Currently there are two ways to limit the vehicle types a relation
tagged as `type=restriction` (turn restriction) affects. The first one
enables the restriction for *all* vehicle types, but then excludes some,
like:
restriction=no_left_turn
except=bicycle
The second one lists the vehicles for which the restriction should be
enabled explicitly using something like
restriction:hgv=no_left_turn
Both ways seem to be useful and also represent what occurs on real
street signs. For example there are some that restrict turns only for
hgv vehicles and others that restrict all vehicles with an exception
like bicycle.
According to this schema one *could* use both approaches in a less
obvious way, i.e. use the restriction tag when the turn is only
restricted for a certain vehicle type like:
// here we like to restrict only hgv
restriction=no_left_turn
except=motorcar;moped;psv;bicycle;
or use the restriction:vehicle tag when the turn is restricted for
almost every vehicle type like:
// here we like to restrict everything except bicycle
restriction:motorcar=no_left_turn
restriction:moped=no_left_turn
restriction:psv=no_left_turn
restriction:hgv=no_left_turn
Quite obviously this isn't ideal and as far as I can tell this is the
exact reason we have the two approaches (one for excluding vehicles and
another for including them). So using the right approach depending on
the situation (or better depending on what is actually stated by the
traffic signs) should be encouraged.
In some very rare cases even both approaches are used simultaneously.
This leads to either redundant information or, even worse, contradicting
tags.
I think the wiki
(https://wiki.openstreetmap.org/wiki/Relation:restriction) should state
that the restriction tag should not be used in combination with the
restriction:vehicle tag. Is there any reason not to do this? For the
except and restriction:vehicle tag maybe there should be a short note
that these should be used when the restriction is valid for all but a
few or only a few vehicles, respectively.
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