[Talk-GB] Maxspeed tagging for the UK

SK53 on OSM SK53_osm at yahoo.co.uk
Tue Apr 12 11:22:54 BST 2011

On 12/04/2011 09:38, Peter Miller wrote:
> On 11 April 2011 23:39, SomeoneElse <lists at mail.atownsend.org.uk 
> <mailto:lists at mail.atownsend.org.uk>> wrote:
>>             On 9 April 2011 08:15, Peter Miller
>>             <peter.miller at itoworld.com
>>             <mailto:peter.miller at itoworld.com>
>>             <mailto:peter.miller at itoworld.com
>>             <mailto:peter.miller at itoworld.com>>> wrote:
>>             ...
>>                We seem to be nudging towards something close to a
>>             conclusion.
>>                Can I suggest that the following two methods are
>>             valid, however
>>                the second one should be considered to be 'better' and
>>             where it is
>>                used then it should be retained to avoid edit warring.
>>             ...
>>                Method 2
>>             maxspeed=60 mph
>>             maxspeed:type=GB:rural
>>             source:maxspeed=survey
>     Great - someone has now changed a bunch of "maxspeed=national"
>     locally to me to to "maxspeed=60 mph".  Next I guess someone will
>     come along and add
>     "source:maxspeed=i_was_sat_in_my_armchair_and_it_seemed_like_a_good_idea"
>     or similar?
>     We've lost the information that the sign is actually NOT a 60 mph
>     sign.  Something like method 2 above would have avoided losing
>     information (although "GB:rural" is meaningless; if pushed,
>     "GB:national" or some variant would be better).
> The general conclusion of the discussion above was that where 
> maxspeed=60mph is applied to a single carriageway road there is also a 
> default 'maxspeed:type=GB:unrestricted' (or whatever value is decided 
> on). This default (and the one for 70mph for motorways and 
> dual-carriageways) was including to avoid burdening the mapper with 
> another tag to add in most situations. The only 60 mph signs that need 
> another tag are those rare cases where a single carriageway road does 
> have a numeric speed limit.
> Fyi, about 95% of currently mapped speed limits in GB at speeds of 
> 60mph and 70mph speed limits were already tagged as 'maxspeed=60' and 
> 'maxspeed=70' when I first looked at this about 4 weeks ago leaving 
> only about 5% tagged as national or nsl.
> I have been converting this remaining 5% over the past 2 weeks (with a 
> brief delay while we discussed the principle on talk-gb after a 
> reversion of one of my edits). I have had no complaints from others to 
> my changes and only one reversion of one section of the A1 as I 
> mentioned in my post. I take this as broad support for the changes.
> By tomorrow there will be next to no remaining 'national' and 'nls' 
> speed limits in Britain other than in your patch around Macclesfied 
> which I won't touch any more.
> There are also a small number (another 5%) of roads that are not in a 
> recognised mph format, either because the mph is missing or because it 
> is in km/h or for some other reason. I will be doing a copy-edit pass 
> on these either fixing them if it is obvious or marking them with a 
> fixme:maxspeed tag if not. I should be finished with that in about a week.
> Regards,
> Peter
>     Cheers,
>     Andy
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I think there has been a discussion: I'm a bit surprised that there was 
also an agreed consensus, let along one which justifies mass edits. And 
if pushed I would have said that the consensus was maxspeed=national.

The discussion never pursued a number of outstanding issues. The most 
important of these being whether it would be useful to identify dual 
carriageways in general, rather than specifically for identifying speed 
limits (I believe that it would, the post-processing effort is high and 
there are sufficient anamolies to make it difficult to identify all 
satisfactorily). Others relate to relevant speed limits for different 
classes of vehicles, and finding suitable names for the additional tags.

I was unhappy with the original mass edits which added unnecessary fixme 
tags and other curious tags to roads. To compound this with assuming 
that further mass edits would be acceptable seems way over the top.

Suitable renders make a big difference by showing what is missing and 
encouraging people to be more proactive in mapping them. Andy Allan's 
new experiemental transport layer is a big bonus in that regard. Using 
such a render to drive tagging is less desirable, simply because it 
results in 'tagging for the renderer' type behaviour such as the 
creation of ways to supress display in the ITO OSM Analysis layer.

I've not pitched my oar in until now. I had been quite happily using a 
numeric value for maxspeed, but the discussion on this list showed me 
the error of my ways. The main reason I'd used a value was a misguided 
belief that it would improve the times calculated by routers. I've been 
playing around with various OSM based routers, and they dont seem to 
make sophisticated use of this information. My impression is that most 
place speeds into buckets, and that they make assumptions along the 
lines of JOSM that a trunk is equivalent to a dual carriageway motorroad.

My summary of what seemed sensible would be:

     maxspeed=national or maxspeed=gb:national  /* with former preferred */
    dual_carriageway=yes (or something similar)   /* preferred over 
variants which just refer to speed */
    national=70 mph (or variants on this)                  /* xxx:type=* 
tags are horribly ambigous */
    source:maxspeed=survey                                   /* don't 
change the meaning of the source namespace */

It is important to keep the basic tagging required as simple as 
possible, maxspeed=national can be added as a preset to things like 
Potlatch with the appropriate sign. The need to add several additional 
tags, adjectival tagging, namespacing should be just that: optional not 
essential in tagging. Outside of people who write programs, who knows 
what a namespace is?

Like it or not OSM tagging will always result in inconsistent values for 
attributes. Inisisting on a data type for tags has in the past resulted 
in horrors like maxspeed=80.467.

Lastly, never assume that absence of messages is approval or 
acquiescence. It is always best to follow up, with "in a few days time I 
plan to perform edits x, y, z: please let me know if you have 
objections". Then do edits in a limited area, to see if you get further 
objections (in this case you have Andy & Steve raising issues).

I recently became aware that the Data Working Group have a draft policy 
in this area: 

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