People who drive slowly would let others pass.
People who drive slowly wouldn’t speed up when others could pass.
People in the vehicles behind slow vehicles would pass when they could or keep back far enough to allow others to pass them.
People wouldn’t try passing when they can’t see what’s coming the other way.
Passing lanes didn’t end on the brow of a hill or blind corner.
Arrows on the road now and then (especially after rest areas in isolated places) showed drivers they should be on the left hand side of the road.