Ah, now I understand what you said.
Some thoughts:
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Where would we get information about the road types? Google Maps has each road number with the appropriate color, but of course we can’t use it as a source.
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Looking on Google Maps, it appears that the road color is almost always what you would expect from the number (2-digit=red, 3-digit=green, 4-digit=brown). The main exceptions are freeways. But freeways are already an exception in the current system.
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So the main difference between your suggestion and the current system is that you move everything up one class (primary->trunk, secondary->primary, tertiary->secondary). The benefit of this is that there is no “gap” with no roads classified as “trunk”. One loss is that the higher classified roads use too much space and stick out in Mapnik. A bigger loss is that relatively small roads outside cities are marked as more significant than big roads in cities, which is bad for route calculation. Overall, I’m not sure the change is worth it.
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And there is something useful we can do with “trunk” - we can use it for road that are “almost but not quite freeways”, like 471 and 531, and 4 near Raanana. This means some interchanges on the route, but some intersections too. This is the convention for the US for “trunk”, while Canada and South Korea have similar conventions.
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I don’t think there should be any debates about what is a freeway - a freeway has interchanges and no traffic lights, a non-freeway has traffic lights.
So my suggestions for non-urban roads:
MOTORWAY - all freeways (interchanges, no traffic lights)
TRUNK - near-freeways (some interchanges, some traffic lights)
PRIMARY - 1/2 digit roads (except motorway/trunk)
SECONDARY - 3 digit roads (except motorway/trunk)
TERTIARY - 4 digit roads (except motorway/trunk)
And for unnumbered urban roads:
MOTORWAY,TRUNK - as above
SECONDARY - very large urban roads (generally around 1 per 100000 inhabitants)
TERTIARY - all significant urban roads, except for above