No structure, wall, fence, shrubbery, or trees shall be erected, maintained, or planted on any lot which will obstruct the view of the driver of a vehicle approaching an intersection, excepting that shrubbery and low retaining walls not exceeding 2.5 feet in height above the curb level and shade trees where all branches are not less than 8 feet above the street level will be permitted. For residential corner lots, this unobstructed area will be a triangular section of land formed by the 2 street curb lines and a line connecting them at points 30 feet from the intersection of those curb lines.