California Code, Streets and Highways Code - SHC § 27173

The work of constructing, repairing, relocating, or otherwise providing any of the highways or approaches composed of the structures or improvements mentioned in Part 3 (commencing with Section 27000) of Division 15 of this code or Article 60 (commencing with Section 20910) of Chapter 1 of Part 3 of Division 2 of the Public Contract Code, may be done by any district in cooperation with a city, county, joint highway district, or other district formed for the improvement or building of roads, or in cooperation with the state.

For the purpose of doing work or acquiring rights-of-way therefor, the board may authorize a contract to be entered into between the district and the political subdivision or agency of the state with which it proposes to cooperate, and full power and authority is granted to the city, county, joint highway, or other district, or the Department of Transportation, to enter into the contracts for cooperative work, construction, or purchase of rights-of-way, in such a manner and upon such apportionment of the cost thereof as may be agreed upon between them.  For this purpose, the contracting parties may designate engineers, county surveyors, or other necessary officials or employees to take charge of the particular work, and provide for the payment of the cost of the work out of funds contributed by the contracting parties.  The plans and specifications for the work shall be approved by both of the contracting parties, and the work may be carried out by either one of the contracting parties as the agent of the other, or it may be carried out under the joint management of both of the contracting parties.

Any project undertaken by a district alone, or in cooperation with any of the agencies mentioned, may be accomplished by federal aid under rules which may be prescribed by the federal agencies involved.

Any work may extend across, through, or within any city, county, joint highway district, or other district, if the work is an extension of an approach to a bridge operated by the district and connects the bridge as a main or alternate route with state highways or other through routes leading to or from the bridge.