Some fragile ego white guy on a Brompton chasing me down after leisurely overtaking them to angrily confront me on what the hand gesture I made just after I passed them was.
After some unprovoked shouting at me he conceded that the left arm out on the left side is the accepted gesture to signal a impending left turn. Which I then proceeded to take.
WAC. Should have known. Full Lycra kit and Brompton.