Positive crankcase valves required starting in 1963 not only cleaned smog enormously and got rid of a major stench in cities, they got rid of the oil stains that used to be common on roads and made a safety hazard after light rain. And they actually extended the life of engines enormously by reducing sludge build up in engines. As emissions technology goes it really was a win win.

When you see old photos of highways, the thing that sticks out is the oil stains. But it's not oil leaks but blow by gasses that used to blow out of cars as they went down the road. Very noticeable on old concrete interstates but it used to be norm on any heavily trafficked road.