And again, from the horse's (?) mouth:
[1992:] [Reynolds] made up Little Boxes [in 1961] when she was driving to Palo Alto to sing for the P.T.A. Driving past Daly City south of San Francisco, she looked up at the hill-side, and said to her husband, "Bud, take the wheel. I feel a song coming on." When she got to Palo Alto, she had the song ready to sing. (Seeger, Flowers 107)