[Talk-GB] Visualising speed limits

[Colin Smale]

On 01/11/2010 19:36, David Earl wrote:
> On Monday, November 1, 2010, Andy Allan<gravitystorm at gmail.com>  wrote:
>> On Fri, Oct 29, 2010 at 9:52 PM, Colin Smale<colin.smale at xs4all.nl>  wrote:
>>> On 29/10/2010 22:22, thomas van der veen wrote:
>>> You might like to take note that nothing is implicit in OSM. There are no
>>> "defaults" as renderers or other consumers of the map data are unconstrained
>>> as to how they handle missing tags. Ideally all roads should have maxspeed
>>> explicitly tagged, even if it could be implied from the road class or other
>>> information.
>> I disagree quite strongly on this. I see little point in pointlessly
>> adding tags when they there is a sensible default. For example, we
>> don't need to tag 98% of roads as "oneway=no", and in the same way we
>> don't need to tag 98% of UK residential roads as "maxspeed=30mph".
>>
>> Life's too short for that kind of tedium. That's why we (actually do)
>> have implicit information in OSM.
> +1
>
> David

I agree with you both too. I was merely stating (for the benefit of a 
new mapper) that there is no working system of defaults. It would be 
very helpful if such a thing were to exist, but right now, there are no 
defaults. "oneway=no" is a bit of a no-brainer but the majority of 
"unwritten rules" ("implicit information") are not quite as obvious. In 
some cases, the wiki states that certain tags may be implied from others 
(e.g. highway=motorway implies oneway=yes) but these are, in general, 
poorly documented and information is fragmented and not easy to find. A 
welcome exception is the wiki page covering access tags for different 
highways[1]. Renderers or other "users" are just as much free to do 
their own thing as mappers are. The only thing that stops the whole 
shooting match descending into chaos is a set of agreements between 
producers and consumers. If it's not written down, and communicated 
appropriately, it might as well not exist. If you want to be 
unambiguous, use an explicit tag.

So why not start documenting all these defaults or implied values? 
Here's a few suggestions to get the ball rolling.

highway=motorway    implies oneway=yes, lanes=2
highway=*, oneway=no    implies    lanes=2
highway=* AND lanes>=2 AND oneway=yes  implies maxspeed=70mph
highway=* AND lanes>=2  AND oneway=no  implies maxspeed=60mph
highway=*    implies maxspeed=60mph
highway=residential    implies maxspeed=30mph
junction=roundabout    implies oneway=yes

Colin

[1] 
http://wiki.openstreetmap.org/wiki/OSM_tags_for_routing/Access-Restrictions