The controls specified in article 3(1) do not apply in respect of a person causing a vehicle which is being loaded or unloaded to stop at the edge of the carriageway on a length of road specified in an item in column (2) of the tables in schedules 3A and 3B during the hours specified in column (3) in that item for as long as is needed to load or unload the vehicle or 20 minutes, whichever is the less, provided that not less than 20 minutes has elapsed since the vehicle was last parked on any part of the same length of road.