Ian,
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.
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.
An interesting contrast between the UK and Holland (maybe other
countries as well), correct me if I'm wrong:
* 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.
* In Holland, the offence is to disobey a sign; if it's not signed
(properly), you haven't committed an offence.
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.
Colin
On 02/11/2010 02:50, Ian Spencer wrote:
Colin Smale wrote on 01/11/2010 19:32:
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
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.
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.
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.
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.
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.
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.
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.
FWIW, Tom Tom maps speed changes pretty accurately, though it is not
completely reliable, but the competition is pretty high.
Spenny
(currently pointless in mpre ways than one!)
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