Are you aware of any other countries that prominently use abbreviations on their subnational routes?
Spain for their autonomous community routes (as michih should know!)
And there's discrepancies between what's on the roads, and ISO has as codes.
Andalusia: ESP-
AN, abbrev
AAragon: ESP-
AR, abbrev
ARA-,
AAsturias: ESP-AS, abbrev AS
Cantabria: ESP-
CB, abbrev
CACastille and Leon: ESP-CL, abbrev CL
Castilla-La Mancha: ESP-CM, abbrev CM
Canary Islands: ESP-CN, abbrevs FV, GC, GM, HI, LP, LZ, TF (done at lower level, islands)
Catalonia: ESP-
CT, abbrev
CGalacia: ESP-
GA, abbrevs
AG (community-wide motorways), AC, LU, OU, PO (done at lower level)
Balearic Islands: ESP-IB, abbrevs EI, Ma (done at lower level, islands)
Region of Murcia: ESP-
MC, abbrev
RMCommunity of Madrid: ESP-
MD, abbrev
MNavarre: ESP-
NC, abbrev
NABasque Country: ESP-PV, abbrevs A, BI, GI (done at lower level)
La Rioja: ESP-
RI, abbrev
LRValencian Community: ESP-
VC, abbrev
CVAlso, for something you might be familiar with: Michigan's state routes are M-xx, not MIxx.