[Tagging] Culverts and Fords

[Vao Matua]

Can someone help me understand two different types of stream and road
crossings?

A ford <https://wiki.openstreetmap.org/wiki/Key:ford>can be a node or a way
(180,749 nodes & 63,842 ways)

However, a culvert <https://wiki.openstreetmap.org/wiki/Tag:tunnel=culvert>can
only be a way (691,972 ways)

In many circumstances from imagery I can see a stream crossing under a road
without a bridge or ford and therefore I assume that it is a culvert, but
typically cannot see the ends to split the watercourse.  Why does a culvert
have to be a way rather than a way OR a node?

What I would like to do is simply merge a node of the road and a node of
the stream and give it the tag tunnel=culvert (when I do this JOSM
complains)

In the case where a culvert carries a watercourse over a road then it would
make sense to create a way and layer=1

Any insights for this apparent inconsistency?
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