[astrange]

Btw, California doesn't use the left lane for passing; you just have to yield to anyone going faster than you.

[godelski]

Most states have keep right laws[1]. Some are definitely stronger than others, but these laws aren't enforced as frequently, though I wish they were.

[1] http://www.mit.edu/%7Ejfc/right.html

tldr: If someone behind you is going faster and you are in the left lane, move right.