California Code, Business and Professions Code - BPC § 5440

(a) Except as otherwise provided in this article, no advertising display may be placed or maintained on property adjacent to a 1,000-foot or greater section of a freeway that has been landscaped with at least an average width of 20 feet of landscaping or that includes trees, on department-owned property at the same or elevated grade of the main-traveled way if the advertising display is designed to be viewed primarily by persons traveling on the main-traveled way of the landscaped freeway.

(b) The department shall determine the average width by dividing the square footage of a landscaped area by its length.

(d) For purposes of this section, “average width of 20 feet” means that over any 1,000-foot section freeway there averages at least 20 feet of total property contiguous to all sides of the main traveled way between the outer edge of the shoulders and the freeway right-of-way boundaries, including median plantings, that otherwise meets the definition of a landscaped freeway in Section 5216.