Two-way segments will also show the small U-turn arrow icon at both ends of the segment. This is used, as its' name suggests, to prevent/enable Waze to use a segment end node as a U-turn, in case of necessity.
When setting the U-turn arrow to green, make sure that U-turn is possible AND LEGAL in real life.
In any other case, set the arrow to red for drivers security.
Just as with other turn restrictions, allowed U-turns must be set on a per-segment-per-end-node basis.