No permit shall hereafter be granted for spur tracks on any public street in the City and County of San Francisco except upon condition that all persons, firms or corporations owning or using property fronting thereon, or owning or using other tracks connecting therewith, shall have the right to use said tracks upon paying a proportionate share of the cost of the construction, maintenance and repair of said tracks and their adjacent pavements, unless upon other terms mutually agreed upon with the person, firm or corporation to whom the permit is granted, it being understood that cars loaded with perishable products shall have the right-of-way if necessary.