Chevrolet 2008 Silverado 1500 Pickup Owner's Manual page 160

To be detected, objects must be at least 10 inches
(25.4 cm) off the ground and below tailgate level.
Objects must also be within 8 feet (2.5 m) from the
rear bumper. This distance may be less during warmer
or humid weather.
A single beep will sound the first time an object is
detected between 20 inches (0.5 m) and 8 feet (2.5 m)
away. Beeping will occur for a short time when the
vehicle is closer than 1 foot (0.3 m) from the object.
The following describes what will occur with the URPA
display as the vehicle gets closer to a detected object:
Description
amber light
amber/amber lights
amber/amber/red lights
amber/amber/red lights
flashing and beep for
three seconds
2-66
English Metric
8 ft 2.5 m
40 in 1.0 m
23 in 0.6 m
1 ft 0.3 m
The indicator light will come on and PARKING ASSIST
OFF displays on the Driver Information Center (DIC)
to indicate that URPA is off, see DIC Warnings
and Messages on page 3-66 for information about
clearing the message.
Notice: If you use URPA while the tailgate is
lowered, it may not detect an object behind your
vehicle, and you might back into the object and
damage your vehicle. Always verify the tailgate is
closed when using URPA or turn off URPA when
driving with the tailgate lowered.
The system can be
disabled by pressing the
rear park aid disable button
located next to the radio.