Lane inversion

I often observe the following when a highway with two lanes per direction approaches congestion: under light traffic conditions, most vehicles stay on the right track so that the left lane is used for passing, as it should be; as traffic increases, the chances are higher that one cannot use the left lane to pass a slow vehicle because a third one is passing both, so users of the passing lane perceive there is a penalty in returning to the right, and ultimately most vehicles stay on the left lane; when this happens, the right lane begins to be used as a passing lane (even though in Spain, and I guess in many other countries, passing on the right is forbidden), and it turns out that the left lane becomes the slowest one. I take this phenomenon of lane inversion to be a sure sign that total congestion is coming on.

I wonder how much the throughput (traffic before congestion) of the highway would be raised if passing on the right was allowed and either lane could be used in cruise conditions.