Safe navigation in restricted waters continues to be a perennial concern for ship owners, operators, navigators, and citizens. The Nieuw Amsterdam, a cruise ship which ran aground in Southeast Alaska in August 1994; the Royal Majesty, another cruise ship which ran aground 10 miles east of Nantucket Island in June 1995; and the Regent Star, which caught fire in Prince William Sound, Alaska on 22 July, 1995, are just a few of the recent incidents which remind us of the importance of safe navigation in restricted waters. Clearly, the Braer, the World Prodigy, and the Exxon Valdez provide other examples of the importance of safe navigation in close waters. Fire, collision, allisions and groundings can result if vessels navigating in such waters stray.

Vessel traffic service systems (VTS systems) have been established, throughout the world, in areas of high traffic density. These systems, operating with such tools as VHF radiotelephony, land-based radar installations and ship's data processing, gather information to respond to collision and grounding danger, to organize traffic flow and assist allied activities. It is expected that VTS systems should effectively reduce the number of collisions in the VTS area. But efforts for collision avoidance can only be successful if the VTS operator in front of the shoreside radar screen is precisely instructed as to which measurements he has to make to identify a risk-of-collision situation. Uncertainties about the direction in which ships must take evasive action arise in VTS areas, where ships are free to engage in encounters.