Marine traffic is a man-machine system, and so not only an extarnally observable quantity but also a mental load on navigators should be evaluated to determine the feasible plans for improving the marine traffic environment. Since a mental load chiefly comes about in decision making process, it is important to make a model to simulate this internal process of a navigator. To achieve it, the maneuvering space concept is useful. This space consists of any action to prevent a collision and attendant value of each action : risk of collision, loss of voyage and so on. Applying these evaluations to all targets around an own ship, traffic environment could be rated by the blocking coefficient. This index, blocking coefficient, is useful to reflect the subjectivity of a navigator and to measure the bounds of human ability.