In some aspects, building of highways was just another feature of "urban renewal", whatever that had come to mean. I was only a child in the 60's growing up in a total white bread neighborhood and school, and I'm still older than most people on this forum, so we really don't have a good understanding of the mindset back then. But there was some sentiment that, if we tear down the slums and build projects, "those people" will be better off. The fact that they were targeting "those" people might reflect on the racial mindset of the time.
Still as someone mentioned, without urban renewal initiatives, many low-income people had no options. If the government would pay you to move out of the path of a freeway or a project, you had an option that you didn't have before when no one wanted the place you were living. And in hindsight we know how the projects didn't work out - they ended up trading one kind of slum for another.