National tier routes can be included, but I'm looking for state/provincial/territorial level routes that cross an International border and back into its home country, akin to MN 23 in Wisconsin, BC 97 and YT 1 in Yukon and BC respectively, NT 5 in Alberta, etc.
My submission: WB 12 in India, with Nepal.
Google, Mapquest, and RMN all agree WB 12 crossed from West Bengal into Nepal and back at least once. OSM shows it doing so three or four times, including an intersection with Nepal F38 on the Nepalese side.
Bing, otoh, doesn't show any crossing.
Is Baarle-Nassau and Baarle-Hertog cheating?
(edited to fix typo)
Strictly speaking yes, as the route changes its respective number with each crossing.
But it's still an amazing example.
Does this road in Estonia and Russia count? Google shows it with the number 178 and OSM shows the number 18178.
https://www.google.com/maps/@57.9058086,27.7285236,13.25z
Ooh, that's close to the spirit, though I'd almost think that's a sanctioned MSR route as Google shows a shield color swap. It does get points for doing crossing into Russia twice, according to OSM (only once in Google).
So far: West Bengal 12, India into Nepal
Estonia 178, into Russia; possibly sanctioned by Russia with a matching corresponding route number