The motel's billboard advertised HBO, which did not exist in 1971 when the movie was set.
There is a scene where we get a look at the neighborhood around Bob's house. We see an elderly lady sweeping her front porch, glancing suspiciously at Bob. That lady is Ruthie, and she owned the house on North Greenwich Avenue used for most of the interior shots. The porch she is sweeping is at the house across the street from hers.