surface=sett: The stones are not entirely flat and typically do not cover the surface completely, unlike surface=paving_stones

Hello. The chosen scheme better represents footway crossings with only one set of traffic lights ("highway=crossing, crossing=traffic_signals"), as opposed to two sets of with an island in between (See Piata Leonardo Da Vinci, TImisoara). Routers optimized for footway that add penalties for traffic-lights benefit from this.