I was looking at Hualien in Google Maps (recent earthquake) and noticed provincial routes 9丙 and 11丙 alongside routes 9 and 11. Street View shows the highways are marked that way as well.
丙 generally means third in rank. Elsewhere I found:
* 20甲 (first rank)
* 9乙 (second)
* 9丁 (4th)
* 21戊 (5th)
* 2庚 (7th)
The numbers 1 thru 10 should be 甲、乙、丙、丁、戊、己、庚、辛、壬、癸.
It's a pretty cool branch numbering system that reminds me of New York State. (I've never been to Taiwan in person.)
English wikipedia doesn't have much detail about Taiwan routes, but Japan wikipedia does: ja.wikipedia.org/wiki/台9線 (https://ja.wikipedia.org/wiki/%E5%8F%B09%E7%B7%9A)
Before 2013, the only roads I recognized in Taiwan were G99 Taiwan ring expressway and G228. Everything else would be Sxxx provincial roads or lower. As of the latest road plan, "Chinese Peking" (i.e. the People's Republic of China, which doesn't control Taiwan) has withdrawn the former and relocated the latter to the mainland.
Other than the Han characters and Arial-ish font, this wouldn't look too out of place in much of New England:
(https://upload.wikimedia.org/wikipedia/en/thumb/8/8c/TaiwanCountyRoad123.jpg/448px-TaiwanCountyRoad123.jpg)
Their green guide signs seem to somewhat mimic MUTCD standards, and the National Freeway signs are cutouts.
The numbering system also follows our "odd = north-south, even = east-west" pattern.