm facing away from the dog to avoid looking 'grabby') for a sniff. A lot of dogs are not at all okay with me, which I don't mind that much since I'm not a dog person and no one in my immediate family wants one, but understanding that is crucial to avoiding bites and other human-aggressive behavior that could get a perfectly decent dog in a world of trouble its family does not need. Basic guideline: no owner in sight, no touching of the dog!