[iainmerrick]

I'm a bit bemused at that "lost to time", as immediately before that it says:

The law requires a bale of straw to be hung from a bridge as a warning to mariners whenever the height between the river and the bridge’s arches is reduced, as it is at Charing Cross at the moment.

That seems clear enough! OK, the reason why it specifically has to be a bale of straw isn't obvious, but apart from that it seems very reasonable, just outdated.

Edit to add: straw does make sense as a makeshift crash barrier -- you'll notice if you hit it, but hopefully won't actually damage your ship. It seems like you would always just plough through and hit the actual bridge, though.

[tlb]

Bales of straw are a cheap, large, soft object you could always find nearby in the pre-motor car days. I can't think of a better object to require.

[bombcar]

Why does everyone think it’s something to run into?

At night it’s a light. It’s obviously a notification system. You visibly see the bale of straw before you get to the bridge and you know to slow down and stop and investigate what the clearance issue is.

[woleium]

Yes, a bale of hay used to be a warning of something to look out for on the road ahead

[raverbashing]

Yeah it's amazing (in a bad sense) how those bridge too low warnings are ignored most of the time

Well of course when you get stuck then it's too late.

[tomxor]

Similar to how people will read a temporary hand written sign on a door, but filter out a (newly placed) professionally made sign, because the latter looks too permanent.

If something has changed, using something out of place or temporary in appearance seems to be the most effective way of getting human attention... A bale of straw feels like it fits the bill.

[mnw21cam]

Absolutely. A straw bale hanging from a bridge over a river is an anachronism. It doesn't normally belong there, and it's more likely to be noticed than a literal notice.

[potato3732842]

They get ignored because there's a fudge factor built into them.

Some states (IL in particular) have absurd fudge factors, so you have 14ft spaces signed as 12.xx and 13'6" trucks drive under them all day every day like it's nothing which basically trains them to ignore the signs.

And that's before you consider all the drivers who can't read english at road speeds so anything that isn't the standardized yellow sign right on/beside the object is going to go unnoticed to them a large amount of the time.