Decades ago, when I was first learning to drive truck, it was ass-plained like this:
First of all, there is NO SUCH THING as a "fast lane." You're required by law to not exceed the posted speed limit...but, of course, nowadays, that's more of a suggestion than an actual law...
Modern highways in the US of A were designed to be used in this way:
On a two lane (each direction) roadway, the outside (#2) lane is for normal traffic...entering the roadway, exiting the roadway, normal driving. The INSIDE (#1) lane is for passing slower traffic, traffic that's exiting the roadway, traffic that's trying to merge onto the roadway, plus, it's used to have room to move over for vehicles that are turning, emergency vehicles on the shoulder, etc.
On a roadway that has 3 or more lanes in each direction, the farthest outside lane is for vehicles that are slower than the normal flow of traffic, (theoretically the posted limit...but...) over-sized vehicles, including large trucks, for vehicles that are about to exit the roadway or vehicles that are merging onto the roadway.
The center lane(s) are for through traffic. Folks who are NOT planning to exit anytime soon.
The far inside lane (#1) lane is strictly for passing folks who are driving slower than the normal flow of traffic, for whatever reason. Of course, even in that lane, you're restricted to driving the posted limit...
In MANY states, larger vehicles (usually 3 axles or more such as semi-trucks, very large motorhomes, (3 axles) even cars/pick-up trucks pulling trailers of any kind) are limited both in applicable speed limit as well as the lanes in which they're permitted to drive.