Streets and Highways Code § 1702
ARTICLE 3. Extension of County Highways Through Cities
Thereafter the board of supervisors of the county may acquire rights-of-way for, construct, maintain, improve, or repair such highway in the same manner as other county highways, and may pay for such work out of one or more of the following:
(a) The county general fund.
(b) The road fund.
(c) The proceeds of any county bond issue voted for the purpose of laying out, constructing, maintaining, improving or repairing county highways.
(d) Any other fund available for such purposes.