Trumpy — Transpositions would be more likely found on longer lines, which are by default transmission voltages.
Another way of looking at it would be to imagine ‘phase rolling,’ and that a dual-circuit line may have one set of 3 conductors twisted clockwise and one twisted counter-clockwise at a transposition point. In Scott35’s pic, though, it looks like both circuits roll in the same direction.
The purpose is to balance phase-to-ground and limit circuit-to-circuit reactances, which helps in voltage balance, makes it easier to sense line-to-ground faults and less susceptible to mutual coupling, which can cause sympathetic line tripping—where two adjacent circuits both trip with one circuit faulted and one healthy.
