Visibility obstructions. In a residential transect, no fence, shrubs or other obstruction to visibility, exclusive of trimmed trees or existing buildings, shall be erected, planted or maintained upon corner lots within 25 feet of any street intersection. Any existing obstruction, exclusive of trimmed trees or existing buildings of this character, which, within the foregoing limits, curtails the view of drivers of vehicles approaching the intersection, shall be removed by and at the expense of the owners of such corner lots within six months of the date of passage of this chapter.