Port of London Authority River Byelaws 1978 (as amended) Part 5 requires a bundle of straw "large enough to be conspicious" [sic] to be hung by day from any bridge reduced in span or height. At night they just specify a white light, but that's much less fun. The farm food has also been hanging from other bridges recently (notably Hammersmith and Blackfriars) but they're not quite as photogenic.
Bundle Of Straw Hanging From Tower Bridge
Last Updated 14 July 2010