The word "empty" appears five times in the story, always in conjunction with streets or public spaces. It occurs three times in the last sentence of the story.
Bradbury is known for being a lyrical, poetic fiction writer. Repetition is a poetic device that adds emphasis to a certain word or group of words. There is a tolling quality to the repetition of the word "empty" at the story's end, as if a bell is ringing or tolling again and again: a tolling bell traditionally represents death, as bells rang in the old days to announce a death.
The arrest of Leonard Mead for his crime of social deviance in taking a walk, and the emphasis on the streets and sidewalks being empty after he is removed, suggests the death of a public culture: he may well have been the last man alive who dared to take a walk....
(The entire section contains 2 answers and 436 words.)