NISSAN 2015 Altima Sedan - Owner's Manual - Manual (Page 318)

The Blind Spot indicator light illuminates if a
vehicle enters the detection zone from behind in
an adjacent lane.
However, if the overtaking vehicle is traveling
much faster than your vehicle, the indicator light
may not illuminate before the detected vehicle is
beside your vehicle. Always use the side and rear
mirrors and turn and look in the direction your
vehicle will move to ensure it is safe to change
lanes.
If the driver activates the turn signal, then the
Blind Spot indicator light flashes and a chime will
sound twice.
NOTE:
If the driver activates the turn signal before
a vehicle enters the detection zone, the
Blind Spot indicator light will flash but no
chime will sound when the other vehicle is
detected.
Overtaking other vehicles
The Blind Spot indicator light illuminates if you
overtake a vehicle and that vehicle stays in the
detection zone for approximately 3 seconds.
The camera unit may not detect slower moving
vehicles if they are passed quickly.
SSD1026 LSD2114 LSD2116
Starting and driving 5-23