## 5 Simulation Models as a Substitute
### Simulation Models
Ship traffic simulation exceeds the ship-handling simulator's ability to consider different ship traffic conditions and situations at the same time. Besides representing single ship behavior, it automatically performs collision avoidances and regulation conformities that are more relevant to collision risks. This enables a safety assessment process in calculating all the possible probabilities in a much shorter time compared to ship-handling simulators. However, a model for automatic collision avoidance is the key to performing various ship behaviors in different situations, rather than the use of expert opinion. Therefore, the successfulness of a collision avoidance model become very important for the simulation model, and it plays a key role in the following risk assessments. There are domain-based models developed for automatic collision avoidances that mimic real ship behaviors. The first traffic simulation model utilizes the widely discussed domain theory for automatic collision avoidance in the model. Ship domain is a predefined area around a ship that should remain clear of other ships. In ship traffic simulations, if the domain of the ship is invaded, action should be taken to avoid collisions. [PERSON], [PERSON] [41], [PERSON] [42], [PERSON] [43], and [PERSON] and [PERSON] [44] gave good examples of various shapes of ship domains. Domain-based ship traffic simulations can count the number of invitations and near misses, which are parameters in a risk estimation. A traffic simulation model using the safety domain for qualitatively analyzing maritime safety in restricted waters, named MARTRAM (Marine Traffic Risk Assessment Model), has been presented by [PERSON] [45], using FoxPro and MapInfo. MARTRAM counts the number of safety domain invasions (encounters) as an indication of collision risk. The OFI (opportunity for incident) model uses a dynamic simulation model to count the occurrences of the various incident scenarios that could happen in the waterway concerned [46]. The simulation platform generates ships on the map and simulates the movements of the ships in the area concerned. The function of the platform is to count the possible incidents by simulating ship encounters on a map [47]. The saved scenarios and the counted data will be further processed by expert judgments in the safety assessment of the number of accidents. Traffic simulation forms an element for counting the incidents. However, the dynamic ship traffic behavior and collision avoidance is not addressed, which makes the model greatly dependent on expert judgment and Bayesian networks [48]. The complete procedure, the model of the system simulation, and the role of the simulations can be found in the Prince William Sound Risk Assessment [49; 50; 51], the Washington State Ferry Risk Assessment [46; 52], and other literature published subsequently based on the previous projects [47; 48; 53; 54]. Ship interactions are simply addressed in SMARTS (Ship Auto-navigation Fuzzy Expert System) [55]. The ship navigation, including collision/grounding avoidance maneuvers, are governed by a fuzzy expert system. This nautical traffic simulation provides ship interactions. However, the interactions based on fuzzy expert system are not described in detail. Therefore, the ability to represent real-word multi-encountering and collision avoidance is unknown. Moreover, the route of the ships is predetermined, while the real ship tracks conform to a lateral spatial distribution rather than a fixed line of route. The Istanbul Strait simulation model investigates ship integrations from a traffic management perspective. Ship traffic behavior is examined based on different scenarios, including ship arrival and waiting time [56]. One important feature is that the passage of a big ship (with a length larger than 200 m) will prevent the opposite traffic from passing the strait. On the whole, this traffic simulation is a good tool to determine the strategy for ship entry into the Istanbul Strait.