If I've understood correctly, ちょうど followed by a counting word usually means something like "just x" or "exactly x" depending on the context, but I can't figure out what it means in this opening sentence of a novel:
バルサが鳥影橋をわたっていたとき、皇族の行列が、ちょうど一本上流の山影橋にさしかかっていたことが、バルサの運命をかえた。
I understand the overall meaning of the sentence, but the exact purpose of ちょうど一本 here eludes me. What does the 一本 count? Or is ちょうど一本 some sort of set phrase beyond counting cylindrical things?