California Code, Streets and Highways Code - SHC § 1728

No city street shall be closed, either directly or indirectly, by the construction of a county highway within a city as a freeway except pursuant to an agreement between the city council and the board of supervisors or while temporarily necessary during construction operations.  No city street or other public highway shall be opened into or connected with any county highway declared to be a freeway by the board of supervisors unless and until the board of supervisors adopts a resolution consenting to the same and fixing the terms and conditions on which such connection shall be made and the board of supervisors may give or withhold its consent or fix such terms and conditions as in its opinion will best serve the public interest.

The board of supervisors is authorized to enter into an agreement with the city council to close any city street at or near the point of its interception with any county highway declared to be a freeway under Section 941.1 or to make provision for carrying such city street over or under or to a connection with the freeway, and the board of supervisors may do any and all work on such city street as is necessary therefor.