TOYOTA 2011 Prius - Manual (Page 251)

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2-4. Using other driving systems
2
When
d
riving
PRIUS_U (OM47685U) DVD
Driving in vehicle-to-vehicle distance control mode
This mode employs a sensor to detect the presence of vehicles up to
approximately 400 ft. (120 m) ahead and to judge the distance
between your vehicle and those the vehicle ahead of you.
Note that vehicle-to-vehicle distance will close in when traveling on long
downhill slopes.