It is based on the split log drag invented by Mr. King back when most roads were dirt.
You can't quite see it, but there is a piece of .187 x 1.75 steel bar welded to the front edge. You can see the dirt scraped up by this bar on the right of the first picture. The bar is welded on so that it hangs about .500" below the bottom of the 5" channel iron.
The dirt that is scraped up flows to the center of the road and is smeared flat by the rest of the 5" channel, and the second 5" channel that follows.