Is "highway" photography a subset of "street" photography? A highway is a kind of street, isn't it? 
Yeah, except street photography hasn't anything to do with streets. I don't know why it got that monicker, but I suspect at least part of it was that with the film speeds available at its beginning you probably needed to be on the street to do much of anything.
On the other hand, these shots definitely show people interacting with their milieu. It wouldn't seem too far a stretch to see highway photography as a subset of street.