According to the liner notes in The Essentials: Harry Chapin, Chapin was inspired to write the song when he happened upon an old lover, as the cabbie in the song does.
However, according to Chapin's biography Taxi: The Harry Chapin Story, by Peter M. Coan, this song was based on a relationship that Chapin had with a Bennett Junior College student named Clare MacIntyre, the inspiration for Sue.