Article 4. Cooperation Between Cities And Counties of California Streets And Highways Code >> Division 2. >> Chapter 9. >> Article 4.

A city and a county may agree that the city shall construct, repair and maintain designated county highways or portions thereof within the unincorporated territory of the county, the cost thereof to be paid by the county to the city.

A county and a city may agree that the county shall construct, repair or maintain designated county highways, or portions thereof, within the unincorporated territory of the county, the cost thereof to be paid in whole or in part by the city to the county.

(a) A county and a city or cities may join in the acquisition of rights-of-way, construction, maintenance, improvement, or repair of bicycle routes or paths, or portions thereof, located within or without the city or cities.
  (b) The cost or expense thereof shall be apportioned among the county and the city or cities as the legislative bodies thereof determine by agreement.
  (c) The authority and responsibility for the acquisition and work shall vest in the county or in the city or cities, or in both, as provided in the agreement apportioning the cost or expense thereof.