Answer: Corn

There is a persistent urban legend around London that cabbies are required to keep a whole bale of hay secreted in the trunk of their taxis. It isn’t true, the cabbies aren’t required to keep hay, grass, corn, or any other livestock feed in their vehicles, but the urban legend has legitimate roots.

Up until the 1970s, there was a law on the books in London that allowed for taxis to be inspected (and drivers to be fined) for not carrying a sufficient supply of… corn, oats, or hay. The law, as you can imagine, was never enforced and was a vestige of the age before automobiles. Per regulations in the 1831 London Hackney Carriage Act, the operators of horse-drawn taxis were required to keep enough livestock feed on their cab to provide for the horse’s midday feeding (and, under the same Act, were required to feed the horses from their hands as to not clutter up the street with feed buckets and piles of hay).

Despite the decline of horse-drawn anything around London, the law sat on the books well into the 20th century. Only in 1976 did a bit of legislative housekeeping strike the law from the statute books and let every London cabby rest easy knowing there was no chance they’d ever lose some shillings over a trunk unladen with feed.