Hacker News

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.

According to the Port of London Thames Byelaws, Clause 36.2, a bale of straw has to be placed under London bridges “when the headroom of an arch or span of a bridge is reduced from its usual limits”.

At night, the bale of straw is harder to see, so some warning lights are also switched on.

Quite why a bale of straw is needed has long since been lost to time, but regardless of its origins, whenever the river bylaws are updated, they kept the medieval bale of hay law intact.