The paper describes and reviews some of the work done by NPL over a period of twelve months in a climate of growing concern about the present and future safety of marine traffic, especially in narrow and restricted waters. The increase in number, size and speed of ships in the last decade emphasizes the need for information on the characteristics of marine traffic affecting safety of navigation, while the use of very large tankers and new types of carrier has increased the potential consequences of pollution to the environment in the event of a serious casualty. The situation in the Dover Strait is further complicated by crossing traffic and by the existence of fishing grounds.