Side yards: 20 feet aggregate total with a minimum of eight feet, provided that when a written agreement is provided by adjoining property owners, no side yard shall be required between properties of separate ownership where two or more commercial uses abut side to side. In the case of a series of abutting structures paralleling a public right-of-way, an open and unobstructed passage of at least 30 feet in width shall be provided at grade level at intervals of not more than 200 feet.