Cadillac 2004 XLR Owner's Manual page 132

Other Vehicle Lane Changes
If another vehicle enters the same lane as you, ACC will
not detect the vehicle until it is completely in the lane.
Be ready to take action and apply the brakes yourself.
3-22
Using ACC on Hills
How well ACC will work on hills depends on your
speed, vehicle load, traffic conditions and the steepness
of the hills. ACC may not detect a vehicle in your
lane while driving on hills. When going up steep hills,
you may want to use the accelerator pedal to maintain
your speed. When going downhill, you may have to
brake to keep your speed down. Applying the brake
disengages ACC. Many drivers find this to be too much
trouble and don't use ACC on steep hills.