What does it mean when a sign says this: "Slower Traffic Keep Right"?

1. Go the same speed as everyone else


2. Stay in the left lane indefinitely, if you want to


3. Get out of the left lane unless you're passing someone


4. Speed limit 55


5. a, b, and d

The answer if you drive in Washington State is apparently e. At least that's what it appears to be any time I spend an extended amount of time on the freeway.