## 1 Introduction  
In ports and adjacent waters, several vessels use a designated route to enter or depart a port and navigate the limited sea area, resulting in a high vessel density. It is known that the composition of the route connecting ports to ports can help predict the congestion level of the harbors and traffic volume based on the prediction of the demands of vessels' calls [1, 2]. This concentration of port logistics results in an increase in the density of ships, which can further cause potential marine accidents due to ship traffic in harbors. The maritime accident statistics of the Korean Maritime Safety Tribunal (KMST) indicate that during the years from 2016 to 2020, there were 13,687 cases of marine accidents in Korean territorial waters. During this period, 1713 cases occurred in ports and adjacent waters [3]. Consequently, several countries have adopted vessel traffic service (VTS) to prevent such potential marine accidents, and vessel traffic is efficiently and safely managed by a vessel traffic service operator (VTSO) through temporal and spatial coordination [4]. It is essential to the process of traffic management that the priority of vessel traffic be determined by recognizing the risk situations related to shipping traffic in advance, such as the surveillance of vessel movements, collisions, and stranding [5]. To monitor the movement of ships and prevent dangerous situations, such as collisions between ships, it is necessary to have the ability to predict future situations from the given information [6]. The VTSO should determine its own priority for efficient management of traffic and subsequently ensure compliance in the work process. In the process of prioritizing traffic management, the VTSO determines the priority based on the ships' dynamic and static information, such as the closest point of approach (CPA) and the time to CPA (TCPA) to the target ships. However, the criteria for determining such priorities are formed in various ways, depending on the environmental factors of the area where the ship navigates. The current VTS operation process relies on subjective judgment of the VTSO to determine such priorities, and research related to this topic is insufficient. Since the current VTS system does not suggest priorities by reflecting the factors according to the navigation environment, it depends on the subjective decision making of the operator, which inevitably increases the task load of the operator. The mutual risk between ships is primarily focused on assessing the risk of collision. This idea is also used in the VTS system to determine any collision risk and to produce ship-oriented information to deliver the information to the ship. Research on collision risk includes the determination of a ship's safety domain and risk. The concept of the domain was proposed by [PERSON] and [PERSON] (1971) [7], and a domain composed of various shapes and sizes was developed considering the encountering relationship based on the International Regulations for Preventing Collisions at Sea (COLREGs) and other navigational conditions [8; 9]. Recently, various calculation methods for collision risk have been proposed, as real-time data can be collected using an automatic identification system (AIS). Further, methods for determining and predicting the risk level using fuzzy theory or other machine learning theories have also been proposed [10; 11]. However, these related studies are efforts to predict the risk of collision based on the self-referent ('own') ship and utilize it as a navigation aid for ships. This study proposed an index for determining the priority of vessel traffic monitoring based on observers. Collision risk has been defined in previous studies by considering the relative value of the risk that a ship may have based on itself. However, the diversity of the safety distance that is formed differently depending on the area the vessel navigates was not considered.