Prius's (and all cars, AFAIK) have a physical braking system. Even if all electronics fail, the brake pedal has a direct mechanical connection to the master cyl

Prius's (and all cars, AFAIK) have a physical braking system. Even if all electronics fail, the brake pedal has a direct mechanical connection to the master cylinder and the brake pistons. Pushing on the brake pedal mechanically forces your brake pads to engage, with or without electronics.