The traffic capacity of a crossing of two fairways is one of the most important problems in marine traffic engineering and a few papers published in Japan deal with this subject, either by theoretical analysis or mathematical simulation. However little is yet known on the size of domain or diameter of evasion, without which any quantitative estimation of capacity is almost impossible. Partly for this purpose a year-long observation was performed in the eastern part of Bisanseto, a strait in the Seto Inland Sea, where the number of crossing encounters is the largest in Japan.