Side yard depth, each: 10 feet. When mutual agreement is provided, in writing, by the adjoining property owners, no side yard shall be required where two or more commercial uses adjoin side to side; however, in no case shall common walls be permitted between properties of separate ownership. In the case of a series of adjoining structures abutting and paralleling a public right-of-way, an open and unobstructed passage of at least 20 feet in width shall be provided at grade level at intervals of not more than 400 feet apart.