Except in the C District, a clear vision area shall be maintained at all intersections so that traffic is clearly visible from two and one-half feet to eight feet above the center grade of the street within an area described by an isosceles triangle in which the equal sides are congruent with the intersection curb lines and are 15 feet in length in all zoning districts, except that in residential zones the length shall be 30 feet. (see Drawing 1.)