In the event of brake failure
If service brakes fail to operate whilst the vehicle is in motion, you can make an emergency stop with the parking brake. The stopping distance, however, will be much greater than normal.
WARNING
Parking brake
Avoid applying the parking brake to stop the vehicle whilst it is moving except in an emergency situation. Applying the parking brake whilst the vehicle is moving at normal speeds can cause a sudden loss of control of the vehicle. If you must use the parking brake to stop the vehicle, use great caution in applying the brake.