Hi Bodo,
I think Garmin is using some kind of hierarchical routing algorithm. It
looks like GPS uses only class 4 and 3 roads for long distances (at
least in case of recent nuvis, hiking GPS can be more flexible). For
proper routing GPS needs a continuous and reasonably dense notwork of
high class roads.
My patch address 2 problems in default style.
First are links and roundabouts, which often get lower road class than
connected roads. This breaks continuity of high class network and breaks
routing. My patch simply increases road class for many links and
roundabout types.
Second problem is low density of good highways in some regions. To make
it better, my patch increases road class for international roads. These
roads are detected by "ref" and "int_ref" tags, both in objects and
relations.
I have added some small general corrections for
"highway=mini_roundabout", "access=hov" and "oneway=reversible".
There is no direct change for "inc\roadspeed". Maybe patch overrides
some of your changes in "lines" file? Anyway, I don't think road speed
breaks routing, you can tune it to your liking.
--
Best regards,
Andrzej
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