On a multi-lane highway, when should you use the leftmost lane?

The 'keep right except to pass' rule is the standard for multi-lane highway driving in every state. The right lane is for normal driving; left lanes are for passing. After completing a pass, return to the right or middle lane. Some states actively enforce left-lane camping with fines. The reason: lane discipline reduces lane-changing, and most multi-vehicle highway crashes involve lane changes. Slow vehicles in the left lane force faster vehicles to pass on the right, which is more dangerous than passing on the left. Always pass on the left when possible. On three-lane highways, the middle lane is for through traffic at the prevailing speed.