"at the Marble Arch they hailed an omnibus"

John Nash designed the Marble Arch in 1828, basing it on the triumphal Arch of Constantine in Rome. It was originally a gateway to Buckingham Palace, but was moved in 1851 to its present location: on a large traffic island at the junction of Oxford Street, Park Lane and Edgware Road. It sits almost directly opposite Speakers' Corner in Hyde Park. 

Historically, only members of the royal family and the King's Troop, Royal Horse Artillery, can pass through the arch in ceremonial procession.

The Arch of Constantine
GNU Free Documentation LicenseThe Arch of Constantine - Credit: Alexander Z.