[w0de0]

This is treated as a whimsical absurdity, but it has a practical purpose that’s hardly obviated by the antiquity of the law.

I’m a sailor myself - I’d certainly want a warning that a bridge is lower than its charted height. The signal is clearly listed in the relevant Port of London notice to mariners [0]. Moreover, the signal also shows the height of the restriction (and in a neat, safe way - nudge the straw and you’ll know you’re too tall):

“Where the headroom is reduced this will be signalled in accordance with the Port of London Authority Thames Byelaw 36.1 namely:

“By Day – A bundle of straw large enough to be easily visible and displayed at the height of restricted headroom”

0 - https://pla.co.uk/notices/M63-25

[lolinder]

This also means that this line is also inaccurate:

> For convenience, they’re actually hanging from the Jubilee footbridges, one on either side of the railway.

It's not for simple convenience: you can see in the photos that it's because the footbridges are taller then the rail bridge and come a bit before it, which allows them to install the straw hanging at the correct height to warn of the constriction before someone hits the bridge itself.

[fsckboy]

i find being warned in advance a most convenient way to avoid hitting the bridge, and if the walkways are already installed in the right place to put the bales, why that's a very convenient place to put them. if the walkways were the same height as the bridge, it would not afford that convenience because then the footbridges would themselves be the obstruction you're in danger of hitting, which would be most inconvenient.

[krick]

Maybe when this was invented, the bundle of straw was the best they could do, I don't know, but surely today you can produce a more visible, more robust, reusable and, ironically, cheaper sign than that.

[tass]

Maybe, but this has a few advantages.

If there were some reusable item (whatever that may be) it would need to be stored somewhere accessible to the construction crews, would eventually need to be repaired or replaced, and depending on how it's made may need to be customized per installation.

Hay and rope are readily available and (depending on the rope) biodegrade so could fall into the river with pretty minimal pollution. I imagine back in the day they were both just borrowed from a local farm or stable.