> I get that leaving the left lane clear for passing is the convention, but is it actually optimal? If everyone stuck to this, then the left lane would be rarel

> I get that leaving the left lane clear for passing is the convention, but is it actually optimal? If everyone stuck to this, then the left lane would be rarely used and we'd be underutilizing the available space, right? Right, it depends on traffic volume. There are situations where "keep right except to pass" obviously doesn't help - e.g. streets with regular left-hand turns and congested highways, most notably.