"Pro" cyclists-- Usually social riders, wearing lycra pants and jerseys, riding bikes worth
more than a
respectable motorcycle, and training on quite possibly the most inconvenient roads while people are attempting to get to work/school/home-- usually major, single lane roads-- and often not in the bike lane.
Not all "pro" (or pro-am) cyclists are bum wigglers but the ones that are, you typically find going up that tight, annoying stretch of hilly road at 15mph/20kph when you're already late for work.