Inattentiveness or misjudgement of the distance and speed of a vehicle approaching from behind can lead the driver into a critical situation when changing lanes. The optional Lane Change Warning System warns BMW 3-Series Sedan drivers of the potential danger of such an overtaking manoeuvre. The system uses a pair of rear-mounted radar sensors to monitor traffic in the adjacent lanes, from the blind spot to a distance of approximately 60 metres (200 ft.) behind the car. This information, indicated to the driver before he actually changes lanes, allows him to prepare for a lane change manoeuvre with confidence and to avoid critical situations from the outset. If other road users are approaching the car in an adjacent lane or moving through the driver's blind spot behind and to the side of the car, a yellow triangle symbol in the housing of the driver's side exterior mirror gives a discreet warning of possible danger. If the driver then still goes ahead and activates the direction indicator, announcing an intention to pull in or out, the LED symbol starts to flash. A further warning is provided in the form of a discreet but unmistakable vibration of the steering wheel rim.