[DocTomoe]

Often, these things are utilitarian, not mystical. So my educated guess: Back in the day, the main problem was river mariners getting hurt by bridge arches that were lower than expected, and the bale of straw was a 'soft buffer' - better to get your head hit by a swinging bale of straw than a rock-solid bridge.

[Sharlin]

More likely that a bale of hay was simply a conspicuous but lightweight thing that was easy to get your hands on back in the day.

[tdeck]

Also if it comes undone and falls in the river, it's not likely to do any harm.

Aside: Although the article makes the same mistake, hay and straw are not the same thing. Hay is dead green grass-like plants. Straw is dead brown grass-like plant matter that has finished it's lifecycle and used up all the sugars and things in it. Hay gets moldy more easily but has nutrients for animals while straw does not decompose as quickly.

[Sharlin]

Yes, I realized that after the edit grace period. Specifically, straw is the dried stalks of cereal that have very little nutritional value to begin with, whereas hay is reaped grass, legumes, whatever herbaceous plants that grazing livestock normally eat.

[oniony]

Indeed. Cheap, readily available, heavy enough to hang, soft enough to bump out of the way. Honestly, it's a mystery to me why it's a mystery. What else would they use, a dead sheep?

[radiorental]

I was wondering about that but it would only work for the sailor standing in the right place on a boat sailing dead center of the river where there is typically two way traffic.

This doesn't seem like a utilitarian solution, more of a signal with a symbolic intention?

[CJefferson]

The law explictly requires "large enough to be conspicuous and by night a white light", which suggests it's about visibility, particularly at night. That makes sense, there is a black metal bridge near where I live, and when it is cloudly I've noticed it's suprisingly difficult to see, even when you know where it is.

[DocTomoe]

Hm, I guess arches were lower back then, and a lot of the riverboats were actually the staked kind (think: Venetian gondolieri)? With different arches for different directions?

Honestly, this is all guesswork. But I can imagine something like that to be the case.