[OSM-talk] talk Digest, Vol 64, Issue 20

Richard Bullock rb357 at cantab.net
Fri Dec 4 02:47:43 GMT 2009


> 2) There is a divided road that has been sketched out roughly, simply
> to indicate the division. (Very common, I think) Converting this to a
> simple divided=* tag doesn't lose information, and better indicates
> the actual level of information stored.

> Here's one that looks like 2):
> http://www.openstreetmap.org/edit?lat=-37.77825&lon=144.830416&zoom=19
>
> There are two lanes to indicate the divided highway, but the mapper
> hasn't marked out the cut throughs, so this results in worse mapping
> than if they'd just made a single road. - the router can't tell that
> you can turn left across the median strip.
>
> If you follow that road south a bit, you'll find cases of 3 and 2:
> painted lines, some of which are marked as separate lanes, some
> aren't. IMHO it would be cleaner to represent all of those with a
> divided=* tag, but tthat's just my opinion.


Right; If you find a road which has been sketched out very roughly - and you 
have excellent NearMap imagery; then perhaps the best solution is to draw it 
in more accurately...

Note that if you follow your 2nd example further south, you'll reach two 
junctions with service roads into the Cinema car park.

http://osm.org/go/uGy6p24ha

The northernmost junction only allows traffic heading South on the road to 
exit to the left, towards the car-park - and traffic entering the car park 
to exit onto the road heading South. Northbound traffic on the road cannot 
turn, traffic from the car park cannot exit to the North.

The southernmost junction allows traffic heading in both directions to enter 
the car park - and allows traffic from the car park to exit in both 
directions.

Using single ways to represent the dual carriageway doesn't distinguish 
between these two junctions - but the possible movements are different - the 
topology is not the same. Using turn-restrictions is not ideal as 
restrictions are really designed for physically possible turnings which are 
legally forbidden - so for instance, an emergency vehicle might be able to 
make the turning - but an ordinary 'civilian' driver could be fined for 
doing so. Mapping as two ways removes any possible doubt here. For the 
northernmost junction, the north-bound carriageway of the dual-carriageway 
has no shared nodes with the access road to the car park - so there is no 
need for any "fudge" of the routing to stop it using those roads.

It's surely a lot simpler to map out separate carriageways as separate 
ways - it really doesn't take that long - and I can't believe there are so 
many to map that the task is overwhelming to map two ways. There are 
probably enough messages and replies on this topic that you could have 
mapped several already. If you don't have two carriageways; some roads have 
something in between the two carriageways - e.g. part of a footpath, there's 
another road (not motorway) between the two carriageways of the UK's M6 for 
a short section in Cumbria. 





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