I never have been to SF. But if I had to guess, It's somewhere in Nob Hill area from the looks of the scene.
I just rewatched the scene on youtube. The bus appears to be route 30. Assuming the route hasn't changed much or reassigned numbers in 27 years, it runs along the bay, between Presido (I think) and downtown. It would be close to Nob Hill from Van Ness to down Columbus Ave then Stockton toward Market ST.