BMW Z4 Roadster Owner's Handbook Manual page 219

Acoustic warning
In addition to the visual display, a warning signal
sounds when your own vehicle is moving in the
corresponding direction.
System limits
The system may have limited functionality in the
following situations:
▷ If the approaching vehicle is travelling very
fast.
▷ In thick fog, wet conditions or snow.
▷ On sharp bends.
▷ If the bumper is dirty, iced up or covered,
for example by stickers.
▷ When a projecting load is being transported.
▷ If crossing objects are moving very slowly.
▷ If there are other objects in the field of view of
the sensors that conceal the crossing traffic.
Driver Assistance Systems
Online Edition for Part no. xxx - X/18
CONTROLS
219