Toyota Prius 2011 Manual page 219

Cruise control can be set when
■
●
The shift lever is in "D".
●
Vehicle speed is above approximately 25 mph (40 km/h).
Accelerating
■
The vehicle can be accelerated normally. After acceleration, the set speed
resumes.
Automatic cruise control cancelation
■
Cruise control will stop maintaining the vehicle speed in any of the following
situations.
●
Actual vehicle speed falls more than approximately 10 mph (16 km/h)
below the preset vehicle speed
At this time, the memorized set speed is not retained.
●
Actual vehicle speed is below approximately 25 mph (40 km/h)
●
Enhanced VSC is activated
If the cruise control indicator light flashes
■
Press the "ON-OFF" button once to deactivate the system, and then press
the button again to reactivate the system. 
If the cruise control speed cannot be set or if the cruise control cancels
immediately after being activated, there may be a malfunction in the cruise
control system. Have the vehicle inspected by your Toyota dealer.
2-4. Using other driving systems
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