Sight triangles. On any corner lot, no wall, fence, structure, sign, berm, or plant growth that obstructs sight lines at elevations between three and six feet above the driving surface of the adjacent roadway shall be permitted in the area formed by measuring 20 feet along both curb lines where they intersect, and connecting the two points to form a triangle, as illustrated in Figure IV-1. This section shall not be construed to apply to existing street trees, provided that no branches are maintained closer than eight feet to the ground.