[OSM-newbies] Highway Types and Speed Limits

[James Ewen]

On Wed, Oct 14, 2009 at 4:53 PM, Morten Kjeldgaard <mok at bioxray.au.dk> wrote:

> I see, obviously you are right in that case. However, here in Denmark,
> we have 3 general speed limits (that may be overruled locally by signs):
>
>   - Inside city limits: 50 km/h
>  - Outside city limits: 80 km/h
>  - Motorways: 130 km/h.
>
> So here, the classification scheme would work quite nicely.

As well as in Canada, by law...

Urban areas 50 km/h, rural areas 80 km/h unless otherwise posted.

There are other default values that are by convention, not law.
Primary highways are normally 100 km/h, restricted access motorways
110 km/h.

Default values does not mean that that is the only value that the way
can have applied. What a default value does, is give a value that can
be used in the absence of an explicitly defined value. If you do not
have a default value, then you have to assume a null value. For speed,
that would be zero. That makes for calculating an ETA pretty
difficult.

We have major arterial roads in urban areas that are posted at 60, 70,
80, 90, and 100 km/h. We have highways posted at 80, 90, 100, and 10
km/h. We also have residential roads posted at 40, 30, 20, 15 and 10
km/h... just because there is a default value associated with a road
type does not mean that you can not tag it with another value.

A default value means that you do not have to explicitly assign a
value to thousands of miles of roadway that are posted at the default
value. You only need assign an explicit value to those segments of
roadway that are posted at a value other than the default.

No default value means that every single way HAS to be tagged.

One thing that can be seen rapidly though, is that default values vary
by jurisdiction. We need style sheets that can be applied to the map
by area. We've talked about style sheets which would allow changes in
the way roads are rendered based on zoom levels on the Talk-CA
reflector. The main OSM slippy map is designed to make nice looking
maps for a very dense area such as Europe, but when you get to the
Canadian prairies, we get a really blank slate, since most of our road
grid is low level priority as per the European definition, but they
cover hundreds of miles of area. To be able to see the roads, you have
to zoom in so close that you can't tell where the road goes.

James
VE6SRV