I've come across a fair number of what I call obvious turn restrictions.
Here's an example: http://www.openstreetmap.org/browse/relation/1566983
The only thing being prevented by this is turning right onto Shelbyville
Road from the motorway_link. But there's a cutoff to the southeast that
you obviously take if you want to turn right.
So what's the point?
Any halfway-decent router will take the obvious route.
Even if a router doesn't do that, it should be clear to someone
following directions that to turn right they need to take the
island-separated right-turn lane(s).
The downside is that these relations lead to clutter, both in the number
of times a way is split and the number of relations on a way. To give
full restrictions at a simple intersection of two undivided roads with
four island-separated right-turn lanes, one would need to split each
road into at least four ways and create twelve relations.
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