CALIFORNIA STATE VEHICLE CODE, SECTION 21650-21664
http://www.leginfo.ca.gov/cgi-bin/displaycode?section=veh&group=21001-22000&file=21650-21664
21650. Upon all highways, a vehicle shall be driven upon the right half of the roadway, except as follows:
(a) When overtaking and passing another vehicle proceeding in the same direction under the rules governing that movement.
(b) When placing a vehicle in a lawful position for, and when the vehicle is lawfully making, a left turn.
(c) When the right half of a roadway is closed to traffic under construction or repair.
(d) Upon a roadway restricted to one-way traffic.
(e) When the roadway is not of sufficient width.
(f) When the vehicle is necessarily traveling so slowly as to impede the normal movement of traffic, that portion of the highway adjacent to the right edge of the roadway may be utilized temporarily when in a condition permitting safe operation.
21654. (a) Notwithstanding the prima facie speed limits, any vehicle proceeding upon a highway at a speed less than the normal speed of traffic moving in the same direction at such time shall be driven in the right-hand lane for traffic or as close as practicable to the right-hand edge or curb, except when overtaking and passing another vehicle proceeding in the same direction or when preparing for a left turn at an intersection or into a private road or driveway.
(b) If a vehicle is being driven at a speed less than the normal speed of traffic moving in the same direction at such time, and is not being driven in the right-hand lane for traffic or as close as practicable to the right-hand edge or curb, it shall constitute prima facie evidence that the driver is operating the vehicle in violation of subdivision (a) of this section.
(c) The Department of Transportation, with respect to state highways, and local authorities, with respect to highways under their jurisdiction, may place and maintain upon highways official signs directing slow-moving traffic to use the right-hand traffic lane except when overtaking and passing another vehicle or preparing for a left turn.
21656. On a two-lane highway where passing is unsafe because of traffic in the opposite direction or other conditions, a slow-moving vehicle, including a passenger vehicle, behind which five or more vehicles are formed in line, shall turn off the roadway at the nearest place designated as a turnout by signs erected by the authority having jurisdiction over the highway, or wherever sufficient area for a safe turnout exists, in order to permit the vehicles following it to proceed. As used in this section a slow-moving vehicle is one which is proceeding at a rate of speed less than the normal flow of traffic at the particular time and place.
21659. Upon a roadway which is divided into three lanes a vehicle shall not be driven in the extreme left lane at any time, nor in the center lane except when overtaking and passing another vehicle where the roadway ahead is clearly visible and the center lane is clear of traffic within a safe distance, or in preparation for a left turn, or where the center lane is at the time allocated exclusively to traffic moving in the direction the vehicle is proceeding and is signposted to give notice of such allocation. This section does not apply upon a one-way roadway.