As you (hopefully) know, most major thoroughfares have two lanes: one for passing and one for everyone else to drive in. This sign should not be foreign to you.
I saw that last month, Nevada enacted a law that would make it illegal to be the jerk clogging up traffic in the left lane. It reads as such: “a driver may not continue to operate a motor vehicle in the extreme left lane if the driver knows, or reasonably should know, that he or she is being overtaken in that lane from the rear by a motor vehicle traveling at a higher rate of speed.”
That basically means that you could be cited if you’re driving below the speed limit in the left or fast lane. Which is good, because slow drivers can be just as hazardous as fast one.
It’s just kind of funny, and I know it’s been a while since I took driver’s ed, but apparently “speed limit” doesn’t mean you the max speed you’re allowed to go. It means “you better be going at least this fast otherwise you’re a hazard to other drivers.”
This is a rule pretty much everywhere, as Jalopnik points out here and in the image below, but a few states are starting to actually enforce fines for people that are going too slow.