New York’s only fraction of a street, (the only street with a fractioned number in New York City) 6 1/2 Avenue was given its official signposts in July 2012 by the Department of Transportation.

The thoroughfare is part of the over 500 areas in the city that are known as Privately Owned Public Spaces. These POPs originated in the 1960s when the city gave property developers incentives for creating public spaces as part of their new buildings. These privately owned public spaces usually took the shape of plazas, atriums, walk through arcades.

https://www.atlasobscura.com/places/6-1-2-avenue-manhattan-s-secret-street