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.
The standard rule in the BeLux: U-turns should be disabled: 
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U-turns explained
A curved green arrow indicates that a u-turn is allowed in the manner and direction indicated by the arrow at that end of the segment.
When you hover the mouse over an allowed u-turn arrow, a small clock icon appears, which, when clicked on, allowed you to set scheduled u-turn restrictions, as well as vehicle type restrictions on the U-turn.
A curved red arrow with the slashed-circle icon indicates that a u-turn is not allowed (is restricted) in the manner and direction indicated by the arrow at that end of the segment.
A curved yellow arrow with a clock icon indicates the presence of scheduled (time-based) turn restrictions for the direction indicated by the arrow at that end of the segment.
Traffic regulations
On this signs (D3, D1) only the displayed directions are allowed. In this case a u-turn is forbidden.
Remarks
- See for U-turn prevention the Road aspects page paragraph Double left and double right.