You can blame Port of London bylaws.
I do whenever someone asks why there’s one hanging beneath Barnes Bridge.
whenever there is a height restriction beneath a bridge over the Thames, a bale of hay must be dangled to warn those passing below.
What made me smile was his justification for the continued use of this approach,
Do you think spending £20-£30 a year on a bale of hay on random occasions is going to be more expensive than paying the lawyers to change the law that requires it?