[Tagging] Another reset on roundabouts

Volker Schmidt voschix at gmail.com
Fri May 18 10:32:32 BST 2012

At the risk that we are going in circles on this argument.

Let us try to figure out what the needs are.

What we have (in theory) at present is


   a roundabout is a road layout where traffic goes around a central
   island. At present a roundabout needs to be drawn as a circular way, not as
   a node.

   The direction of traffic depends on the country (left-hand or right-hand

   a mini-roundabout is a road layout where traffic goes around a central
   traversable area. This is to allow long vehicle to use it.
   A mini-roundabout is inserted as a node, not a way.

The distinction between the two types of roundabout is clear-cut in some
countries, where there are different rood signs for the two types, and is
less clear-cut in others where the same road sign are used for both, only
the physical implementation is different.

Where does our problem stem from?

Many mappers have used the mini-roundabout node for physically small
roundabouts with an non-traversable centre island. In some cases (I have
done it myself) it has been used to mark even a larger roundabout in a
provisional way.

What we are missing in this scheme is most obviously a “roundabout” node.

Let's look at the new roundabout node:

We should give it parameter to indicate its size, for example est_diameter.

Now we have to consider the routing: If we look at the end product we want
be able to deliver, i.e. the vocal instructions, they would have to
distinguish between two categories of roundabouts:


   “small” roundabouts, i.e. mini-roundabouts and roundabout nodes with
   est_diameter less than 12 meters (tbd): “turn left”, “turn right” etc

   “large” roundabouts that are drawn as circular ways and node-roundabouts
   with est_diameter over 12 meters (tbd): “enter roundabout”, “take third
   exit” etc.

So, if we look at it from the routing point of view it looks to me that we
do not need to distinguish between a mini-roundabout and a small
roundabout, but what we seem to need is a way to indicate the size of the
roundabout in order to distinguish the routing behaviour.

What I am arriving at is that we need a new node “roundabout” with a
mandatory size parameter of some kind. The distinction between the present
“mini_roundabout” and a small roundabout would no longer be relevant.

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