Thanks Phil. I have to say although I live in UK, I've never noticed
until now that some A roads are green and others are black.
Incidentally, would this information be gleaned just by driving around
on the ground and looking at road signs, or is there another source of
classification?
Sebastian
On 16/12/12 23:58, Philip Barnes wrote:
Hi Sebastian
Thanks for the extra info. I will download a fresh copy of maps for
Navit with the restrictions in place to make sure that junction works as
expected. I kind of figured out Navit's navigation must have thought
that route would have been shorter or better somehow - but in real life
it wouldn't have been a viable one.
Routers do seem to overuse road classification, it works with motorways
but in built up areas classification is largely irrelevant. In the
example you gave Navit is assuming that the trunk road is faster, even
if its just by a second or two. In this case it wont be faster. Even if
it was legal, there is a 90 degree turn.
I must admit I haven't figured out yet the business with trunk roads - I
always sort of assumed based on OSM's wiki that "A" roads in UK are
primary highways - clearly I must go back and read the OSM highway
classification again.
You do need to read the OSM wiki page, WikiProject United Kingdom A and
B Roads at
http://wiki.openstreetmap.org/wiki/WikiProject_United_Kingdom_A_and_B_Roads
Quote from the page:
In the UK, the OSM rule goes like this:
green signed A roads: highway=trunk
black-and-white signed A roads: highway=primary
B roads: highway=secondary
(It might sound confusing if you're used to hearing A roads described
differently. Sorry, it's just how we do things round here.)
/end quote
It doesn't quite help the fact that tertiary
highways correspond to "C" roads, and secondary highways correspond to
"B" roads - thus living primary highways without a direct correspondent
in UK.
Tertiary roads are a bit of a black art, there are very few C roads
numbered on the ground and unless you have access to council info you
will not easily get hold of the numbers.
I tend to tag the main unclassified roads through an area as tertiary,
the ones that are used by through traffic, but I may be wrong.
Regards Phil
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