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Revision as of 15:02, 8 June 2016 by Kartografer (talk | contribs) (I added material from the "Interactive junction instruction algorithm" by PesachZ to further explain roundabout angles.)

Understanding navigation instructions

Normal roundabouts give a "turn right" / "continue straight" instruction. Non-normal roundabouts give a "take the X exit" instruction. This section will explain the difference between a normal and a non-normal roundabout.
A normal roundabout:

  • Will give you a "turn right", "turn left" or "continue straight" instruction.
  • Has up to 4 nodes.
  • All exit nodes are aligned within "normal" range of the entrance node.

A non-normal roundabout:

  • Has 5 or more nodes (Ex. Each segment of a split road connects at its own node.)
  • Has nodes outside the normal roundabout angle range.
  • Will give you a "take the X exit" (with arrows around it) instruction.

A roundabout can be both normal and non-normal at the same time depending on your entrance node, because the roundabout angle is calculated from where one enters the roundabout.

Roundabout Angle

The Roundabout angle is the angle of any segment connected to the roundabout from the perspective of the origin. The actual turn angle of the segments is irrelevant.
To explain;

    • Draw an imaginary line through the center of the roundabout to the other side of the roundabout from junction node where the origin joins the roundabout. This will represent a 0° Roundabout angle, any deviation to the right or left will increase the angle.
    • Draw another imaginary line across the roundabout through the center and perpendicular to the first imaginary line. This will represent a 90° Roundabout angle to either side.
Normal Non-Normal
Roundabout 3 normal.png

Roundabout 3 non-normal.png

All other segments are within the allowed ±15° from either 0° or 90°. Not all other segments are within the allowed ±15° from either 0° or 90°.


  • Since the angles of the segments approaching the nodes do not matter, both of these layouts will give the same instructions. Only the angles between the nodes and the center of the roundabout (roundabout angles) determine which instructions are given.


For the sake of simplicity, let’s look at a roundabout with three exits.

  • Coming from node #1, this is how we see the roundabout:

  • Both exits 2 and 3 have roundabout angles within the normal range (90°±15° & 0°±15°).

  • These exits will get an instruction to "turn right" or "continue straight".

  • Coming from node #3, this is how we see the roundabout:

  • Exit 1 is within the normal roundabout angle range, but exit 2 is not within the normal roundabout angle range.
  • Even though one of these exits is within normal range, the roundabout itself is ‘non-normal’ and both exits will get the "take the X exit" prompt. The entire roundabout must be normal to generate "turn left", "turn right" or "continue straight" instructions.