[OSM-talk] Divided roads proposal

Morten Kjeldgaard mok at bioxray.dk
Fri Dec 4 15:03:03 GMT 2009


Peter Childs wrote:

> There are two types of Dual Carrage-way.which I think is the problem here.
> 
> Type One.
> 
> Large Motor Way Type Roads (but also large Primary roads), with Slip
> Roads, and the two separate carriageways are never joined, or maybe by
> a roundabout from time to time or other major junction. Here its best
> to map each carriage way separately as they are separate roads.
> 
> Type Two
> 
> Smaller roads where the Dual Carriage way exists but is split at
> regular intervals by gaps to turn right (Sorry I'm in the Uk (Left in
> most countries)), Often created due to the lay of the land, Bridges,
> Tunnels, Crossing Islands, Safety Island, etc. May exist for a few
> meters upto a major junction to help traffic flow etc
> 
> I think we actually need both, and use the right one for the logic of
> the road/junction.

+1

There is a very important point here: there's not a single unifying idea
that we can come up with that describes every situation. Ultimately, beauty
is in simplicity.

The example with the motorway is interesting, because on one hand, we think
of it as "the motorway from A to B", but when analysing its topology, it is
perhaps more correct to think of it as one road from B to A and another road
from A to B... that happens to be next to it. The distinctive feature is
that the two roads are actually not connected directly, i.e. you need to
travel along another road in order to reverse directions.

The second example you mention is on the other end of the spectrum. A split
road is from a topological standpoint a single road, since you could make a
U-turn on it (some places even without breaking the law) if you want to
reverse directions.

I think these are useful rules-of-thumb to keep in mind. In practice, there
are other issues, such as the number of T- and X- intersections etc. that
may influence how a particular segment of road is best mapped as simply as
possible.


Jean-Marc Liotier wrote:

> But although I'm learning the ways of OpenStreetMap, I'm learning by
> observing what others do and experimenting on my own. It works and in
> due time I'm sure I'll become a decent mapper, but that process could be
> more efficient. Some people on this list, such as you, seem to have a
> good grasp of the concepts. Would you be so kind as to start a wiki page
> that explains good practices and discernment in choosing what should be
> mapped as a tagged line and what really requires an area ?

In connection with a discussion we are currently having on the talk-dk list
about how to map cycleways I recently made a page on the wiki [0]. The
purpose of the page is to explore the different problems/benefits you
encounter when mapping cycleways as separate ways, vs. mapping them as
tagged onto the road itself.

These problems are similar to the current discussion on dual carriageways.

Making this wiki page was an exercise to enlighten myself and (hopefully)
others, and I learned a lot from making these diagrams. The page is in
danish, but you can probably grasp the meaning by looking at the diagrams.
If there's interest, I can translate the text to english.

(You can't use the diagrams as "best practice" examples, because some of
them are actually incorrect or "bad practice" :-) )

Wrt. the line vs. area problem, it's interesting, but a little bit OT in
this thread, so let's discuss it separately (so not to lose focus.)

Cheers,
Morten

[0] http://wiki.openstreetmap.org/wiki/Da:Cykelstier




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