Edit: I edited the second half of this post about 1.5 hours after posting as I initially confused CAs and CMAs. My apologies.
Yes, and the route to its major CMA (Quebec City) is a motorway, so even higher than a trunk.
The question is whether the route between Saint-Georges and the Maine border should be a trunk, and that is much less clear to me, since the ties between Saint-Georges and any Maine metropolitan areas appear much weaker due to: 1. shared cultural and administrative ties with Quebec City instead, 2. international border in the way, and 3. sheer distance.
The other interpretation could be that a route between Quebec City and Maine’s metro areas is of enough transit importance to warrant a trunk (in that case we’d have motorway on autoroute 73, and trunk on route 173). Again I have my doubts. The fact that it was not included in NHS core, unlike e.g. Ontario’s Highway 71 to Fort Frances border crossing, suggests to me that that’s not the case. But I’ll happily defer to Quebec editors on that subject.
hoserab linked to the talk-ca discussion thread when the wiki draft version was introduced, that should give you some ideas.
On a higher level, there’s two approaches to road classification below motorway in OSM:
- Classifying based on built form. For example in Poland
trunk is used for expressways that are of slightly lower design than motorways. Gaps in these expressways, no matter how important or busy, are tagged primary, as with almost all other national roads. In this scheme, network importance is indicated by important roads belonging to the proper classification. This depends on this classification being sensible, i.e., all important roads really are national roads.
- Classifying based on network importance. For example, deciding that main roads connecting Census Metropolitan Areas are to be
trunk or higher, or deciding that NHS core routes are all trunk or higher. In this case, even a two-lane highway can be a trunk. In this scheme, built form is indicated by tags like expressway=yes.
As I understand it, in Canada so far we’re largely using the network importance schema, not the least because cross-country routes like the NHS and Trans-Canada really are important, even though they might be a thousand-kilometre two-lane highway with traffic volumes in low thousands per day. Then the problem is deciding which roads are important to the network, considering that we don’t have a true national system, and some provinces’ classification can leave something to be desired (ask me about Ontario’s Connecting Link).
Personally I think that using the criteria: 1. all NHS core routes are at least trunk, and 2. roads connecting CMAs are at least trunk, results in a fairly good network in Ontario and Quebec. My understanding was that this is essentially what was settled on in the talk-ca thread and the wiki discussion for Ontario, since these are basically the only objective definitions of “highways of national importance” and “population centres of regional importance” that we have. In Ontario, all centres of regional importance is connected by motorways or trunk right now, and I don’t see any important routes that should be trunk but aren’t.
(There are currently some deviations in Ontario from these guidelines in case of city bypasses, for example Caledonia Bypass or Veterans Memorial in London or Highway 26 outside Collingwood. Personally I don’t like these, and think they should be primary and expressway=yes if appropriate. But my interpretation of the main argument in favour is that they have implicit network importance by being designed to take through traffic, and I find that convincing enough to not get into edit wars about them.)
To summarize: route 173 between Saint-Georges and the U.S. border is neither an NHS core route nor connecting to a nearby population centre, thus personally I don’t see why it should be upgraded to trunk.
It makes perfect sense if the road from St-Georges to the border isn’t all that important.
