California Code, Business and Professions Code - BPC § 5300

(a) A person engages in the business of outdoor advertising whenever, personally or through employees, that person places an advertising display, changes the advertising message of an advertising display that does not pertain exclusively to that person's business and is visible to a state highway or freeway.

(b) A manufacturer or distributor of a product for sale to the general public does not engage in the business of outdoor advertising when furnishing a sign pertaining to the product to a retailer of that product for installation on the retailer's place of business or when installing on the retailer's place of business a sign containing advertising pertaining to the product, the name or the business of the retailer.