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  13 September 2015 Last updated at 09:11 BST
  While they sing artist William Mackrell draws the bumps and swerves.
  His installation Gaps, Glitches and Speed Bumps on the No. 9 bus is part of his residency for the Royal British Society of Sculptors.
target: Four professional singers interpret their journey on a London bus guerrilla-style to the surprise of passengers.