[OSM-talk] zones for motorway/in town/outof town?

[Radomir Cernoch]

Florian Lohoff píše v St 20. 05. 2009 v 21:39 +0200:
> On Wed, May 20, 2009 at 11:59:05AM +0100, Radomir Cernoch wrote:
> > 1) Every road is by default 'rural' road (speed limit 90 km/h).
> > 2) Every highway has speed limit 130 km/h by default.
> > 3) If a road is inside a polygon tagging the city limits,
> >    then its speed limit is set to 50 km/h. Such polygon might be tagged 
> >    by 'place', but the actual name of the tag is not important here.
> > 4) If a road has different speed limit from rules 1) - 3), it is tagged
> >    with 'maxspeed=...'.

Hi,

I'm not sure whether I completely understand you. Please correct me, if
I misinterpret you...

> The problem with implicit speed limits is that one day someone will rely on
> your output and probably just someone only forgot to map at that end of the
> road. The best thing to do is to make maxspeed/zone explicit only.

The idea is not to create a node "here starts city" and a node "here
ends the city". The idea is to create a polygon which defines the border
of 50 km/h speed limit. The problem of forgotten "end node", which
causes cities to leak all over the planet, does not apply.

> The advantage of a traffic "zone" to maxspeed is that one can derive the
> source of the speed limit (and possibly other limits)...

By "traffic zone" you mean a tag "zone=XXX" on each individual road in
the city? I'm afraid that such system is very prone to forgotten tags.

If you use a polygon around a city, you can also derive the source of
the speed limit. Rules 1) - 3)  mean that being "on highway"/"in
city"/"outside city" is the main limitation. Applying rule 4) means
there is a physical sign that causes the regulation.

> ...In Germany we have a very
> widespread "zone 30" culture where not all streets have individual speed limits
> but rather the sign limits the speed until you pass the end sign - probably on
> a very different road.  Much like city limit signs.

Yes, in the "polygon" system you would encircle the whole area and every
road inside of it has automatically 30 km/h limit.

> The problem now is that you cant actually work with polygons as for
> example a motorway could pass over a zone-30.

Sorry, I maybe didn't make myself clear. "Polygon" rules do not apply
for motorways. Is there any country, where a highway inside a city has
different speed limit from the highway outside of the city? Even if yes,
this can be specified in the set of country-specific rules...

I can imagine a situation, where a normal 50 km/h road goes through the
middle of a zone-30. Then there are two options:
1) You split the zone-30 polygon into 2 polygons.
2) You tag the 50 km/h road with "maxspeed=50".

Yours,
Radek