The real easy way to understand culverts and fords for OpenStreetMap is about
the layers they are on and this dictates the nodes they use.
For a ford the stream/river is at the same level as the road (effectively
layer=0) and therefore they are able to share a node.
Because a culvert (layer=-1) is not on the same level as the road but passes
underneath so it cannot share a node with the road and therefore the culvert is
attributed to the river/stream with a node either side of the road.
With a bridge the road (layer 1) is not on the same level with the
stream/river so again cannot share a node and therefore the bridge is
attributed to the road with a node at each end of the bridge.
Hope this will be of help in understanding the problem.
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