Design (including shape and style) as well as location of the street address numbers shall be such that they are legible and easily readable. Irrespective of whether the street address number was established before or after June 1, 1973, the Chief shall have the authority to prescribe a street address number larger in size than the dimensions stated in Section 57.505.1.1 when it is determined that due to lighting conditions, building location, shadows, background, or other reasons, street address numbers of minimum dimensions will not be easily readable from the street or approach to the entrance. A street address number of unusual design or placement may not be employed without prior written approval of the Chief.