Decision making and strategies in an interaction situation: Collision avoidance at sea

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Transportation Research Part F 11 (2008) 259–269

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Decision making and strategies in an interactionsituation: Collision avoidance at sea

Christine Chauvin a,*, Salim Lardjane b

a University of South Brittany (UBS), GESTIC, Centre de Recherche, rue de Saint-Maude, 56321 Lorient Cedex, Franceb Institute of Biomathematics and Biometry, GSF – National Research Center for Environment and Health,

Ingolstadter Landstrasse 1, 85764 Neuherberg, Germany

Received 9 March 2007; received in revised form 4 December 2007; accepted 13 January 2008

Abstract

This paper aims at analysing decisions which are actually made by watch officers onboard ferries in the Dover Strait.More precisely, it aims at characterizing the generic situations in which several courses of actions are available and iden-tifying the strategy underlying an action choice. Relying on the RPD model of Klein [Klein, G. (1997). The recognition-primed decision (RPD) model: Looking back, looking forward. In C. E. Zsambok & G. Klein (Eds.), Naturalistic decision

making (pp. 285–292). Mahwah: Lawrence Erlbaum Associates], it points out the critical cues, the goals of actors and therules they use.

Two sets of data were processed: motions of vessels observed from the vessel traffic system and verbal protocolsrecorded onboard a ferry with three watch officers.

Logistic regression models show that different types of ships do not act in the same way: the slowest vessels tend to keeptheir course and speed, even if they have to move. The faster cargo ships such as ferries alter their course in compliancewith the regulations. In some situations, a ferry may nevertheless follow informal rules. Onboard a ‘give way’ ferry, awatch officer may – in some conditions – alter his course to port rather than to starboard to reduce the course alterationand the loss of time. On board the ‘stand on’ vessel, he may perform an action in order to master the situation, even if therule requires him to keep his course and speed.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Interaction; Decision making; Strategies; Ship handling; Collision avoidance

1. Introduction

Maritime transport and travel has a relatively low death and injury rate when compared to road travel; in2000, fatality risks assessed on the basis of the distances travelled in a particular mode was estimated at 1.1 forroad travel (per 108 person km) and at 0.33 for ferry travel in Europe (Mackay, 2000). However, within a

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doi:10.1016/j.trf.2008.01.001

* Corresponding author. Tel.: +33 0297874521; fax: +33 0297874500.E-mail addresses: [email protected] (C. Chauvin), [email protected] (S. Lardjane).

mailto:[email protected]

mailto:[email protected]

260 C. Chauvin, S. Lardjane / Transportation Research Part F 11 (2008) 259–269

context of rapid development of the world merchant fleet (the number of vessels is up 30% since 1998), onemay observe an increase in major or total losses of vessels (225 in 1998 and 700 in 2006 all around the world),as well as in groundings and collisions as causes of these losses (Robertie, 2007). In this context, the questionof risk and more precisely the question of human factor appear to be crucial points. Human error is, in fact,the main factor in maritime accidents (Hetherington, Flin, & Mearns, 2006). Hetherington et al. (2006)described factors that may contribute to maritime incidents and accidents: human performance factors, (fati-gue, stress, and health), personnel issues including technical skills, cognitive skills (situation awareness anddecision making) and interpersonal skills (communication, language, and teamwork), organizational issues(safety training, bridge team management, safety climate and safety culture). Pourzanjani (2001) focussedon the human error in collision avoidance and identified cognitive errors such as diagnosis and decision errorsas significant contributors to failure in collision avoidance.

Risks of collision are particularly important in heavy traffic areas, such as the Dover Strait for northwestEurope. Four hundred vessels per day pass in the Dover Strait, 70 ferries and 240 other vessels cross them. Inthe northwest Europe area there are about 6–7 collisions every year. The 2002 collision between the Norwe-gian car carrier Tricolor and the Bahamian container vessel Kariba, which later resulted in both the Dutchcoaster Nicola and the Turkish oil tanker Vicky running into the wreckage of the Tricolor, is still brandedin everyone’s minds. Such events, even if they are rare, have very real, far reaching and costly consequencesand must, as far as possible, be avoided.

In the maritime field, as in other transportation modes, the major aim of traffic psychology is to explain andto predict the behaviour of system users (Brown, 1997; Summala, 1997), particularly in interaction situationsor ‘traffic conflicts’ where a risk of collision does exist. If one could predict the ‘normal’ vessel behaviour, onecould monitor the traffic, point out dangerous behaviour and prevent accidents. Our aim is, therefore, tounderstand the cognitive processes of the watch officers engaged in ‘normal interaction situations’. Two meth-ods have been used:

– External observations of behaviour; as Saad noted (1991), is an approach which makes it possible to eval-uate the frequency with which a type of behaviour occurs, as well as the effect of different variables whichshould be estimated on a global level. However, it does not allow one to study the mechanisms by which thedriver controls his driving, nor can the regulating actions he carries out be studied in depth.

– Observations of officers’ behaviour during journeys onboard one vessel, as in the studies carried out bySaad (1991, 1996), were complemented by the recording of their verbalizations. Verbalizations were ana-lysed in order to determine the officers’ objectives and strategies, their knowledge and representations ofthe current situations. We chose to observe watch officers onboard a ferry operating in the Dover Straitbetween France and England, because ferries are often involved in ‘interaction situations’ and officersonboard these vessels may be considered as experts of encounter situations.

2. Decision making in maritime interaction situations

Interaction situations that occur in maritime traffic are, in some respect, similar to road situations and poseidentical problems (Chauvin & Saad, 2004).

– One officer of the watch working alone does the navigation on board merchant ships, in open water. He isresponsible for navigation and bridge management activities but also for collision avoidance.

– In ship handling, as in car driving, the two participants involved do not communicate or communicate verylittle.

– In the most congested areas, vessels have to follow predetermined routes. Traffic separation schemes (TSS)separate opposing streams of traffic by the establishment of traffic lanes. In the Dover Strait, a TSS wasestablished in 1967.

– In both cases, conflict resolution is regulated. In fact, on board ships, each officer has to take the collisionregulations (HMSO, 1972) into account. Collision regulations define different kinds of interaction situa-tions (crossing, overtaking and head-on situations) and different status of vessels (the ‘give way’ vessel shall

C. Chauvin, S. Lardjane / Transportation Research Part F 11 (2008) 259–269 261

keep out of the way and the ‘stand on’ vessel shall keep her course and speed). Concerning the crossingsituations, the collision regulations mention, in Rule 15, that the vessel which has the other one on her

own starboard1 side shall keep out of the way and shall, if the circumstances of the case admit, avoid crossing

ahead of the other vessel (HMSO, 1972). The ‘stand on’ vessel shall keep her course and speed. She may, how-ever, take action to avoid collision by her action alone, as soon as it becomes apparent to her that the vessel

required to keep out of the way is not taking appropriate action in compliance with these rules. In this case, sheshall, if the circumstances of the case admit, not alter course to port for a vessel on her own port side.2

Studies dealing with collision avoidance show that different interpretations of the regulations generateuncertainty concerning the actions of vessels (Habberley & Taylor, 1989; Hinsch, 1996).

This paper aims at describing decisions that are actually made onboard ferries during crossing situations, inorder to avoid collision and at identifying the different strategies that are likely to be used.

Two decision theories are available (Kobus, Proctor, & Holste, 2001). On one hand, there are the analyticalor computational decision making models that describe the strategies available to decision makers when theirtask involves selecting one course of action (or ‘option’) from several possible ones. On the other hand, thereare models of naturalistic decision making which rely on the notion of ‘situation assessment’.

Analytical decision making consists in selecting an option regarding its attributes. The basic model of ana-lytical decision making is the maximisation of expected value or utility (EV). It postulates that the process ofdecision making is made up of several stages:

(1) all alternatives and all attributes of each alternative are identified,(2) the value of all attributes are considered, as well as their relative importance (in terms of weights or

probabilities),(3) an alternative is evaluated after having multiplied the weight or probability by the value and summing up

the new values,(4) decision making consists of choosing the alternative which has the best evaluation.

Game Theory uses this model to analyse decision making in interaction situations. It considers interactionbetween agents as a strategic game, in which each ‘player’ strives to maximise his outcome. It takes intoaccount the notion of ‘risk’ and ‘incertitude’; in a game, uncertainty is essentially due to the fact that a playerdoes not know what the other is going to do. To reduce uncertainty and considering the payoff of each com-bination of actions, the players have to choose a strategy. A ‘safe’ strategy consists in minimising the lossregardless of what the other does. Thus, each player should examine the maximum possible loss associatedwith each strategy and then pick the strategy which minimises this maximum associated loss as his best strat-egy. This is called a minimax approach. Prentice (1974), analysing a car crash, gave an example of applicationof such a strategy, as well as Cannell (1981), analysing collision avoidance at sea.

Game theory seems, therefore, appealing for the purpose of analysing decision making in an interactionsituation. It is nevertheless difficult to apply when one wants to account for naturalistic decision making.In fact, one is faced with several questions:

� How can one take into account the spatial and temporal features of dynamic situations?� What is the criterion used to evaluate an option (safety or time saving or both)?� Does option selection really consist in considering utilities? Are options really evaluated in terms of winning

and losing?

It is now well known that experts, in operational settings, do not have enough time to generate manyoptions and that the analytical models of decision making do not take into account the complexity of naturalsituations (Kobus et al., 2001).

1 Starboard: the right side of a ship when you are looking towards the front.2 Port: the left side of a ship when you are looking towards the front.


Klein (1997) has presented the recognition-primed decision (RPD) model to define ‘‘what people actuallydo under conditions of time pressure, ambiguous information, ill-defined goals, and changing conditions”. Itpointed out that people do not carry out formal comparison between options and that experienced people gen-erally use experience to generate a single, plausible and satisfying (but not optimal) option. In naturalistic deci-sion making, experts are able to make quick and satisfactory decisions because they match the environmentfeatures with a generic situation. The RPD model contains three functions, labelled: simple match, evaluatethe course of action, and diagnosis of a situation. Pattern matching or ‘simple match’ represents the case inwhich a decision maker identifies a situation: ‘‘the goals are obvious, the critical cues are being attendedto, expectations about future states are formed and a typical course of action is recognized” (Klein, 1997,p. 285). The function of ‘evaluate a course of action’ represents a case ‘‘in which the course of action is delib-erately assessed by conducting a mental simulation to see if it runs into any difficulties and whether these canbe remedied, or whether a new course of action is needed” (Klein, 1997, p. 285). Diagnosis is ‘‘the attempt tolink the observed events to causal factors” (Klein, 1997, p. 290).

This theoretical framework is used, here, to identify the situations where several courses of actions areavailable and to analyse the actual decision made.

3. Method

Quantitative data describing the manoeuvres undertaken by ferries and cargo ships, and verbal reportsrecorded on board a car-ferry were collected in the Dover Strait, which is one of the most crowded maritimeareas in the world.

3.1. Features of manoeuvres performed in interaction situations

Vessel motions were observed at the Gris-Nez vessel traffic system. This centre watches over the traffic inthe Dover Strait. Traffic in this area is heavy; it consists of cargo ships operating in the traffic separationscheme and of ferries crossing between England and France, so that four interaction situations are possible(cf. Fig. 1); in these situations, the vessel which has the other on her starboard side is the ‘give way’ vessel.Traffic in this area was observed for one month, and 62 interaction situations between cargo ships and ferriescrossing between Dover and Calais were recorded; all these situations took place in good weather conditions(good visibility and soft wind).

50:57:07

50:58:34

51:00:00

51:01:26

51:02:53

51:04:19

51:05:46

51:07:12

51:08:38

1:17:00 1:24:12 1:31:24 1:38:36

1

2

3

4

Fig. 1. The different kinds of interaction situations analysed. (1) The ferry is leaving Calais. She crosses the cargo ships heading to theports of Northern Europe. She is the ‘stand on’ vessel. (2) The ferry is leaving Dover. She crosses the cargo ships heading to the south. Sheis the ‘stand on’ vessel. (3) The ferry is heading to Dover. She crosses the cargo ships heading to the south. She is the ‘give way’ vessel. (4)The ferry is heading to Calais. She crosses the cargo ship heading to the ports of Northern Europe. She is the ‘give way’ vessel.


For each of these situations, priority was defined (the ferry being either the stand on vessel or the give wayvessel); every 36 s for about 15 min the values of the following variables were noted:

� speed and course of each vessel,� distance between vessels (in nautical mile3),� DCPA (the distance at closest point of approach, in nautical mile),� TCPA (time to closest point of approach).

Quantitative data were analysed with the R statistical software (version 2.4.1, 2006) using logistic regressionmodels aiming at pointing out relations between situation features and manoeuvre features (distance at whichthe manoeuvre is performed, direction of the manoeuvre, course alteration). Logistic regression is a type ofpredictive model that can be used when the target variable is a categorical variable with two categories –for example, acting or not, altering course to port or to starboard. The logistic model formula computesthe probability of the selected response as a function of the values of the predictor variables.

3.2. Cognitive activities of ferry watch officers

Following the approach defined by Ericsson and Simon (1993), three watch officers onboard a ferry wereasked to ‘think aloud’ during seven interactions between their vessel and a cargo ship: in four of these situa-tions, the ferry is the ‘give way’ vessel and in three situations she is the ‘stand on’ vessel. All watch officers wereexperts, whose ages were 30 to 40. They had different ranks, namely: master, first officer and second officer.

Each verbal report was audio taped for transcription, coding and analysis.The verbal report recording began when an encounter situation was identified (a risk exists when the pro-

jected courses and speeds of two vessels place them at or near the same location simultaneously) and finishedwhen the vessels had crossed and the risk was over.

The coding scheme defined by Klein, Vincent, and Isaacson (2001) to analyse driving skills was adapted tothe maritime situation and was used for coding. It is divided into four main categories: cues (what the watchofficer is looking for in other vessels, in the situation, in his vessel), tactical rules (specific IF/THEN statementsfor dealing with simple situations, often linked to cues), knowledge of limitations (limitations due to their owncharacteristics or to the characteristics of their vessel), strategies (IF/THEN decision rules for dealing with theperceived long-term limitations). A last category was not used because it did not seem appropriate for experts;it is labelled ‘changes over time’ (participants report increases or decreases in confidence, patience, risk takingand comprehension of traffic flow or other users).

4. Results

Statistical analysis allows:

– the informal rules applied by ferries crossing the Dover Strait and by cargo ships operating in the trafficseparation scheme to be distinguished, and

– those rules to be compared with the formal rules (namely the Collision Avoidance Rules).

As said before, the formal rules describe the behaviour of the ‘give way vessel’ but also take into accountthe behaviour of the ‘stand on’ vessel. This section investigates three questions:

� Which vessel manoeuvres first? Is she always the ‘give way’ vessel, according to the formal rule? In the caseof a negative answer, what are the features of the situations in which informal rules may apply?� When the ‘give way’ vessel is the ferry, does she follow the formal rule? If not, in what kind of situations

does the officer use informal rules and for what kind of purpose?

3 Nautical mile (nm): a measure of distance used at sea, equal to 1853 m.


� When the ‘stand on’ vessel is the ferry, does the officer conform to the formal rule? If not, in what kind ofsituation does the officer use informal rules and for what kind of purpose?

4.1. The manoeuvring vessel

It is written in the Collision Avoidance Rules that:

– When two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other one

on her own starboard side shall keep out of the way (Rule 15).

When the cargo ship is the give way vessel, we note that she does not always perform a manoeuvre. On thecontrary, ‘give way’ ferries almost always alter their course; either to starboard (64.5%) or to port (23.5%),with mean amplitude of 18�, at an average distance of around 3.5 nautical miles to cross astern of the cargoship at a distance of 0.7 nautical miles (nm) or ahead at a distance of 1 nm.

Concerning the type of ship that takes action first, the most satisfying logistic model was found to be thefollowing:

4 Kn

P ðY ¼ 1Þ ¼ 1=ð1þ e�0:69�2:11ðferryÞÞ
where Y is the type of the ship which acts first, Y = 1 if she is the ‘give way’ vessel and (ferry) = 1 if the ‘giveway’ vessel is a ferry and 0 if she is a cargo ship.
This model provides the best fit to our data among a variety of tested models, including those obtainedusing various methods of variable selection. The tested models were compared using Akaike’s InformationCriterion (AIC) and standard statistical tests available under the R software (The R Development Core Team,2006; Fox, 2002; Venables & Ripley, 2002). The same methodology was used for all the logistic models pre-sented in this work.

According to the selected model, there is a probability of 0.67 that the ‘give way’ vessel takes action if she isa cargo ship and a probability of 0.94 that she takes action if she is a ferry.

The ‘give way’ vessel features seem, therefore, to determine the decision to manoeuvre more than the sit-uation characteristics.

The most satisfying model to represent the action of a ‘give way’ cargo ship, regarding Akaike’s Informa-tion Criterion (AIC) and several statistical tests, was found to be the following:

P ðY ¼ 1Þ ¼ 1=ð1þ e2:72�0:25�V Þ
where Y is the type of the manoeuvring vessel, with Y = 1 if she is the ‘give way’ cargo ship and V indicates thespeed of the ‘give way’ vessel (in knot4).
According to this model, the probability – for a cargo ship – to take action first when she is the ‘give way’vessel is:

� 0.19 if her speed is around 5 knots,� 0.45 if her speed is around 10 knots,� 0.74 if her speed is around 15 knots,� 0.91 if her speed is around 20 knots.

The variation is almost linear between 5 and 15 knots, with a gradient of about 0.05.From the two previous models, and given the fact that ferries are generally quite fast (their average speed is

about 19 knots, and more than 80% of the observed speeds are not below 18 knots), the models fitted to ourdata show that the faster the ‘give way’ vessel, the more likely she is to conform to the formal rule.

ot: measure of speed used for ships that is about 1853 m/h.


4.2. Behaviour of the ‘give way’ ferry

An examination of our data showed that ferries almost always took action first when they were ‘give way’vessels. However, some of them altered their course to port, even though the rule recommends to avoid cross-ing ahead of the other vessel (Rule 15). Alteration of course to port is inconsistent with the formal rules, since itleads to crossing ahead of the stand on vessel.

4.2.1. Logistic regression model

Concerning the direction of the course alteration undertaken by the manoeuvring ‘give way’ ferry, the mostsatisfying model regarding Akaike’s Information Criterion (AIC) and several statistical tests was found to bethe following:

P ðY ¼ 1Þ ¼ 1=ð1þ expf�19:57ðrisk of collisionÞ� 1:61ðthe ‘give way’ ferry will cross astern the ‘stand on’ vesselÞ� 0:56ðthe ‘give way’ ferry will cross ahead the stand on vesselÞgÞ

where Y is the direction of the course alteration, with Y = 1 if the ‘give way’ vessel alters her course to star-board and 0 if she alters her course to port, (risk of collision) = 1 if the collision is certain and 0 if not, and theother expressions involving brackets are to be understood in the same way.

Thus, if the vessel taking action first is the ‘give way’ ferry, then the probability that she alters her course tostarboard is:

� Almost 1 if the DCPA is less than 0.2 nm (collision is certain if no action is taken),� 0.83 in case of a ‘give way’ ferry crossing astern the ‘stand on’ vessel (if no action is undertaken),� 0.64 in case of a ‘give way’ ferry crossing ahead the ‘stand on’ vessel (if no action is undertaken).

The best predictor of a course alteration to port is the fact that the ‘give way’ ferry will cross ahead theother one if no action is performed (cf. Fig. 2). In this case, alteration of course to port is less important thanan alteration of course to starboard. An alteration of course to port is, thus, the most ‘economical’ action.

4.2.2. Cues, goals and rules mentioned in verbal protocolsWhen the ferry is the ‘give way’ vessel, watch officers first mentioned the direction of the chosen manoeuvre

and then its amplitude. The first step of the decision process seems, therefore, to be the choice of the direction(to alter the course to port or to starboard).

The mentioned cues are the following ones: DCPA of the target ship, bow crossing range, presence of athird vessel in the vicinity, distance of the target vessel, speed of the target ship, current.

Two sets of rules are indicated.The first set established a relationship between some cues and the choice of a direction. In this case, the

mentioned cues are: (i) the target speed (ii) the presence, in the vicinity, of other vessels, (iii) the time.

‘Give way’ vessel

‘Stand-on’ vessel

Fig. 2. The ‘give way’ vessel will cross ahead the ‘stand on’ vessel.


� The target speed is mentioned in terms of the differential speed between the ferry and the cargo ship. A suf-ficient speed ratio is necessary to alter course to port. Such a manoeuvre is performed in case of a bowcrossing range; that is to say that the ferry would cross ahead of the cargo vessel if no action was under-taken. This option is not considered if the cargo ship is faster than the ferry (‘‘It is certainly a containership. 23 knots. For such a vessel, with such a speed, we can’t cross ahead”).� The presence of a third vessel is considered to assess an option (‘‘I’ve enough room to cross astern without

hindering the vessel that follows him”, ‘‘I’ll alter my course to starboard to cross him at a range of 1 nm andso that the other one isn’t afraid”). It may also prevent an option choice.� The possibility of losing time is mentioned, as well as the possibility of gaining it, especially when taking the

current into account (‘‘We will not lose too much time, because the current will push us easily”).

In the second set of rules, a goal is associated with the mention of the direction or with the mention of themanoeuvre amplitude. The decision may aim at satisfying two goals:

� To cross the other vessel at a sufficient distance (‘‘I’m going to alter my course to starboard so that we crossat 1 nm”),� To make the action apparent to another vessel as is recommended by Rule 8 and to comply with the target

expectation (‘‘He will be sure that we will cross ahead at more than 1 nm”, ‘‘We’ll show him clearly that wewill cross astern”, ‘‘We’ll alter our course a little bit to show him that we saw him”, ‘‘I’m beginning to altermy course to show him that I’ll take action”).

The distances of 3 or 3.5 nm are mentioned as being the proper distances to take action. The first officeradopts a strategy which consists of: (i) beginning his manoeuvre very early (at a range of 6 nm) to indicatethat he identified the risk and will take action and (ii) making a large course alteration at a range of 3 nm.

4.3. Behaviour of the ‘stand on’ vessel

The ‘stand on’ vessel may take action to avoid a collision by her manoeuvre alone, as soon as it becomes appar-ent to her that the vessel required to keep out of the way is not taking appropriate action in accordance with these

rules (Rule 17). In 29 interaction situations, the cargo ship was the ‘give way’ vessel. In 16 of these situations,the cargo ship took action (altering her course to starboard in 14 cases at a mean distance of 2.68 nm and toport in two cases).

We observed, therefore, that 13 of the 29 stand on ferries altered their course. For seven of these cases, theaction was carried out at a short distance (less than 2.6 nautical miles, the average distance at which the cargoships habitually manoeuvre). In the other cases, the manoeuvre was carried out very early.

4.3.1. Logistic regression model

When it comes to the direction of the course alteration undertaken by the ‘stand on’ vessel, the most sat-isfying model regarding Akaike’s Information Criterion (AIC) and several statistical tests was found to be thefollowing:

P ðY ¼ 1Þ ¼ 1=ð1þ expf�2:93ðrisk of collisionÞ�2:13ðthe ‘give way’ vessel will cross astern the ‘stand on’ vesselÞ�0:42ðthe ‘give way’ vessel will cross ahead the ‘stand on’ vesselÞþ1:17ðthe manoeuvring vessel is the ‘stand on’ oneÞgÞ

Thus, if the vessel taking action first is the ‘stand on’ vessel, then the probability that she alters her course tostarboard is about:

� 0.85 if the risk of collision is certain,� 0.72 if the ‘give way’ vessel will cross astern the ‘stand on’ vessel,� 0.32 if the ‘give way’ vessel will cross ahead the ‘stand on’ vessel.


4.3.2. Cues, goals and rules mentioned in verbal protocols

Three protocols illustrate the case where the car ferry takes action, although she is the ‘stand on’ vessel.The cues noted by the expert are the following: DCPA, the target crossing position (she crosses ahead of

their vessel), distance, speed of the two vessels (his own and the target vessel), course of other neighbouringvessels, direction of the currents, position of the vessel in relation to its route.

Two different rules are mentioned in these protocols. One rule justified the action (an alteration of thecourse to starboard, at a range of 2.5 nm) with regard to the bow crossing range and to the intention to forcethe target to do something (‘‘a bow crossing range of 0.4 nm, I do not want him to cross so close to me, so Iprefer to show him where to cross”). In this case, the watch officer altered the ferry’s course to starboard toreduce the DCPA and to oblige the other to alter his course to starboard too and to cross astern. The otherrule relies on a stereotype and may be expressed in the following terms: this type of vessel (a slow cargo ship)does not follow the rules; therefore I will take action and will do it early enough to be sure that he will notmove.

5. Discussion and conclusion

Two interaction situations exist in which watch officers onboard ferries may generate an option that doesnot comply with the formal rule or consists in a specific interpretation of the rule.

We first noted that the observed behaviour of ferries may not correspond to Rule 15 because the ‘give way’ferry alters her course to port. Logistic regression shows that this option is chosen in situations where the ferrywould cross ahead the other vessel if no action were performed. Verbal protocols confirm the importance ofthis cue and point out that this action is chosen on conditions that: this manoeuvre will not generate anotherinteraction situation and the ferry is much faster than the target. This action aims at limiting the course alter-ation and the loss of time. This result supports the statement of Rothengatter (2002), who says that it is nec-essary to take external motivations (such as time pressure) into account in the study and modeling of drivingactivity.

Behaviour of ‘stand on’ ferries also consists in different applications of Rule 17, each of them aiming atmastering the situation: actions performed very early and the anticipation of the fact that the ‘give way’ cargoship will not respect the formal rule or actions taken to force the cargo ship to perform an action.

These actions are two possible interpretations of Rule 17 (Cockroft & Lameijer, 1996). In fact, Rule 17defines four stages relating to the permitted or required action for each vessel (cf. Fig. 3):

Action required

Action permitted

Action permitted

Keep course and speed

Fig. 3. The four stages relating to the permitted or required action for the ‘stand on’ vessel.


(1) at long range, both vessels are free to take any action;(2) when a risk of collision first begins to apply, the ‘give way’ vessel is required to take proper action to

achieve a safe passing distance and the ‘stand on’ vessel must keep her course and speed;(3) when it becomes apparent that the ‘give way’ vessel is not taking appropriate action, the ‘stand on’ vessel

is required to give the whistle signal and is permitted to take action to avoid collision by her manoeuvrealone;

(4) when collision cannot be avoided by the ‘give way’ vessel alone, the ‘stand on’ vessel is required to takesuch action.

The interpretation of the rule relates to the distances at which the various stages begin. In the observed sit-uations, the outer limits of the second stage seem to be of the order of 3 miles and the outer limits of the thirdstage seem to be of the order of 2 miles.

In most cases, watch officers on board ferries try to master the interaction by manoeuvring very early, whenthey are on board the ‘give way’ vessel but also when they are on the ‘stand on’ vessel.

This general strategy is efficient to avoid accidents, as well as incidents, whatever the behaviour of the cargoships. It is widely shared among the ferries operating in the Dover Strait. Belcher (2003) analysing the traffic in theDover Strait made the same findings. He pointed out the great number of near miss encounters occurring in theDover Strait, but noticed that very close near misses particularly concern overtaking situations and very fewcrossing situations. The author explained that this result is due to the early action taken by the officers on the ferryfor vessels crossing from their own port side and confirm the efficiency of the current interpretation of Rule 17.

This strategy is in accordance with the task-capability model of Fuller (Fuller, 2000, 2005), which suggeststhat drivers attempt to match task demands with their capability to maintain control. It also illustrated theidea developed by Brehmer (1992) and Amalberti (2001), that cognitive behaviour is a compromise betweenthe demands of the tasks and the need to conserve one’s cognitive resources. As was observed in car driving(Van der Hulst, Rothengatter, & Meijman, 1998), watch officers on board ferries use predictions about futureevents to perform anticipatory actions. These actions allow them to keep their cognitive resources to face otherpotential situations and to avoid the increase of workload as pointed out in the situation of the ‘stand on’vessel (Hockey, Healey, Crawshaw, Wastell, & Sauer, 2003).

The statistical analysis show that observed behaviour is not haphazard, even if it cannot be predicted fromthe formal rules. It depends on the context: on generic situations defined by the speed ratio between ships andby the positive or negative bow centre range. The psychological analysis, relying on verbal reports, explain thetypes of behaviour and show that they follow strategies that are shared among certain groups of ships, ferriesplying in the Dover Strait and in other areas. In fact, these strategies are also consistent with the results of anactivity analysis performed onboard ferries operating in the English Channel (Chauvin, 2000) and with anactivity analysis performed in several countries onboard high-speed ferries (Olsson & Jansson, 2006). Theyrepresent two general rules: a tactical rule aiming at achieving a safe passing distance and a strategy aimingat mastering the interaction situations.

From a practical point of view, such results – revealing the strategies available in a given situation – couldbe implemented in software used to monitor the traffic and to detect abnormal behaviour. It is also useful forthe training of young watch officers, since it gives a basis for the design of exercises helping trainees to acquireexpertise. In fact, according to Cannon-Bowers and Bell (1997), such exercises must be designed allowing themto define relevant cue patterns, and to build up schemata to which they can associate satisfying answers.

Acknowledgements

We thank the Gris-Nez vessel traffic system operators and the company that helped us to perform this studyand the watch officers that told us about their work and welcomed us on board their vessels.

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Decision making and strategies in an interaction situation: Collision avoidance at sea