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.
(Amended by Stats. 1975, Ch. 450.)