[OSM-talk] [Tagging] - RFC -Motorway_linkimpliesoneway=??

[Philip Homburg]

In your letter dated Wed, 08 Oct 2008 12:25:53 -0400 you wrote:
>Philip Homburg <pch-osm-talk at u-1.phicoh.com> writes:
>> But seriously, what I expect is a set of rules of the form "if in-germany
>> then highway=trunk implies oneway=yes".
>
>This leads to a nightmare.  Those rules would need to be implemented
>in every tool that works with OSM data (and cares about oneway
>properties).

And? All code also has to have an XML parser.

I think it can be just a library that gets as input a way (or a node) and
returns a set of default tags.

>I am for the simple aproach: If it is oneway - tag it as oneway.

I've no problem with that. Just that the absence of the oneway is taken to
be 'oneway=no'.

Anyhow, I think that three types application applications that need to know
about defaults:
- rendering engines, 
- consistency checkers.
- routing programs

For rendering engines it is not all that important if the defaults are not
exactly right (we don't really need any arrows in dual-carriage roads, most
people know they are oneway anyhow).

Consistency checkers are complicated anyhow. And may also have a lot of
heuristics.

>If we really must have implied values for certain properties, keep
>that simple as well and don't make in dependant on too many tags
>(like: it's oneway if it is in Germany, but not in Saxony or on an
>island and only if the speed limit is above 80 km/h, or, if it is in
>France and ...) [I know I am exaggerating.]

And the third class, the routing programs, really needs information about
speed limits. And that is going to require some kind of localization. 
And once you got the mechanism in place, you can just as well use it for
all kinds of access restrictions.