On every corner lot in an R, B-A, B-A1, B-B and M-1 District, within the triangle formed by the street lines on such lot and a line drawn between points on such street lines at the distance from their intersection specified below, there shall be no fence or wall higher than three feet six inches above the street grade, nor any other obstruction to vision other than a post, tree or column not exceeding in cross section one square foot or one foot in diameter between a height of three feet six inches above the street grade and a height of 10 feet above the top of the curb of either street:
A. For a lot having an interior angle of 90º or more at the street corner thereof: 20 feet.
B. For a lot having an interior angle of less than 90º at the street corner thereof: 20 feet plus one foot for every 10º or major fraction thereof by which such interior angle is less than 90º.