There is no "Fast Lane" the speed limit applies to all lanes of traffic. There is also a "Keep right except to pass Law in Colorado.
And you are also legally obligated to keep up with the flow of traffic within reason, even if that means exceeding the speed limit to a certain degree.
If the speed limit is 55, everyone is going 65, you can be found as a road hazard.
Laws are tricky.
Whenever my wife asks me why I never wear pants around the house I simply remind her:
"A lie gets halfway around the world before the truth has a chance to get its pants on.” - Winston Churchill