Based on the updated ship movement records, the vessel speeds in the Strait are assessed to be relatively low with the exception of only 3.18% records reflecting speeds in excess of 25 knots. The probable explanation could be that the speed limit in most legs of the Strait is 12 knots. Therefore, absolute speed may not be qualified as a reliable risk index in contrast to speed dispersion. Based on the consultations and discussions with several experienced ship captains, we concluded that it was reasonable that greater differences in speed among vessels are related to a higher collision risk.
The speed dispersion estimation procedure is as follows. First, we give a ship positioning procedure to group those records into 15 groups (records with respect to 15 legs) based on their position records. In the following, we calculate the time mean speed of vessels transiting through the leg. Third, the speed dispersion can be calculated for each leg.
Ship positioning is to identify in which leg the ship is navigating. In this step, we firstly identify the vertices and edges of each leg. For example, as can be seen in Fig. 2, there are five vertices and five edges in Leg 7W.