I think you were simply mistaken in this one case. https://www.osm.org/node/13011166798. Bridges typically have headwalls or are paved. This is an unpaved road which has neither. 

I think I may agree with you in that my initial assessment was wrong and that the road seems to be at a higher elevation than the wash so there is probably a culvert which you may be able to see on Esri World Imagery.

I just do not like iD because of this because it allows users to pick between a bridge and culvert where it places endpoints arbitrarily. You can essentially just click the button and it "fixes" it, leaving accuracy behind.