Roundabout lines

It depends. Often roundabouts can be drawn as a single way, but sometimes they need to be split such as where short sections cross bridges over another highway below. All sections will be tagged as roundabout and be the same type of highway and when considered together will form a closed loop. In the split case it is unlikely to be at the junction nodes with the joining highways; usually at other nodes that are part of the roundabout, though you might split at the junction nodes for example when mapping bus routes that only use a section of the roundabout. I can't be certain from this phone but think http://osm.org/go/0EFDla6d is a split example.

I draw roundabouts as a singular circular closed way and the guidance here seems to suggest the same wiki about roundabout

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Haven't seen one like this yet but I would consider this incorrect :)

Edit: On further consideration and checking some of the different kinds of roundabouts on the wiki link above, I can see that the more complex roundabouts, would be best mapped by splitting into several sections and be separately named and may be of different class too.