Toyota prius Owner's Manual page 323

CAUTION
 Hill-start assist control
 Do not overly rely on the hill-start assist control. Hill-start assist control
may not operate effectively on extremely steep inclines or roads covered
in ice.
 Unlike the parking brake, hill-start assist control is not intended to hold the
vehicle stationary for an extended period of time. Do not attempt to use
hill-start assist control to hold the vehicle on an incline for an extended
period of time, as doing so may lead to an accident.
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2-4. Using other driving systems
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