Ford Explorer 2011 Owner's Manual page 312

Driving
The CTA system detects vehicles approaching up to 45 feet (14 meters)
away. Coverage decreases when vehicles and objects in close proximity
block the CTA sensors (refer to figure for approximate zone coverage
areas [sensor obstructed for vehicle on left]). Backing slowly from
the parking spot in these situations helps to increase the sensor coverage
and effectiveness.
CTA coverage also decreases when parking at shallow angles (refer to
figure for approximate zone coverage areas [sensor obstructed for
vehicle on left]).
312
2011 Explorer (exp)
Owners Guide, 3rd Printing
USA (fus)