A Roundabout is a type of road intersection at which traffic enters a one-way stream around a central island.
Roundabout Segmentation Options
Councils are required to include all of their assets in an asset register. To accomplish this network assets, like roads typically need to be broken up into a large number of segments. In the case of roads, intersections are normally an obvious place to start and end segments, but roundabouts can and do complicate things somewhat.
The simplest way to deal with the issue geometrically is to treat the roundabout as a standalone segment as per (A) above. The downside to this option is that in order to faithfully display road segments in the GIS it is necessary to add dozens, hundreds or even thousands of circles to the road segment map. It also wrecks the "line node line" nature of a road centreline map, which isn't ideal from a GIS perspective.
The simplest way to deal with roundabouts from a GIS perspective is to basically ignore them and just apportion a quarter of the value of the roundabout to each road segment that connects to it (B), but this means the true length of segment doesn't match the length of the line segment that represents it.
(C) is a compromise solution, where half of the roundabout is attached to either leg of the major road on which the roundabout is located. This allows for nodes between all segments, but is pretty messy.
Unfortunately none of the first three segmentation methods have much to do with, the physical reality of roundabout construction, as the roundabout approaches and accompanying splitter islands are typically constructed at the same time as the roundabout itself. Option (D) is therefore the best representation of how the roundabout is constructed, but it is neither GIS friendly or geometrically simple.
Option (E) is simple geometrically and logical. It could work pretty well for large roundabouts, but will result in four very short road segments if applied to smaller roundabouts.
None of the above or other possible options are ideal, so often it is a case of choosing the least bad option for a particular Council.
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External Links & References
- Wikipedia
- Google Search
- Explaining the Dutch Roundabout Abroad - Bicycle Dutch