In a C Residence District, a fence, as defined in this chapter, not exceeding six feet in height, may be erected on the rear lot line and such portion of the side lot lines as shall be lot lines enclosing the required rear yard, and, with respect to all other lot lines, any such fence shall not exceed four feet in height, provided that, if such fence shall be erected along any road or highway, the permitted height thereof shall be measured from the existing elevation of the center line of such highway opposite such fence and provided, further, that such height so measured shall not exceed two and one-half (2 1/2) feet at any point within a radius of 15 feet of the corner formed by any intersecting roads or highways unless otherwise permitted by the Board of Appeals. The provisions hereof shall also apply to hedges and other densely growing shrubbery.