Thomas (t-i) has a bicycle access map that a number of you may have already seen http://osm.t-i.ch/bicycle/map/
If you have a look at the map you will notice large areas of red: http://osm.t-i.ch/bicycle/map/?zoom=12&lat=47.49081&lon=8.06874&...
Complete forests that are forbidden for bicycles. The reason for this is widespread incorrect use of
access=forestry or access=agricultural
on these ways. This restricts ANY usage of the way in question to forestry and agricultural use, so even by foot entrance is not allowed.
What people are mostly tagging is the sign "Motorfahrzeuge verboten" in combination with "Waldstrasse" or "Land- und Forstwirtschaftliche Verkehr gestattet" see http://wiki.openstreetmap.org/wiki/Switzerland/Map_Features#Other_common_Sig... (note on the side: sign 2.01 http://de.wikipedia.org/wiki/Bildtafel_der_Verkehrszeichen_in_der_Schweiz should be tagged verhicle=....... ).
In principle it is naturally possible to tag access=forestry and then foot=yes, bicycle=yes, horse=yes and so on, but that is silly and error prone (and wouldn't cover usage for which OSM currently doesn't have a tag), the only sensible way to tag this restriction (which is universal by law in all of Switzerland) is motor_vehicle=forestry.
Correct tagging of this situation is important if we want to provide correct bicycle and walking routing, don't forget that there are (forest) areas in which access is really not allowed, these can only be correctly determined if we don't have to ignore wrongly tagged objects.
I've fixed some of this tagging in my immediate vicinity, and a lot of the access=forestry stuff, however there are roughly 3000 ways tagged with access=agricultural left of which a lot are in forests, where we can suspect the same mistagging has happened. The preferred way of handling this is for local mappers to correct the tagging, I would be prepared to change this in a conservative fashion in a massedit, however that will only so a 90% job and leave a lot of stuff that still will have to be individually corrected.
Simon