California Code, Business and Professions Code - BPC § 5404

(a) If within a distance of 300 feet from the point of intersection of highway or of highway and railroad right-of-way lines, except that this does not prevent the placing of advertising display on that side of an intercepted highway that is opposite the point of interception. But in case any permanent building, structure or other object prevents any traveler on any such highway from obtaining a clear view of approaching vehicles for a distance of 300 feet, then advertising displays may be placed on such buildings, structure or other object if such displays will not further obstruct the vision of those approaching the intersection or interception, or if any such display does not project more than one foot therefrom.

(b) If placed in such a manner as to prevent any traveler on any highway from obtaining a clear view of approaching vehicles for a distance of 500 feet along the highway.