I thought that in the US, freeways meant that you could overtake slower traffic on either side. In the UK, you're not allowed to overtake on the left.

(Temporarily translating the drive-on-the-opposite-side-of-the-road thing back again, and making the point that traffic laws vary from state to state so I can only really speak for California...)

No, that's an incorrect assumption. If you ever go driving in California, if you look closely, you'll see the "Slower Traffic Keep Right" signs posted even on 6-lane freeways. So the laws are the same here in the states as they are in England, and they apply to freeways as well as highways.

Many people think as you did, and it's one of the root problems. You'd think people would understand the traffic laws, or at least be able to read the road signs, but they don't.

In England (and all of Europe), as I understand it, it's a common practice to flash your lights at someone in the fast lane to indicate you want them to move over so you can pass. You wouldn't believe how angry U.S. drivers get when you do this. Sometimes there's been room for me to use the slow lane to pass them, and they get pissed and wave me around on the slow side. They don't realize that if I have room to pass them on the right, they have no business being in the left lane at all. By law.

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Tony Fabris