If we are talking about a modern HGV then If the parking brake is applied all the time, then wedging the brake pedal down with the ignition on to check the stoplights is not going to make the vehicle roll away. The brakes will still be on all the time . There is the remotest possibility that doing the same thing with a van or some of the Asian made under 7.5t vehicles might be different if the handbrake quadrant and pawl is worn but it is unlikely.