Cal. Sts. & High. Code § 2214

Federal-aid secondary funds apportioned to any county under Section 2208 and not claimed or used as provided in Sections 2211 to 2213, inclusive, shall be expended on construction of projects upon the state highway system in the county group, as specified in Section 187, which includes the county from which released, to the extent that such may be done in conformity with federal law. The money apportioned by the state to match the unexpended balance of the federal aid in any county, in accordance with this section and the federal law, shall be deemed an expenditure to meet the requirements of Sections 188 and 188.8.