A vehicle is about to merge into traffic. When must it yield?

The driver of a vehicle merging into traffic shall always yield. They must check that the road is clear and that no vehicles are approaching, or that the distance, position, trajectory and speed of approaching vehicles is such that the merging manoeuvre can be performed without danger. Otherwise, they must desist and postpone the manoeuvre.
Other road users should facilitate the manoeuvre as much as possible.