man in the street vs John Doe

man in the street and John Doe both are nouns.

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

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

Other synonyms of man in the street include Joe Bloggs, Joe Blow.

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