Route Design & TSS(Traffic Separation Scheme} establishment are often used to improve the maritime traffic safety in the heavily congested traffic area. There are several domestic and international route design guidelines, but there is no guideline of TSS design As a result, TSS is usually designed based on the subjective opinions of professionals in this field. Until now, there exist a few papers dealing with the effects of TSS on the reduction of maritime accidents, however, it is hard to find a paper on the design guidelines of TSS. In this paper, TSS design guidelines are proposed based on the quantitative assessment of navigational difficulty felt by mariners, which is expected to improve navigational safety of vessels in congested area.