On Mon, 24 Aug 2009, Tobias Knerr wrote:
> Christiaan Welvaart wrote:
>>> http://wiki.openstreetmap.org/wiki/Proposed_features/Conditions_for_access_tags
>>
>> For example, with this proposal it is
>> possible to create both bicycle:backward and oneway:bicycle, while I
>> would really prefer to only have the former.
>
> If we don't try to abolish oneway completely, I would prefer the latter
> in most situations.
>
> My opinion is that a base key should not be able to remove a restriction
> introduced by another base key. For example, hgv=yes should not be able
> to remove a maxweight=3.5 restriction. Similarly, an access tag (such as
> access:bicycle:*=* or short bicycle:*=*) should not be able to remove an
> oneway tag.
Interesting, wouldn't it then be better to always use maxweight instead of
hgv, since AFAIK the only property of hgv is its weight?
> One example why I think oneway and access (including the transport mode
> and category tags) should not affect each other:
>
> In front of a station, there is a road that must not be used by motor
> vehicles except busses. This road also is an oneway road, with no
> exceptions. Therefore, I consider it natural to tag this
> - oneway = yes
> - (access:)motor_vehicle = no
> - (access:)bus = yes
> This can easily be understood if oneway isn't influenced by the other tags.
> If, however, we consider oneway=yes just another way of saying
> (access:)vehicle:backward=no, then we suddenly have a problem: Neither
> of the two conditional expressions "vehicle:backward" and "bus" is more
> specific than the other one, so we cannot determine whether the yes from
> "bus" or the no from "vehicle:backward" is relevant here.
This can be defined. As I described it one would have to write
bus:forward=yes , but people may indeed expect bus=yes to work.
> To sum up: Yes, both bicycle:backward and oneway:bicycle are
> direction-dependent restrictions for bicycles. However, they are still
> different because only oneway:* keys should be able to overwrite other,
> less specific oneway keys.
It is not clear from the text on the proposal page that
oneway:<transportation mode> is more specific than
<transportation mode>:forward ... It would be nice to have an explicit
description of how all the different tags can be evaluated.
One thing I don't like about using the oneway tag in complex situations is
that oneway works the opposite way of regular access restrictions:
oneway=no allows access in both directions, while access=no denies access.
This could be a reason why having *both* oneway:* and
access:*:forward/:backward is not such a good idea.
Christiaan
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