Fig.8 The distribution chart of a risk is shown in (a) and that of utility
of each maneuver is shown in (b). And the encounter situation
for this trial calculation is shown in (c).

4.2 BLOCKING COEFFIENT AS AN INDEX TO ASSESS TRAFFIC ENVIRONMENT

When there is a high risk with any maneuver, such a traffic environment should be difficult to decide the maneuver to prevent a collision and put too much load on a navigator. Then it can be considered that such a environment is closely related with a distribution of a risk in the maneuvering space shown as figure 6. Taking account of a preference of each maneuver as a weighting function, the index to evaluate a marine traffic environment, Blocking Coefficient, can be defined as follows.

In order to confirm validity of the model, this index was compared with the results obtained from the experiment using a ship handling simulator. Two pilots and two captains who are in active service had tried to maneuver under four scenarios; encounter situations. Since every pilot and captain had tried under every scenario, sixteen data which was subjectively evaluated about the difficulty of maneuver were obtained.

The results of comparison are shown in Fig.9 (a) and (b). The longitudinal axis of both figure is a subjective sample value of navigators in experiment and ten rate were used to evaluate the subjective difficulty; one means very easy encounter situation and

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