[OSM-talk] Indiscrimate layering

David Earl david at frankieandshadow.com
Sat Dec 20 17:46:53 GMT 2008


On 20/12/2008 17:25, Scott Atwood wrote:
> I'm guessing you don't map in many heavily urbanized areas then, because 
> around where I live, freeway interchanges with two, there, four, or even 
> more layers are not uncommon, e.g:

Indeed I don't. And these multilevel structures are pretty uncommon even 
in urban areas and on the motorway network in the UK - instead a typical 
model here is a grade separated roundabout which involves just two 
bridges (not on top of each other), e.g:
http://www.openstreetmap.org/?lat=52.34052&lon=-0.20714&zoom=17&layers=B000FTF
(sometimes two roundabouts and one bridge 
http://www.openstreetmap.org/?lat=52.329684&lon=-0.192102&zoom=18&layers=B000FTFTTT 
  ). Even at the intersection of two motorways, the roundabout is 
usually the link between the two rather than bridges on bridges (e.g. 
http://www.openstreetmap.org/?lat=52.86544&lon=-1.56859&zoom=16&layers=B000FTF 
which looks to me like it needs some bridges marking. IIRC the 
roundabout is on top, but it may be the A38).

No doubt this is because it is expensive to build bridges, especially 
the soaring structures found in the US, and also because Americans have 
  never really got the hang of roundabouts (sorry, "traffic circles").

Not that we don't occasionally have them here, e.g. 
http://www.openstreetmap.org/?lat=51.59227&lon=0.03767&zoom=16&layers=B000FTF
(though even here, there's a roundabout at the bottom!)

But they are sufficiently rare that the vast majority of Britons will 
know exactly what you mean if you say "Spaghetti Junction" (the junction 
of the A38(M) and M6 near  Birmingham): 
http://www.openstreetmap.org/?lat=52.51121&lon=-1.8626&zoom=16&layers=B000FTF
(note, even then, there's still a roundabout!)

However, interesting as these cultural differences may be, I don't think 
that changes my point. There are more bridges anyway in these areas, and 
most of them are common-or-garden road passes over stream type things 
which in many cases aren't even marked as bridges.

David




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