[Tagging] Key: turn

[Philip Barnes]

On Wed, 2012-12-26 at 12:57 +0100, Johan C wrote:
> The key:turn has values for slight_left/right and sharp left/right.
> There are no signs associated to these four values. I also couldn't
> find any corresponding signs on this page:
> http://en.wikipedia.org/wiki/Comparison_of_European_road_signs. The
> rationale of these values is missing on the key:turn page. What is the
> rationale?
> 
> 
> One rationale could be to help the router in the following
> instructions: turn sharp right, turn right, bear right, take the exit
> right However, these values should then have to be applied on every
> turn. And routers do have a calculation for these: turn sharp right =
> turn angle < 90 degrees, turn right = about 90 degrees, bear right = >
> 90 degrees, take the exit = the _link on motorway/trunk exits. 
In terms of restrictions, they are relations of ways you cannot pass
between of must pass between. Am not sure that no right or no left turn
makes any difference. 

In terms of voice directions, I have long since switched them off and
just use an occasional glance at the screen. Turn instructions can often
be confusing.

Is slight left the road at 20 degrees or the one at 40 degrees, easy
when you look at the screen.

Again take the 3rd exit from a roundabout can be confusing, without
knowing how far around it is, and therefore which lane to be in. 3rd
exit, may be anything between 90 degrees, or a full 360 degrees. You
need to see a picture, as they say a picture paints 1000 words.

> I suppose routers also use streetnames, to determine automatically if
> there's a turn onto a different street since nobody likes to hear
> 'straight on' too often when there's a slight angle in the road ahead.
One problem here is the straight ahead road often changes name, but the
road name you are on takes the side road turning. This often happens
where sections of road are bypassed.

There is a relation, type=through_route, which allows a mapper to
specify which road is the through route and specify turns better. I have
not managed to use it yet and am not sure how widely supported it is,
but it does seem the way to go.

I have certainly missed turns, because a satnav has ignored a sideroad
that is straight ahead, and I have continued on the main road.

Using street names is not especially helpful in navigation, in an
unfamiliar area you will not know the names and street signs are often
difficult to read, or even find when driving. It may be behind the turn,
high on a building or anywhere. 

Phil