Before 1972 the Collision Regulations were drafted so as to enable a court to decide which vessel was to be blamed rather than to assist the mariner to avoid collision. The new Regulations have been drafted primarily to assist mariners to prevent collisions at sea. Even so, actions to avoid collision required by the Rules are described in abstract terms because of the variety of visibility, sea-room available, manoeuvring capability of the vessels and the nature of the encounters. In order to help mariners understand the requirements of action to avoid collision both in quality and quantity, some reference data are recommended by experienced mariners and researchers. This paper is an attempt to introduce a study on quantification of the term substantial action to avoid collision and the distance of last-minute action in turning to avoid collision.