In clear-weather conditions well trained mariners experience little difficulty in dealing with multi-ship encounters in all circumstances. In restricted visibility they do not display the same confidence in manœuvring even in the open sea, and the problem is amplified as the waters narrow and the traffic density increases. The plea of the mariner would be to find an anti-collision system that would make manœuvring in fog at least as straightforward as in clear weather. To satisfy such a request it might seem that there would need to be two systems, one for fog and another for clear, but it seems probable that a system evolved to work safely in fog would also be acceptable in the less demanding conditions of clear weather; this would in time be the only system adopted.