I saw a last-man-on-Earth movie on TV some time in the '70s. I don't know the title. It takes place in the US. It opens with a black man trapped underground (mine? construction site?). He manages to free himself only to find that all other people are gone: cars sit empty in motionless traffic jams, etc. After a while he meets a white woman who thought she was alone in the world. Later on, another (white) man shows up in a boat having traveled from somewhere in the southern hemisphere where he is, apparently, the last remaining human.
The general plot outline is reminiscent of "The Quiet Earth" (1985), which I've not seen, which was based on a 1981 novel. I saw this movie before 1981. There are no zombies, so it can't be "The Omega Man" (Charlton Heston). A friend once said it was called "This Quiet Earth" but a search for that title only turns up "THE Quiet Earth" and it's not that film.