Ford 2013 Flex Owner's Manual page 237

Using the Systems
BLIS turns on when the engine is started and the vehicle is driven
forward above 3 mph (5 km/h); it remains on while the transmission is in
D (Drive) and N (Neutral). If shifted out of D (Drive) or N (Neutral),
the system enters CTA mode. Once shifted back into D (Drive), BLIS
turns back on when the vehicle is driven above 3 mph (5 km/h).
Note: BLIS does not function in R (Reverse) or P (Park) or provide
any additional warning when a turn signal is on.
CTA detects approaching vehicles from up to 45 feet (14 meters) away
though coverage decreases when the sensors are blocked. Reversing
slowly helps increase the coverage area and effectiveness.
WARNING: To help avoid personal injury, NEVER use the CTA
system as a replacement for using the interior and exterior
mirrors and looking over your shoulder before backing out of a parking
space. CTA is not a replacement for careful driving and only an assist.
In this first example, the left sensor is only partially obstructed; zone
coverage is nearly maximized.
2013 Flex (471)
Owners Guide gf, 2nd Printing
USA (fus)
Driving Aids 237