(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.
This is a pretty typical law - I know that PA has pretty much the same code. Doing a little quick research, State "keep right" laws - you can see that most states (31/50) have the exact same laws. Six states (IL,KS,KY,ME,MA,NJ) actually forbid use of the left lane, and two states (PA, WA) have a slight ban on the using the left. The rest of the states have no specific law, though all of these states require you not to obstruct the flow of traffic.
...though his invention worked superbly -- his theory was a crock of sewage from beginning to end. -- Vernor Vinge, "The Peace War"