[OSM-talk] Roundabouts and routing

[David Earl]

On 09/09/2011 11:00, Graham Stewart (GrahamS) wrote:
>
> David Earl wrote:
>>
>> In areas where it has been important for me (where I've been producing a
>> high quality paper map), I have tagged these as junction=approach.
>>
>> The reason I needed such a tag was to avoid one way arrows cluttering up
>> the map on those little Y-shaped approaches to roundabouts
>
> This seems like a bad approach to me. (pardon the pun)
>
> If the road flares like that then those two road sections ARE one way. If
> you do not tag them as such then you will confuse routing  software, which
> will see two possible exits from the roundabout, rather than one on and one
> off.

As I said, I didn't invent this, only added a tag to identify the kind 
of feature. But nearly all roundabouts in the UK are done like this or 
similar, by lots and lots of different people, because they are 
significant geographical features. On major roads they can be many tens 
of metres long and the gap between the ends can be 10 or 20 metres on 
some big roundabouts. It's almost a special case of dual carriageway.

If they are explicitly marked one-way then the problem is with routing 
algorithms if they could them as an exit when they aren't. If you didn't 
mark them as one-way then there would be some excuse for counting them 
all as exits (except that you could tell, if they are marked 
junction=approach, but that's not nearly so widely used).

David