Lincoln 2010 Town Car Owner's Manual page 194

Driving
The system detects obstacles within approximately 16 feet (five meters),
at speeds above 2.5 mph (4 km/h) behind the rear bumper with a
decreased coverage area at the outer corners of the bumper. If vehicle
speed is below 2.5 mph (4 km/h), the system detects obstacles within
approximately 9 feet (2.6 meters) behind the rear bumper with a
decreased coverage area at the outer corners of the bumper. As you
move closer to the obstacle, the rate of the tone increases. When the
distance to the obstacle is approximately less than 18 inches (45.0 cm),
the tone will sound continuously. If the system detects an object that is
approaching the vehicle at such a rate that rapid braking is required, a
very high rate tone will sound. If this tone is heard while reversing, the
driver is advised to slow down immediately until the tone either changes
to a slower rate or stops.
Distance to Object
up to 1.6 feet (.5 meters)
1.6 feet (.5 meters) to 3.1 feet
(1 meter)
3.1 feet (1 meter) to 5.7 feet
(1.8 meters)
5.7 feet (1.8 meters) to 9 feet
(2.8 meters)
9 feet (2.8 meters) to 13 feet
(4 meters)
194
2010 Town Car (tow)
Owners Guide, 1st Printing
USA (fus)
Audible Warnings
Speed above 2.5 mph (4 km/h)
Solid
Fast beep
Medium/fast beep
Medium beep
Medium/slow beep
Warning