Do we tag these roadside gutters / drainage channels (not sure what the right word is) that were posted by @Vonter and @SekeRob with waterway=drain at all?
The Wiki page for drain shows a relatively wide waterway, like a stream. Same for @mcliquid’s examples.
The small channels that run along many streets look quite different to me.
See also Tagging of a steel gutter inlet where one suggestion was to use waterway=drainage_gutter for them.