After taking a few snaps of Marble Arch and the front of the bronze horse's head statue by Nic Fiddian-Green, I turned back and saw two buses parked on either side of this strange sight. I lined myself up and took this photo. About 2 seconds after the final exposure had finished, the bus on the right drove off.
It's not my usual style of photograph, but it's weird enough to warrant putting up!
Canon EOS 500D / ISO 200 / f/6.3 / 18-55mm @ 41mm / 20 seconds 0EV, 5 seconds -2EV, 30 seconds +2EV