The word "back street" typically refers to a narrow, low-traffic road behind the main thoroughfares. Antonyms for this phrase can vary depending on the context in which it is used. In terms of location, the opposite of a back street would be a front or main street. In terms of desirability, antonyms might include words like "prestigious," "expansive," or "modern." Additionally, if we consider "back street" to carry a negative connotation, antonyms could include words like "prestigious," "residential," "upscale," or "luxurious." Ultimately, the antonyms for "back street" will depend on the context and intended meaning of the speaker or writer.