Toyota 2013 Prius Owner's Manual page 343

Driving in vehicle-to-vehicle distance control mode
This mode employs a radar sensor to detect the presence of vehicles
up to approximately 400 ft. (120 m) ahead, determines the current
vehicle-to-vehicle following distance, and operates to maintain a suit-
able following distance from the vehicle ahead.
Note that vehicle-to-vehicle distance will close in when traveling on long
downhill slopes.
PRIUS PHV_OM_OM47787U_(U)
2-4. Using other driving systems
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