California Code, Vehicle Code - VEH § 235

A “business district” is that portion of a highway and the property contiguous thereto (a) upon one side of which highway, for a distance of 600 feet, 50 percent or more of the contiguous property fronting thereon is occupied by buildings in use for business, or (b) upon both sides of which highway, collectively, for a distance of 300 feet, 50 percent or more of the contiguous property fronting thereon is so occupied.  A business district may be longer than the distances specified in this section if the above ratio of buildings in use for business to the length of the highway exists.