I'm a little perplexed by the fact street and road roundabouts are 2x2 while one-way road roundabouts are 3x3. The latter seems like it'd be by far the preferable size:
- You can pick what building to put in the middle.
- You can get more tributary roads around it within the confines of the grid.
- You can get at it from the four cardinal directions that a lot of roads in SimCity are going to tend to run at rather than a slightly awkward rotated angle (i.e. it can integrate more nicely into heavily gridded layouts).
Is there some technical reason for the difference?
They are simply different sizes of roundabout that just happen to be built with different network tools. You can connect road, street, OWR, even RHW-2 and MIS to the OWR roundabout.
It's not limited to OWR, but simply called that because that's the tool used to build it and to distinguish it from the smaller roundabouts drawn by the road and street tools.
That makes sense, and would also explain the presence of the "reduced arrows on roundabouts" option. So they all count as being the same capacity?
The Street Roundabout uses Street, so it has Street's capacity, the Road roundabout uses Road's capacity and the OWR roundabout uses OWR's capacity.