On most US highways (i.e. multi-lane limited access roads), it's customary to leave a path in the left 'passing lane' for any traffic that wants/needs to go faster than you. If cars match speeds across lanes, it impedes faster traffic.
The speed limit itself is a separate convention and regulation. In some places you can be cited for obstructing traffic by going the speed limit in the passing lane if you are matching the speed of cars to your right, effectively blocking the road.
> (b) An operator of a vehicle on a roadway moving more slowly than the normal speed of other vehicles at the time and place under the existing conditions shall drive in the right-hand lane available for vehicles, or as close as practicable to the
right-hand curb or edge of the roadway, unless the operator is:
> (1) passing another vehicle; or
> (2) preparing for a left turn at an intersection or into a private road or driveway.
Note this law specifically mentions "normal speed of other vehicles at the time and place" and doesn't directly mention speed limits. So by the text of this law, if you're driving the speed limit and hanging out in the left lane while the normal speed at that time is like 10 over you're technically breaking this law.
We have specific signage for highways where this is supposed to be the law.
Sometimes I an appreciate wanting to cruise in the middle lane, because ADAS level 2 systems common on cars today is far more comfortable when it does not have to deal with regular merging traffic. But aside from that, I really don't like it when people camp in the middle lane because they tend to form a pretty tight line and manage to effectively turn a three-lane highway into two single-lane highways -- hard to get through from one side to the other.
Sorry, I didn't fully explain the idiom: "centre lane" refers to the right-most (what in the US would be left-most) "fast" lane - the one closest to the centre of the highway as a whole.
To your point, I think the middle lane of a three-lane road is, ideally, the correct travel lane. Cruising there at the prevailing speed leaves one lane more lightly traveled for entering and exiting, and the other for passing. Predominantly using that lane minimizes lane changes, which are the most dangerous driving moments. You're right, though, that the strategy breaks down as traffic gets heavier, and gets ruined entirely when (as under discussion in this thread) people gum up the supposed "fast lane".
> If cars match speeds across lanes, it impedes faster traffic.
I think this undersells it a little. It does not just impede faster traffic, when the lanes are pacing each other it makes navigating harder -- simply switching lanes is more difficult. The highway moves so much more efficiently with a small but steady difference in speed between each lane.
> Results: A 5-mph increase in the maximum state speed limit was associated with an 8.5% increase in fatality rates on interstates/freeways and a 2.8% increase on other roads. In total during the 25-year study period, there were an estimated 36,760 more traffic fatalities than would have been expected if maximum speed limits had not increased—13,638 on interstates/freeways and 23,122 on other roads.
That doesn't make sense to me. If you want to change lanes, and worst case scenario you're right next to someone, go 2mph slower for 20 seconds and they'll be shifted by 60 feet. I'm sure you can plan your lane changes on a freeway 20 seconds in advance.
It is the dynamics. When lanes are pacing each other the gaps all tighten up. So sure, you can slow down a bit to find the gap behind you, except that gap is not big enough to fit in. So you turn on your signal and wait for someone polite enough to let you in, meanwhile the guy behind you is riding you like a pony because you are no longer keeping up with traffic.
When traffic isn't balled up so tight, you can plan for a lane change in advance and accomplish it without having to slow down traffic. Everything flows better.
The speed limit itself is a separate convention and regulation. In some places you can be cited for obstructing traffic by going the speed limit in the passing lane if you are matching the speed of cars to your right, effectively blocking the road.