I think that this cannot be discussed here. This has to be done locally. In Belgium, primary roads are roads with a reference in the form of an "N" or "R" followed by one or two numbers. The reference of a secondary road contains one letter and three numbers and a tertiary has no reference but has at least two lanes. Speed limits can go from 30 kmh to 90 kmh.
A residential road is one in a village or city center with one lane, where there is a speed limit of 50 kmh or less and an unclassified road is a road with one lane in the countryside. Unclassified roads have a speed limit of 90 kmh but only very good (or very bad) drivers can reach that limit. This tagging schema may be rubbish in other parts of the world, but it works good in Belgium. So what I suggest is to put clear links to the local communities on the highway page. And give a good local definition of those highway types on the local community pages.
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