(a) A junk yard lawfully in existence on August 6, 1968, that is within 1,000 feet of the nearest edge of the right-of-way and visible from the main-traveled way of any highway on the interstate or primary system shall be screened, if feasible, so as not to be visible from the main-traveled way of the highways.
(b) A junk yard lawfully in existence on July 1, 1970, that is within 1,000 feet of the nearest edge of the right-of-way and visible from the main-traveled way of any highway on the secondary system, or a junk yard lawfully in existence on September 14, 1976, that is within 1,000 feet of the nearest edge of the right-of-way and visible from the main-traveled way of a road maintained by the state, shall be screened, if feasible, so as not to be visible from the main-traveled way of the highway.