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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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