There was a large discussion about friendly animals riding in vehicles, and it seemed to be the general consensus by the devs that it just wasn’t possible within constraints that would be reasonable within the game.
Players wanted some sort of storage tile for animals, where they could activate it and place the animal in it, but the devs said that it would basically be a pokeball, and that this isn’t that type of game.
Personally, I think it would be awesome to flip a switch on a semi and release an army of friendly dogs from the back, but this probably will never be implemented.
As for NPCs, there has been a lot of discussion lately about adding boosts to the AI, some of which has been implemented already.
The problem specifically with getting NPCs to use seatbelts is two sided: getting the AI to choose to ride in a seat rather than a random tile on the vehicle, and getting the seatbelt then to function with the NPC and not only with the character (which I think is currently the case).
I would say that this is a good note to add to the list of things that we need for NPCs, but I don’t think this is at the top of the list.
For now, we will have to deal with NPCs just riding on random parts of vehicles and getting smacked around like stupid idiots.
In general, I think it’s pretty safe to say that NPCs aren’t particularly concerned with their personal safety in big picture situations, like for example car safety.