|roundabout = turbo
|A turbo roundabout allows drivers to select their direction before entering, which is then enforced throughout the roundabout.
|Used on these elements
|Status: in use
|Tools for this tag
A turbo roundabout is a specialized type of roundabout that is designed to increase traffic capacity for specific directions. Unlike regular roundabouts, a turbo roundabout typically features multiple lanes that are physically separated by kerbs, making it impossible or illegal to change lanes while driving on the roundabout.
Identifying a turbo roundabout
A key feature of a turbo roundabout is its spiral shape, which gradually pushes traffic from the inside to the outside lanes as they approach their desired exit. To achieve the spiral shape a turbo roundabout has at least one lane that starts from the center island, every lane that starts at the center island has a corresponding forced exit. This prevents drivers from continuously driving around the roundabout.
Turbo roundabouts are not to be confused with multi-lane roundabouts. Although a standard multi-lane roundabout also requires the driver to select a lane before entering it, it still physically allows them to weave between the lanes inside the roundabout, creating a safety hazard.  Some standard multi-lane roundabouts have been retrofitted with physical lane dividers but do not feature the spiral shape. 
How to map
Turbo roundabouts can be mapped in two LODs (level of detail). The first method is very similar to normal roundabouts, with the addition of change:lanes=no|no and lanes=2 or lanes=1 where appropriate.
The second level of detail is to map the individual separated lanes. This method more accurately describes the situation but requires significantly more work. On this page, the detailed mapping of turbo roundabouts is covered.
Detailed mapping of turbo roundabouts
To properly map a turbo roundabout some skills are required, including knowledge of turn restrictions, bus routes and advanced geometry editing.
General mapping information
- Buses, if they have a choice of lanes, usually take the outside lane.
- Split multi-lane approaches at the last moment that switching lanes is possible.
- Use turn:lanes=* both on the approach where the multiple lanes start and on the single lanes right before entering the roundabout.
- Add roundabout=turbo to the same parts as the parts that have junction=roundabout.
- If desired, the kerbs between the lanes can themselves also be mapped. You can use barrier=kerb and (possibly) two_sided=yes.
Turn restrictions are required on all the parts where switching lanes is illegal or impossible. For a standard turbo roundabout this would require 6 turn restrictions. 4 straight through on the crosses in the main roadway and 2 no left turn on the outside lane to the inside connection of the turbo roundabout.
How to get accurate geometry
Because turbo roundabout lanes are not exactly circular, getting smooth and accurate geometry requires some extra work. The steps (in JOSM) of getting smooth geometry are:
- Split up all the round parts.
- Add enough nodes to the parts.
- Aline the nodes
- Press “O” to make the parts round.
- Select the joint between two parts and between 2 and 4 before and after the join
- Press “O” on this selection
- Repeat steps 2-6 util the geometry is consistent, accurate and detailed.
- Join all round parts that can be joined, taking relations and tags into account.