Cal. BPC § 5408.1

(a) No advertising display shall be placed or maintained beyond 660 feet from the edge of the right-of-way of an interstate or primary highway if such advertising display is located outside of an urban area or within that portion of an urban area that is not a business area, is visible from the main traveled way of such highway, and is placed with the purpose of its message being read from such main traveled way, unless such advertising display is included within one of the classes of displays permitted by Section 5405 to be placed within 660 feet from the edge of such highway. Such display may be placed or maintained within the portion of an urban area that is also a business area if such display conforms to the criteria for size, spacing and lighting set forth in Section 5408.

(b) Any advertising display which was lawfully in existence on the effective date of the enactment of this section, but which does not conform to the provisions of this section, shall not be required to be removed until January 1, 1980. If federal law requires the state to pay just compensation for the removal of any such display, it may remain in place after January 1, 1980, and until just compensation is paid for its removal pursuant to Section 5412.

(c) For purposes of this section, an urban area means an area so designated in accordance with the provisions of Section 101 of Title 23 of the United States Code.