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Ian,<br>
<br>
You're right, my detailed knowledge of these things is definitely
below par! I spend too much time outside the UK to follow all this.
My "suggestions" were more-or-less intended to catalyze a discussion
so I am not surprised they are not correct.<br>
<br>
I think your post proves the point I was trying to make. Defaults
and the rules surrounding their use can be complex. If the rules are
simple enough and can be documented, we might get away with it, but
then some of the subtleties might get optimised out of the rules
leading to incorrect inferences if mappers don't put the subtleties
back in with explicit tags. So once again I will make a plea for
documenting the defaults very clearly, and/or explicit tagging. And
to be clear, I am *not* talking about adding oneway=no to almost
every way.<br>
<br>
An interesting contrast between the UK and Holland (maybe other
countries as well), correct me if I'm wrong:<br>
* In the UK, you commit an offence (e.g. parking where it is
forbidden) based on a local authority by-law, although the absence
of decent signing could be a valid mitigation.<br>
* In Holland, the offence is to disobey a sign; if it's not signed
(properly), you haven't committed an offence.<br>
<br>
A significant difference, which leads to more consistent, more
explicit, less confusing signage in NL, without having to e.g.
measure the distance between street lights. The "built-up area"
starts where the sign says it does.<br>
<br>
Colin<br>
<br>
On 02/11/2010 02:50, Ian Spencer wrote:
<blockquote cite="mid:4CCF6E7C.6030700@gmail.com" type="cite">
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<br>
<br>
Colin Smale wrote on 01/11/2010 19:32:
<blockquote cite="mid:4CCF15B2.2020303@xs4all.nl" type="cite">So
why not start documenting all these defaults or implied values?
Here's a few suggestions to get the ball rolling. <br>
<br>
highway=motorway implies oneway=yes, lanes=2 <br>
highway=*, oneway=no implies lanes=2 <br>
highway=* AND lanes>=2 AND oneway=yes implies maxspeed=70mph
<br>
highway=* AND lanes>=2 AND oneway=no implies maxspeed=60mph
<br>
highway=* implies maxspeed=60mph <br>
highway=residential implies maxspeed=30mph <br>
junction=roundabout implies oneway=yes <br>
<br>
Colin <br>
<br>
</blockquote>
<br>
Someone has not been on their speed awareness course, have they?
(ahem!) 70mph is based on dual carriageway. A dual carriageway
does not need two lanes to qualify, and this is more common these
days as you find that on certain dual carriageways a lane has been
blanked out for some distance. <br>
<br>
Also, to nitpick, your implications are not all correct,
oneway=true and lanes >=2 does not imply any sort of maximum
speed, as any decent one-way system in a town would match this,
and you cannot rely on a trunk road to identify this as truck
roads often run through built up areas. In other words, there are
sensible defaults, but you cannot imply speed limits as easily as
you suggest from the tags you present there. Similarly, although
again a sensible default, residential roads with 40mph or more are
not uncommon.<br>
<br>
However, that is pedantic, and I'd agree in principle with
defaults being sensible, indeed road signage in the UK is based on
the principle that you can infer the speed limit from the presence
or absence of street lighting if there are no contradictory speed
limit signs (motorways being an explicit exception to the rule).
This has saved me from a speed camera incident or two in the past.<br>
<br>
So to go down a proper mapping exercise to determine actual speed
limits, we should be mapping the limits of street lighting as that
is the relevant attribute, though to be fair, I can barely recall
an example where a council has relied on the presence of street
lighting alone to control traffic speed.<br>
<br>
Changes in legislation a few years ago make understanding this
implicit speed limit more relevant, as it is now no longer a
requirement to signpost increases in speed limits in certain
scenarios, not is it a requirement to have signs either side of
the road in all cases and your only clue might well be that you
see a repeater sign somewhere up the road - the theory being that
when you exit a minor road, you carry on at the same limit until
you are informed otherwise (e.g. exiting on an unlit country lane
which was 30 mph onto another unlit country lane you might find a
repeater sign a bit down the road saying 40mph without an
intervening speed limit sign to show the increased speed. So a
little bit of care is needed in mapping speed limits to ensure you
map based on the subtleties. <br>
<br>
Of course, these speeds do not apply to vans which are not car
based, nor buses and lorries. A surprising number of white van men
drive at a licence losing 30mph over their 60mph speed limit on
dual carriageways and 50mph on single carriageways; and perhaps
you should not curse the lorry on a typical single carriage A road
where they are limited to 40mph by law, 50mph on a dual
carriageway. It gets more interesting for lorries on a motorway,
because although they are allowed to do 60mph in terms of speed
limit, lorries now have to have a speed restrictor and digital
tachograph that limits them to 56mph to comply with EU legislation
so this is the de facto limit.<br>
<br>
So any maxspeed needs to account for the type of vehicle, (or it
should be a code). This is even more the case in Europe where it
is common to see sections of autobahn or trunk road where the
speed limit is explicitly varied by type of vehicle.<br>
<br>
FWIW, Tom Tom maps speed changes pretty accurately, though it is
not completely reliable, but the competition is pretty high.<br>
<br>
Spenny<br>
(currently pointless in mpre ways than one!)<br>
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