Toyota Prius+ Owner's Manual page 290

2-4. Using other driving systems
Automatic cancelation of constant speed control

The cruise control will stop maintaining the vehicle speed in the following sit-
uations:
 Actual vehicle speed is more than approximately 16 km/h (10 mph)
below the set vehicle speed.
At this time, the memorized set speed is not retained.
 Vehicle speed falls below approximately 40 km/h (25 mph).
 VSC is activated.
 Radar sensor and grille cover
Always keep the sensor and grille cover clean to ensure that the vehicle-to-
vehicle distance control operates properly. (Some obstructions, such as
snow, ice and plastic objects, cannot be detected by the obstruction sensor.)
Dynamic radar cruise control (vehicle-to-vehicle distance control mode) will
be canceled if dirt is detected. (Constant speed control mode can be used).
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PRIUS +_OM_OM47821E_(EE)
Grille cover
Radar sensor