Hey @Scupper , welcome! Noticed you’re posting in the cycling forum for the first time. Feel free to add yourself to the LINE group if you have any other questions about cycling in Taiwan.
As for your question about passing etiquette, the translation of “on your left” is
主意右邊 / Zhǔyì yòubiān = Attention to your left
I normally go with the two below:
小心後面 / Xiǎoxīn hòumiàn = Careful behind you
不好意思 / Bù hǎoyìsi = Sorry
Just as long as your tone of voice is not too loud, sudden or in a bad tone, all the above are polite ways to say “on your left”