Where an entry to a subway station exists in the sidewalk area of a street on which a public plaza fronts and such entry is not replaced within the public plaza itself, the public plaza shall be at the same elevation as the adjacent sidewalk for a distance of at least 15 feet in all directions from the entry superstructure. Such public plaza area around a subway entry shall be free of all obstructions and may count towards the required clear area requirements as specified in Section 37-721 (Sidewalk frontage).