In any district where a front yard of 25 feet or more is required by this chapter, no sign, fence, wall, hedge, shrub, planting, tree foliage or vehicle which obstructs vision at elevations between three and seven feet above the street level shall be placed or maintained within the triangular area formed by two intersecting right-of-way lines, as defined herein, and a line connecting points on such right-of-way lines 30 feet distant from their point of intersection. This regulation shall not apply to any necessary retaining wall or to buildings existing on the effective date of this chapter.