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 party walls be permitted between properties of separate ownership. In the case of a series of abutting structures abutting and paralleling a public right-of-way, an open and unobstructed passage for vehicles and pedestrians, of at least 30 feet in width, shall be provided at grade level at intervals of not more than 1,000 feet.