As a fellow European with experience driving in California (who prefers driving in the right-most lane), I believe it boils down to a few ways in which CA freeways are different to those in Europe:
- posted speed limits fairly low (typically 65mph)
- *much* higher traffic volume
- traffic consisting of ~95% cars
- left-lane freeway ramps
- ramps usually merging directly into the freeway (mentioned by another comment here)
A combination of these factors results in all lanes moving at roughly the same speed (a few mph above the posted limit), which may feel bizarre to a European driver, but certainly improves traffic flow in this specific scenario.
In general, freeways are windy (similar to most European 'dual carriageways'), which requires low speed limits, which reduces throughput. Since the traffic volume is very high, to increase throughput left-lane ramps and ramps merging directly into the freeway are introduced, which may cause occasional hiccups, but on average works well. Since the vast majority of vehicles are cars, all travelling at roughly the same speed, the highest throughput can be achieved by fully occupying all lanes.
There was an interesting related discussion on HN on a few occasions about the best way to utilise escalators at metro stations - see e.g. https://news.ycombinator.com/item?id=10916704
> which requires low speed limits, which reduces throughput
Lower speed limits usually increase the density on the road which does increase the Fotoalbum throughput. It takes longer for an individual car to make a given distance, but the number of cars that pass any given point at a specific point in time is increased.
In general, freeways are windy (similar to most European 'dual carriageways'), which requires low speed limits, which reduces throughput. Since the traffic volume is very high, to increase throughput left-lane ramps and ramps merging directly into the freeway are introduced, which may cause occasional hiccups, but on average works well. Since the vast majority of vehicles are cars, all travelling at roughly the same speed, the highest throughput can be achieved by fully occupying all lanes.
There was an interesting related discussion on HN on a few occasions about the best way to utilise escalators at metro stations - see e.g. https://news.ycombinator.com/item?id=10916704