Why are you in the lefter lane if you can't pass?
I've been in rush hour (where keep-to-the-right-unless-passing is very strictly enforced) in bumper-to-bumper traffic and the left two (out of 6) are completely empty and everyone is doing 'around' the speed limit. Some are in the right lane doing a few below the limit, some are in the left-most lane doing a few above.
Occasionally, someone who is late to work, emergency services, or whatever goes flying by in one of the left-most lanes.