Cadillac 2008 XLR Owner's Manual page 144

Approaching and Following a Vehicle
If this symbol does not appear, or disappears briefly,
Adaptive Cruise Control will not respond to vehicles
you may see ahead.
3-22
The vehicle ahead symbol
only appears on the HUD
when a vehicle ahead
is detected in your path.
CAUTION:
{
When the Adaptive Cruise Control radar is
blocked by snow, ice, or dirt, it may not detect
a vehicle ahead. Adaptive Cruise Control may
not have time to slow your vehicle enough to
avoid a collision. Do not use Adaptive Cruise
Control when the radar is blocked by snow,
ice, or dirt. Keep your radar clean. See
"Cleaning the System" later in this section.
Adaptive Cruise Control automatically slows your
vehicle down when approaching a slower moving vehicle.
It then adjusts your speed to follow the vehicle in front at
the selected following distance. Your speed increases
or decreases to follow the vehicle in front of you, but will
not exceed the set speed. It may apply limited braking,
if necessary. When braking is active, your brake lights
come on. It may feel or sound different than if you were
applying the brakes yourself. This is normal.