Developments of autonomous ships are progressed in the world and algorithms for automatic collision avoidance or its decision support are widely required. Following factors are essential for the algorithm such as, selection of a target ship to avoid collision risks; determination of a start time of collision avoidance manoeuvre; and determination of a way to avoid the target ship. Authors analysed collision avoidance cases of a coastal sea area, which are extracted from an AIS database. The four moments during the collision avoidance manoeuvre, e.g. start time of collision avoidance, is identified with the AIS database. Then, conditions of a give-way and a stand-on ships at the moments are analysed by behaviour analysis. In this paper, the conditions at the start time of collision avoidance manoeuvre is focused. As the result, it is found that the condition at the start time is expressed by a simple equation composed of two parameters: the distance to the give-way and the stand-on ships; and the rate of change in relative bearing to the stand-on ship. Additionally, an assessment diagram to evaluate the collision risk is developed.
