Drivers are supposed to keep right (outside/slower lanes) except to pass. That's the law in WA state and probably in most parts of the country. As far as I know it's also the law in other countries too; offhand on the Autobon.
In theory, the lanes further left should be at about the speed limit and the lanes to the right could be for slower long haul or dangerous loads driving, such as large and heavy trucks which might have a lower speed limit on some sections of road. In practice, please keep right except to pass and don't cause a traffic hazard trying to enforce slowness upon speeders with a rolling roadblock.
In theory, the lanes further left should be at about the speed limit and the lanes to the right could be for slower long haul or dangerous loads driving, such as large and heavy trucks which might have a lower speed limit on some sections of road. In practice, please keep right except to pass and don't cause a traffic hazard trying to enforce slowness upon speeders with a rolling roadblock.