(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.
CVC 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.
(AKA "Slower traffic, keep right" law)