CITROËN C4 Handbook page 198

Driving
The alert is given by a warning lamp which
comes on in the door mirror on the side in
question as soon as a vehicle - car, lorry,
cycle - is detected.
For this, the following conditions must be met:
- all the vehicles must be moving in the
same direction,
- the speed of your vehicle is between 7 and
75 mph (12 and 140 km/h),
- when you overtake a vehicle with a speed
difference of less than 6 mph (10 km/h),
- when a vehicle overtakes you with a speed
difference of less than 15 mph (25 km/h),
- the traffic must be flowing normally,
- in the case of an overtaking manoeuvre,
if this is prolonged and the vehicle being
overtaken remains in the blind spot,
- you are driving on a straight or slightly
curved road,
- your vehicle is not pulling a trailer, a
caravan...
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No alert will be given in the following situations:
- in the presence of immobile objects
(parked vehicles, barriers, street lamps,
road signs...),
- with vehicles moving in the opposite
direction,
- driving on a sinuous road or a sharp
corner,
- when overtaking (or being overtaken by) a
very long vehicle (lorry, coach...) which is
at the same time detected at the rear in the
blind spot angle and present in the driver's
forward field of vision,
- in very heavy traffic: vehicles detected in
front and behind are confused with a lorry
or a stationary object,
- when overtaking quickly.