Preventing collision accidents is essential issue in safety navigation at sea. However, collision accidents still account for high proportion of all marine accidents. It is effective to alert risk of collision to operators. However, the timing of alert needs to match the operators’ sense, otherwise they may feel it somewhat strange or doubtful.

The purpose of this study is to evaluate the collision risk quantitatively between two ships in order to recommend initiating the collision avoidance manoeuvre to operators. The authors have analyzed the actual manoeuvres extracted from measured AIS data. Then, a new index to quantify the collision risk between two ships in a coastal sea area is proposed.