Traffic control at crossing points will be a important problem for the future marine traffic control system in congested waters. Fairways necessarily cross each other in large harbours. Crossing points must be time shared by traffic from different directions. Fairway network system was studied by Koyama and Arai using GPSS simulation. According to that study, traffics at crossing can be controled successfully by reservation system (booking the specified time to pass the crossings) as long as the traffics remain not too many. GPSS simulation provides with the estimation of the maximum time which must be adjusted by speed control of ships to pass a crossing under the traffic control. The required distance to adjust the time schedule at crossings was studied in this paper in relation with the maneuvering characteristics of ships. A large ship needs longer distance than a small ship to adjust the time. The evaluation of this distance is one of the most important parameter in designing a control system at crossings. Minimum time control theory was employed as a base index of this distance. The obtained value ( L _min) by this method is the minimum in which the adjustment can be attained, so that a human operator can not follow this control law. Simulator studies were made to know the relation between the minimum distance ( L _min) and the practically necessary distance by human operators by the ship maneuvering simulator at The University of Tokyo Marcantile Marine. According to this study, the minimum distance is a very good index for our purpose. The practically necessary distance seems to be 1.5 and 2 times longer than ( L _min) for smaller and larger ships respectively.