| +/- junction=roundabout
A road junction where the traffic goes around a non-traversable island and has right of way.
A roundabout is a road junction where the traffic goes around a non-traversable island (or a void in case of elevated roads) in the middle, and the traffic on the roundabout has right of way. Roundabouts can have any size from tiny to huge with several lanes.
Please pay attention to the section "Possible misinterpretations" to differentiate between roundabouts, mini-roundabouts, turning circles and further road features.
How to map
Start by drawing a circular shape, which would represent the roundabout. Furthermore:
- The OSM ways of the roundabout itself must be drawn in the direction the traffic flows.
- Tag the OSM way(s) of the roundabout with junction=roundabout.
- oneway=yes is implied, but can be added for clarity.
- Connect it with the roads approaching the roundabout. If a road is split into two separated carriageways (one approaching, one departing), draw those explicitly and tag these carriageways (not the roundabout) with oneway=yes. Pay attention to their correct orientation and use separate nodes of the roundabout for the approaching and departing road.
- Also tag the way(s) with highway=*, the same as the connecting roads. If there are several roads of different importance connecting to the roundabout, you should usually use the one with the greatest importance, that does not begin/end at the roundabout.
Editor support for circles
There exist two ways to draw a circle in JOSM:
- Draw a closed way and press the key 'o' or select Tools -> Align Nodes in a Circle in the menu
- Select two or three nodes (e.g. ending nodes of ways leading to a roundabout) and press 'shift-o' or select Tools -> Create Circle in the menu
In Potlatch 2, draw a closed way with at least three nodes and use Align in a Circle in the toolbar.
Roundabouts are usually indicated by signposts, like e.g. one of these:
The following table shows a selection of classical roundabouts: they all are one-ways with a non-traversable center island, and the traffic within the roundabout has right-of-way. For the determination of the correct highway=* see the next section.
Selection of the right highway tag
The following examples focus on the determination of the correct highway=* for the roundabout and therefore only this tag is listed. When tagging a roundabout, junction=roundabout always has to be specified.
|highway=trunk||A dual carriageway trunk passes through the roundabout with two unclassified roads also connecting to it.||The trunk which passes through the roundabout has the highest importance, and therefore the roundabout itself should also be tagged as trunk. Also use two nodes on each side to connect the trunk to the roundabout - one for each driving direction.|
|highway=unclassified||A secondary road ends at the roundabout, the other three roads are all unclassified.||The secondary road has the highest importance, but it ends at the roundabout. The unclassified roads pass through the roundabout and therefore the roundabout is also tagged as unclassified.|
|highway=primary||A primary road connecting from north and west, a secondary road ending east and a service road ending south.||The primary road has the highest importance and does not end at the roundabout. Therefore the roundabout is tagged as primary.|
In OpenStreetMap we have several tags that describe different circular and widened road features. Perhaps the three tags which are most commonly confused with each other are:
- junction=roundabout for roundabouts
- highway=mini_roundabout for mini-roundabouts
- highway=turning_circle for turning circles
Because of incomplete or ambiguous documentation, misinterpretation or just for the sake of simplicity they were often used in situations, where they do not apply. The major differences between the three are:
- A roundabout is a one-way street with right-of-way and a non-traversable center island.
- A mini-roundabout is a one-way street with right-of-way and a traversable center island. In particular, large vehicles are allowed to drive across the center island if otherwise not possible due to their dimensions, i.e. it might be impossible for a large vehicle to drive through a roundabout but possible to drive through a mini-roundabout with the same dimensions.
- A turning circle is a widened space on a two-way street without any center island (or at least a fully traversable island). The absence of a center island allows also large vehicles to turn around.
Another term sometimes mistaken for a roundabout is a traffic circle: contrary to roundabouts, the entering traffic is controlled by stop signs, traffic signals, or is not formally controlled. This wikipedia article explains the difference of a roundabout and a traffic circle according to the United States traffic engineers. In OSM traffic circles are usually not indicated by specific tags.
A very common question is: "Why is it important to follow this tagging guideline if the map looks good even when tagged differently?"
The answer is: The map is not everything. There exist a lot more applications which process our data and these tags have different implications e.g. for routers. So if you tag a small roundabout as a mini-roundabout or a loop at the end of a street as a turning circle, it might look ok on the map but will break other applications. So please don't think of this as annoyance and try to use the most fitting tags in each situation.
The following table presents the preferred tagging for the most common situations and also tries to clarify some ambiguities.
|A standard roundabout with a large, non-traversable center island.|
|Small, but not a mini-roundabout. Although it is preferred to tag this the same way as a large roundabout, for the sake of simplicity it is acceptable to tag this with a single node with junction=roundabout. In this case the tag direction=* should be specified, as its default value is anti-clockwise when used in conjunction with a roundabout.|
|A classical mini-roundabout where the center is only a painted circle. The tag direction=* should be specified, as its default value is anti-clockwise when used in conjunction with a mini-roundabout.|
|Large, but a mini-roundabout. The center island is traversable and without signposts.|
|Also a classical mini-roundabout with a slightly raised but still fully traversable center island.|
|This photo shows a round place with a traversable island in the middle, but this is neither a mini-roundabout nor a roundabout, but instead a turning circle, which allows large vehicles to turn around. Please bear in mind that a turning circle can have any shape, but never a non-traversable island.|
|Don't let the island confuse you: there are indications neither of a one-way nor of right-of-way, so this is just a junction with an island used for traffic calming.|
|Watch the arrows closely - this is also just a junction with traffic calming.|
|Obviously a place for vehicles to turn around. But because of the island, it is not a turning circle (which must not have an island), so this should be mapped as a loop. For the sake of simplicity the tag highway=turning_loop on a single node is currently under discussion.|
Many roundabouts with heavy traffic are modified with traffic signals controlling traffic on the roundabout itself. Traffic joining the roundabout may not have to obey standard rules of priority at some entrances to the roundabout because of the signalling. In many cases some entry roads (those with lighter traffic) on these roundabouts are not signal controlled so that the whole junction still behaves as a roundabout (e.g., http://osm.org/go/eu5aUXR98--). Even when all junctions are signal-controlled, it is suggested that junction=roundabout be retained.
- Wikipedia article about roundabouts
- UK Highways Agency Design Manual for Roads and Bridges - Volume 6, Section 2, Part 3: Geometric Design of Roundabouts
- US Federal Highway Administration Safety Program - Technical Summary - Roundabouts
- How to change an intersection into a roundabout in JOSM
- Proposal to group the ways and/or nodes comprising a road junction
- Wikipedia article about traffic circles