In recent years it is probably true to say that the main pressures for inprovements in navigational safety have come from public and hence political sources. The pressure has usually taken the form of proposals for increased regulation of marine traffic. Often there is a defensive reaction from the professional mariner on the grounds that there is no real evidence that the proposals will produce a significant improvement in safety. Shipowners are also worried because of potential increased costs and curtailment of operating freedom.

It is therefore important that more information is made available so that both the safety aspects and economic aspects of additional regulation may be assessed. This paper is an account of a study into the development of danger criteria for marine traffic flows. This study was the key part of a larger one which was funded by the Department of Industry through its Ship and Marine Technology Requirements Board, now superseded by the Marine Technology Committee.

Collision rates on their own are usually not suitable for measuring safety risk, as collisions in a specific area are normally a very rare event. It is also not advisable to use data collected over several years as the traffic is likely to change, e.g. size of ship. Another approach might be the reporting of near misses, but there are considerable problems if navigators are asked to do this especially as there might be subjective judgments on whether to report an incident or not.