lane assist Citroen DS4 2010.5 1.G Owner's Manual

209Driving
Blind spot sensors
This driving assistance system warns the driver of the presence of another vehicle in the blind spot angle of their vehicle, as soon as this presents a potential danger.
Operation
This system is designed to improve safety when driving and is in no circumstances a substitute for the use of the interior rear view mirror and door mirrors. It is the driver's responsibility to constantly check the traffic, to ass es the distances and relative speeds of other vehicles and to predict their movements before de ciding whether to change lane. The blind spot sensor system can never replace the vigilance and responsibility of the driver.
 On switching on the ignition, or engine running, press this button to activate the function; the warning lamp comes on.
Sensors fitted in the front and rear bumpers monitor obscured zones in the driver's field of vision.
A warning lamp appears in the door mirror on the side in question: - immediately, when being over taken, - after a delay of one second, when over taking another vehicle too slowly.