That's why I find it sad that virtually everybody continues to let their dogs go unrestrained in their vehicles, often with windows open so that the dogs can experience the great outdoors. Even people who would never think of not wearing a seat belt or letting a child ride without proper restraints do this. I wonder if this is simple ignorance or casual, reckless disregard. I haven't asked any dog owners about this recently but I suspect their reasons would have something to do with a dog's biological imperative to move around and get fresh air. (Perhaps people think that in the wild, wolves ride motorcycles without helmets.) Virtually any explanation of this sort would echo what people used to say about restraining children in what now seems like the distant past. I for one remember getting to ride in the tailgate area of station wagons.
A valid but outdated excuse would be the lack of suitable solutions. A simple search query on the subject yields plenty of results. Canine Auto is one of the first and best sites on the topic.
My belief is that it's a matter of education and acculturation. I'd like to think that in another decade or so, letting a dog go unrestrained in a vehicle will be as socially acceptable as it has become for a child.