https://www.fedlex.admin.ch/eli/cc/1962/1364_1409_1420/de#art_38 has the actual rules btw.

Thanks. It seems only paragraph 3 of that article (de: Art. 38 VRV, fr: Art. 38 OCR, it: Art. 38 ONC) is specific to mountain postal routes, as paragraph 1 is about steep streets in general and paragraph 2 has been revoked.

Here's what paragraph 3 says:

Auf Bergpoststrassen müssen die Fahrzeugführer bei schwierigem Kreuzen und Überholen die Zeichen und Weisungen der Führer von Fahrzeugen im Linienverkehr beachten.

Lorsqu’il est difficile de croiser ou de dépasser sur les routes posta­les de montagne, il faut suivre les instructions et les signes donnés par les conducteurs des véhicules publics en trafic de ligne.

Sulle strade postali di montagna, qualora l’incrocio o il sorpasso sia difficile, è obbligo attenersi ai cenni e alle istruzioni dei conducenti dei veicoli di linea.

This is in accordance with paragraph 2 of Art.s 45 (de, fr, it) and 67 (de, fr, it) SSV / OSR / OSStr:

Das Signal «Bergpoststrasse» (4.05) kennzeichnet Strassen, auf denen der Führer bei schwierigem Kreuzen und Überholen die Zeichen und Weisungen der Führer von Fahrzeugen im öffentlichen Linienverkehr beachten muss (Art. 38 Abs. 3 VRV). Wo diese Pflicht aufhört, steht das Signal «Ende der Bergpoststrasse» (4.06)

Für das Verhalten auf der Strasse verbindlich sind die Zeichen und Weisungen:
[...]
g. der Führer von Fahrzeugen im öffentlichen Linienverkehr auf Berg­post­stras­sen (Art. 38 Abs. 3 VRV)

Le signal «Route postale de montagne» (4.05) désigne les routes sur lesquelles les conducteurs doivent observer, lorsqu’il est difficile de croiser ou de dépasser, les signes et les indications donnés par les conducteurs des véhicules publics en trafic de ligne (art. 38, al. 3, OCR). Le signal «Fin de la route postale de montagne» (4.06) est placé là où cette obligation prend fin. ...

Les usagers de la route sont tenus de se conformer aux signes et instructions don­nés:
[...]
g. par les conducteurs des véhicules publics en trafic de ligne sur les routes posta­les de montagne (art. 38, al. 3, OCR);

Il segnale «Strada postale di montagna» (4.05) designa le strade sulle quale i con­ducenti devono osservare, quando è difficile di incrociare o di sorpassare, i segni e le indicazioni date dal conducente di veicoli pubblici del servizio di linea (art. 38 cpv. 3 ONC). Il segnale «Fine della strada postale di montagna» (4.06) è collocato dove cessa questo obbligo. ...

Per il comportamento sulla strada, hanno carattere obbligatorio i segni e le istru­zioni dati:
[...]
g. dai conducenti dei veicoli pubblici in servizio di linea sulle strade postali di montagna (art. 38 cpv. 3 ONC);

Because of that, I think a "give_way" or "priority" tagging as discussed in chat on #osm-ch would not accurately capture the implications for thusly signed roads.

I would as a tendency suggest

mountain_postal_route=yes

Sounds good to me.

But in discussion Manfred suggested combinations of existing tags, and an argument can be made that width or lanes=1 should be tagged on the road to make the relevant available outside of apps that are aware of the specific tagging.

If I remember correctly, some mountain postal routes I've seen aren't single-lane, but postal buses will (due to their length) have to use both lanes to get around tight curves where the road bends sharply, while two normal-size cars may be able to pass each other even there.

So I guess lanes=1 should be used (in addition to a new tag) if (and only if / where) the mountain postal route is actually a single-lane road.

If the actual width is known, that can be tagged (also in addition), of course. Though be aware that it can vary quite a bit in mountain roads, even when not accounting for passing places (Ausweichstellen).

Cheers,
Raphael (das-g)