Highways within urban areas

I am trying to help map Minneapolis, (Minnesota, United States), and suburbs.

I am having a little trouble classifying highways. Consider Olson Memorial Highway, for example: it is a divided highway without ramps, and speed limits of 40-55mph. It is thus too slow to be a trunk highway, and too divided to be a primary highway. The problem as I see it is that trunk highways as defined, (in http://wiki.openstreetmap.org/index.php/Highway_tag_usage, for example, or http://wiki.openstreetmap.org/index.php/Mapping/Features/Road)), simply don’t exist here. There are no highways having speed limits of 65 mph or greater in the urban area: the possible exceptions (in far suburbs) all have ramps.

So, here is how I’m thinking of classifying highways:

  • United States interstate highways are classified as motorways, no question.

  • County highways must be classified as at least secondary roads, presumably, (even those which are merely busy city streets), since tertiary roads don’t seem to support the ref tag.

  • For other highways, I suppose:

    • Local highways which are functionally equivalent to United States interstate highways, (divided highway, ramps, speed limits of at least 45 mph), will be classified as motorways.
    • Other highways will be classified as trunk highways, (divided), or primary highways, (undivided).

Sound reasonable?