Didn't this result from the mid-air collsion between two airliners over France in 1922?
They were flying in opposite directions in poor weather following the same railway line and collided head-on. After this the regulation was introduced that, if following a linear feature on the ground, you keep it on your left -hence opposite direction traffic would be on opposite sides of the feature.
When two pilot aircraft came along then the PIC sat on the left to keep visual with the feature.