Human behaviours are difficult to predict in traffic situations, because they result from different interpretations of the rules and even from rule violations. This statement is widely accepted both for road users and mariners. In nautical environments, problems in identifying intentions or predicting the actions of other vessels are a major source of incidents or accidents (i.e.?collisions).
This study aims to explain mariners? behaviours in collision avoidance situations. It is based on Ajzen?s Theory of Planned Behaviour. This theory postulates that behaviour is a function of three kinds of salient information, or beliefs: behavioural beliefs (referring to the outcome or to the cost incurred by performing the behaviour), normative beliefs (referring to the perceived social pressure to perform or not to perform the behaviour), and control beliefs (referring to the perceived ease or difficulty of performing the behaviour).
In this study, 103 trainee Officers of the Watch (OOW) in a maritime school examined one of two collision avoidance situations. In one situation, their ship is the " give way " vessel, whilst in the other, she is the " stand-on " vessel. Both situations are ambiguous, because they allow different actions to be taken. Participants were required to indicate the action they would choose if they were in that situation and to answer questions relating to:

• the consequences (desirable and undesirable) of each of the possible actions (behavioural beliefs);
• the individuals who would approve or disapprove of performing a given action and the individuals who would or wouldn?t perform this action (normative beliefs);
• the features which could facilitate or inhibit the performance of each action (control beliefs).