IE - in many parts of the US, there are many roads where 'everyone speeds'. Because 'everyone knows' cops won't pull you over until you are going some arbitrary speed faster.
From the Wikipedia, and I know its true because I live there. Quote-mined for clarity:
In California...Drivers moving slower than the general flow of traffic are required to stay in the right-most lanes (by California Vehicle Code (CVC) 21654) to keep the way clear for faster vehicles and thus speed up traffic. However, faster drivers may legally pass in the slower lanes if conditions allow (by CVC 21754). But the CVC also requires trucks to stay in the right lane, or in the right two lanes if the roadway has four or more lanes going in their direction. The oldest freeways in California, and some freeway interchanges, often have ramps on the left, making signs like "TRUCKS OK ON LEFT LANE" or "TRUCKS MAY USE ALL LANES" necessary to override the default rule. Lane splitting, or riding motorcycles in the space between cars in traffic, is permitted as long as it is done in a safe and prudent manner.[2]
As long as you are an average driver, you can abide by the choice phrase "flow of traffic" and that's the easiest way to cope with it. Otherwise the whole thing looks like a group of nested if-else statements gone horribly wrong.
That's because of a bunch of stupid laws that get passed many years apart with no care of the previous law. It use to be very simple. Slower traffic stay to the right (it is still marked like this on many four lane highways, but hardly anyone follows it). Then the speed limit got reduced from 65 to 55 (I know it went back up many years ago, but there are still freeways marked with 55) and now the "fast lane" is no longer fast. I've seen stories on the news where a policeman is giving a reporter a ride and they're watching someone tailgate because they're doing 55 and the other driver wants to go faster. Do they move to the right? No. They ask why the other driver doesn't slow down.
The reason for the "TRUCKS MAY USE ALL LANES" is because on some freeways, and this is mostly coming into LA County from the north (as far as I've ever seen) there are lanes specifically for the trucks. You won't see very many cars on them and they're usually pretty empty, even when the freeway is filled with cars. It's because those roads usually have only one or two lanes and they're specifically meant to get the big trucks out of the traffic. Having an 18 wheeler stuck in stop and go traffic is a lot worse than letting them take a different route (the route is usually longer anyway).