Such automobile runways from the roadways of improved public streets in the City and County of San Francisco to and upon portions of the sidewalk areas adjacent to the curb lines of the roadways of such streets, as have heretofore been constructed under sanction and to the satisfaction of the Department of Public Works and are now being maintained as so constructed, and such other automobile runways as have already been constructed and which, as to method of construction, are within the restrictions denoted on the approved and adopted plan referred to in Section 715 of this Article, are hereby permitted to be so maintained, subject however to the obligation imposed by the provisions of Section 717 of this Article.