Toyota PRIUS Manual page 355

 Cruise control can be set when
The shift position is in D.

 Vehicle speed is above approximately 40 km/h (25 mph).
 Accelerating after setting the vehicle speed
 The vehicle can be accelerated normally. After acceleration, the set
speed resumes.
 Even without canceling the cruise control, the set speed can be
increased by first accelerating the vehicle to the desired speed and then
pushing the lever down to set the new speed.
 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 16 km/h (10 mph)
below the preset vehicle speed.
At this time, the memorized set speed is not retained.
Actual vehicle speed is below approximately 40 km/h (25 mph).

 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 any authorized Toyota dealer
or repairer, or another duly qualified and equipped professional.
PRIUS PHV_OM_OM47831E_(EE)
2-4. Using other driving systems
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