California Code, Streets and Highways Code - SHC § 976

The board of supervisors of any county may purchase or lease with the consent of the owner, or may obtain by gift, and may hold, improve or maintain any real property or interest therein for the uses and purposes of county highways. Such real property or interest need not be within the boundaries of the county acquiring it, if it is convenient thereto in an adjoining county. In all such cases such property or interest is subject to the control of the county acquiring it and to the ordinances, rules, and regulations of such county.

Nothing in this section shall affect the power of any county to acquire property within its boundaries by proceedings in eminent domain in the manner prescribed by law.