[Tagging] Key: turn

[Johan C]

The key:turn <http://wiki.openstreetmap.org/wiki/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. 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.
Since the routers can calculate these instructions based on a turning
angle, it's somewhat strange to use values for it.

A second rationale could be to help the router in displaying a lane assist.
But, lane assist is correlated with traffic signs. So through, right and
left are perfect values for a lane assistant. But slight_left/right and
sharp left/right?

So, I can't figure out a proper rationale for these four values. Is there a
rationale for these four values?
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