Ford 2011 Econoline Owner's Manual page 204

Driving
WARNING: To help avoid personal injury, please read and
understand the limitations of the reverse sensing system as
contained in this section. Reverse sensing is only an aid for some
(generally large and fixed) objects when moving in reverse on a flat
surface at "parking speeds". Inclement weather may also affect the
function of the RSS; this may include reduced performance or a false
activation.
WARNING: To help avoid personal injury, always use caution
when in reverse and when using the RSS.
WARNING: This system is not designed to prevent contact with
small or moving objects. The system is designed to provide a
warning to assist the driver in detecting large stationary objects to
avoid damaging the vehicle. The system may not detect smaller
objects, particularly those close to the ground.
WARNING: Certain add-on devices such as large trailer hitches,
bike or surfboard racks and any device that may block the
normal detection zone of the RSS system may create false beeps.
204
2011 Econoline (eco)
Owners Guide, 1st Printing
USA (fus)