In business districts, signs visible from the public way shall be permitted only when subject to the following conditions: The gross area in square feet of all signs on a business shall not exceed 10 times the lot frontage in lineal feet. Flashing signs shall be limited to 100 square feet. All flashing signs and illuminated signs shall be shielded from park areas and residence districts, and no sign shall be within 20 feet of a residence district. No sign shall project more than 18 inches into the public way, except nonilluminated signs indicating name and/or address only, mounted flat on marquees, canopies and awnings. Roof signs are not permitted.