Side yards, two, width: 10 feet each side of a principal building; provided, that when a written agreement is provided by adjoining property owners, no side yard shall be required where two or more commercial uses abut side to side; however, in no case shall common walls be permitted between properties of separate ownership. In the case of a series of abutting structures adjacent to and paralleling a public right-of-way, and open and unobstructed passage for vehicles and pedestrians, of at least 20 feet in width, shall be provided at grade level at intervals of not more than 400 feet.