CA Veh Code Section 21658

Whenever any roadway has been divided into two or more clearly marked lanes for traffic in one direction, the following rules apply:

(a)

A vehicle shall be driven as nearly as practical entirely within a single lane and shall not be moved from the lane until such movement can be made with reasonable safety.

(b)

Official signs may be erected directing slow-moving traffic to use a designated lane or allocating specified lanes to traffic moving in the same direction, and drivers of vehicles shall obey the directions of the traffic device.