Billy Joe Martin

A man was one of at least two police officers riding motorbikes in the motorcade to the left of John F. Kennedy's presidential limousine on 22 November 1963, the date of the President's assassination. Shortly before the assassination, the driver was photographed with the occupants of the limousine in Dallas, Texas. This photograph was retrieved from the Washington public archive in 2004 by Clive Finch, of interest to him because it featured the Ninth Doctor observing events. (TV: Rose) He was also visible in the version of the picture later obtained by Adam Mitchell. (COMIC: Mystery Date)

The driver followed the route of the motorcade down Main Street towards Dealey Plaza, turning right into Houston Street past the Dallas County Criminal Courts Building. He then performed a difficult left-hand turn onto Elm Street drastically lowering the vehicle's speed and increasing the length of time required for the journey towards the underpass, on which Kennedy was fatally shot. (PROSE: Who Killed Kennedy)

Behind the scenes
The identity of the officer, in the real world William Joseph "Billy Joe" Martin, is not revealed in Rose or Mystery Date.