John Doe vs man in the street

John Doe and man in the street both are nouns.

Word	Noun	Adjective	Verb	Adverb
John Doe	Yes	No	No	No
man in the street	Yes	No	No	No

As nouns, John Doe and man in the street are synonyms defined as:

Other synonyms of John Doe include Joe Bloggs, Joe Blow.

John Doe (noun)	man in the street (noun)
an unknown or fictitious man who is a party to legal proceedings	a hypothetical average man
a hypothetical average man