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1 INTRODUCTION

Maritime navigation, particularly in Traffic Separation

Schemes (TSS), poses significant challenges for

watchkeeping crews due to the density of traffic and

the increased likelihood of close-quarter situations and

collisions. TSS are designated areas in busy waterways

where traffic is organized into lanes to enhance safety

and efficiency, governed by the International

Regulations for Preventing Collisions at Sea

(COLREG). Despite these regulations, ambiguities in

interpretation and the dynamic nature of dense traffic

often complicate collision avoidance efforts. The

COLREG, specifically Rule 8, emphasizes that actions

to avoid collisions should be positive, made in ample

time, and result in a safe passing distance, typically

prioritizing course alterations as the primary

maneuver (IMO, 1972). However, in TSS environments

characterized by constrained space and heavy traffic,

altering course may not always be feasible or sufficient,

prompting the need to explore alternative strategies

such as reducing ship speed.

Speed reduction, though explicitly permitted under

COLREG Rule 8 as an effective avoidance action, is

often underutilized by navigators. This reluctance may

stem from operational pressures, misjudgments of

situational dynamics, or a lack of familiarity with its

efficacy in specific scenarios. Existing studies on

collision avoidance in TSS have predominantly focused

on course alterations and the application of COLREG

rules in simulated and real-world settings (Chin &

Debnath, 2009; Goerlandt & Kujala, 2011). However,

the potential of speed reduction as a standalone or

complementary action remains underexplored.

Reducing speed may increase reaction time and

provide greater maneuverability in confined waters.

This paper investigates the efficiency of speed

reduction as a collision avoidance action in TSS by

analyzing historical collision and close-quarter

Consideration of Ship Reducing Speed as Collision

Avoidance Action

M. Barić, V. Pavić, L. Grbić & I. Mislov

University of Zadar, Zadar, Croatia

ABSTRACT: Navigation through the TSS represent a certain amount of stress for watch crew. In such areas the

traffic is heavier and close quarter situations and collisions are more likely to occur. Despite the COLREG

regulations for navigation in TSS there are always situations which are differently interpretated by navigators

and due to dense traffic avoiding collision can be challenging. As the COLREG rules state the collision can be

most efficiently avoided by altering ship course, however sometimes that can be difficult in TSS during heavy

traffic. This paper analyses the collisions and close quarter situations in TSS which could be avoided by reducing

ship speed and reproduce similar situations in navigation simulator. Such reproduction on navigation simulator

is used to analyses behaviors of navigator in collision situations and what prevents them to use speed reduction

in collision avoidance. The results of the research may be used to familiarize the navigators that in certain

situations reducing ship speed could be most efficient action to avoid collision.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 19

Number 2

June 2025

DOI: 10.12716/1001.19.02.07

396

incidents that could have been mitigated through this

approach. To further understand navigators’ decision-

making processes, the study employs a navigation

simulator to replicate TSS scenarios where speed

reduction could prove advantageous. The simulator-

based approach allows for controlled experimentation,

offering insights into how training and familiarity with

speed adjustment can enhance collision avoidance

outcomes.

The results could bring changes in existing training

programs, encouraging navigators to consider speed

adjustments as part of their decision-making toolkit in

dense traffic scenarios.

2 LITERATURE REVIEW

Maritime navigation within Traffic Separation

Schemes (TSS) is a critical area of study due to the

heightened risk of collisions stemming from dense

vessel traffic and complex navigational demands. This

literature review examines existing research on

collisions and close-quarter situations in TSS, with a

particular focus on collision avoidance strategies,

including the role of speed reduction.

The COLREG framework, particularly Rule 8,

provides the foundational guidelines for collision

avoidance, mandating actions that are positive, timely

and result in a safe passing. However, in TSS

environments, where space is limited and traffic

density is high, the feasibility of course changes

diminish. Research by Chin and Debnath (2009)

analyzed collision incidents in the Singapore Strait TSS,

identifying those misjudgments in applying COLREG

rules, particularly in multi-vessel encounters, often

lead to near-misses or collisions. Similarly, Goerlandt

and Kujala (2011) utilized probabilistic risk models to

demonstrate that the dynamic interactions in TSS

amplify the likelihood of close-quarter situations,

suggesting that strict adherence to course-based

avoidance may not always suffice.

Human decision-making plays a pivotal role in

collision avoidance, yet it is frequently undermined by

operational pressures and situational

misinterpretation. Statheros et al. (2008) reviewed

navigational practices and found that watchkeeping

officers often hesitate to deviate from planned routes

or speeds due to commercial schedules or uncertainty

about other vessels’ intentions. This reluctance is

particularly pronounced in TSS, where navigators

must balance compliance with lane discipline and the

need to avoid collisions. A study by Hetherington et al.

(2006) on maritime human factors highlighted that

inadequate training and over-reliance on automated

systems, such as the Automatic Identification System

(AIS), can exacerbate decision-making errors in high-

traffic zones. These findings underscore the need for

alternative strategies that enhance navigators’

flexibility in TSS scenarios.

While course alteration dominates collision

avoidance literature, speed reduction remains

underexplored. However, empirical studies on its

application in TSS are limited. Debnath and Chin

(2010) conducted a statistical analysis of near-miss

incidents in TSS and noted that speed adjustments

were rarely employed, despite their potential to de-

escalate critical situations. The authors attributed this

to a lack of awareness or training, as well as a cultural

preference for maintaining speed to meet operational

deadlines.

Simulation-based research offers further insights

into speed reduction’s efficacy. Szlapczynski and

Szlapczynska (2017) used navigational simulators to

test collision avoidance strategies and found that speed

reduction, when combined with course changes,

significantly improved outcomes in multi-vessel TSS

scenarios. However, their study focused on hybrid

maneuvers rather than speed reduction as a standalone

action.

The literature reveals a predominant focus on

course alterations and COLREG compliance, with

insufficient exploration of speed reduction as a

primary or complementary strategy in TSS. Existing

studies often rely on theoretical models or post-

incident analyses, with limited experimental data from

controlled simulations. Furthermore, navigators’

reluctance to use a speed adjustment remains a

challenging barrier. The proposed study’s emphasis on

simulator-based experimentation in order to provide

providing empirical evidence on speed reduction’s

efficacy.

Speed reduction emerges as a promising strategy,

offering increased reaction time and maneuverability.

This review supports the introduction’s focus on

investigating speed reduction through simulation and

historical analysis, with potential implications for

improving navigational safety in TSS environments.

3 METHODOLOGY

This research investigates navigational decision-

making in a simulated Traffic Separation Scheme (TSS)

under heavy traffic conditions, with a specific focus on

the use of speed reductions as a collision avoidance

strategy. The methodology combines a controlled

simulation environment, participant observation, and

quantitative data analysis to assess behavioral patterns

among participants with varying levels of maritime

experience.

A total of 19 simulations were done. Participants

were experienced seafarers attending regular Ship

maneuvering and handling course at our training

facility and also two group of students currently

enrolled in our nautical undergraduate studies with

limited or no practical sea experience. The inclusion of

both experienced professionals and students allows for

a comparative analysis of decision-making strategies

influenced by practical expertise. Participants were

selected based on availability and willingness to

participate.

Bridge team was consisted of 4 persons acting as

Master, Chief Mate, OOW and Helmsman similar to

real situations on ships. The experiment utilized a full

mission Transas NTPro navigational simulator capable

of replicating a realistic TSS environment. The

simulated scenario was designed to reflect a busy

westbound traffic in TSS in Singapore Strait with heavy

traffic of various kind of vessels moving at varying

speeds and trajectories and based on real incidents. The

scenario incorporated dynamic factors such as tidal

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currents and a high density of vessels to increase

navigational complexity and pressure. Each group of

participants were tasked with navigating a predefined

route through the TSS while adhering to the

International Regulations for Preventing Collisions at

Sea (COLREGs).

Participants were very familiar with the simulator

controls and also with the objectives of the exercise,

which included safely navigating through the TSS

while avoiding collisions. No explicit instructions were

provided regarding speed reductions to ensure that

any use of this strategy emerged from their decision-

making process. Each group completed the simulation

in a single session lasting approximately 45 to 55

minutes. The instructor recorded key navigational

parameters such as vessel speed, course alterations,

closest point of approach (CPA) to other vessels, and

time to closest point of approach (TCPA). Used vessel

was laden AFRAMAX crude oil tanker where the

reducing speed is often considered as less effective

collision avoiding action.

Vessel speed (in knots) was logged at 10-second

intervals throughout the simulation. Speed reduction

events were defined as a deliberate decrease in speed

using telegraph command by at least 15-20% from the

initial speed, sustained for a minimum of 60 seconds,

and not attributable to external factors. Instructor

recorded also cases where bridge team explicitly

mentioned speed adjustments or appeared to prioritize

speed reduction over other maneuvers based on their

interaction with the simulator interface.

The study adhered to ethical guidelines, ensuring

participant anonymity and voluntary participation. No

personal data beyond experience level and simulation

performance were collected, and participants were

debriefed on the study’s purpose following their

session.

The simulation environment, while realistic, may

not fully replicate the stress or unpredictability of real-

world TSS navigation. Additionally, the small sample

size (n=20) limits generalizability, though it provides a

foundation for preliminary insights due to the range of

participants experience.

4 RESULTS

In the research there was 19 participant groups, with

different sea experience. Three research groups were

composed of senior officers, fourteen research groups

were composed of junior officers and two research

groups were students (Figure 1).

The simulator instructor, who was conducting the

exercise, monitored the results of the exercise. The safe

action was considered as course alteration, speed

reduction or both, as long as the collision avoiding

action had the minimum CPA larger than 0,5 M. Also,

the safe collision avoidance is considered in CPA less

than 0,5 miles as long it was passing astern of the vessel

being avoided but not less than 0,3 M. The results from

the simulations showed that 7 research groups made

the safe collision avoidance action, 9 research groups

came in close quarter situation, and 3 research groups

collided or made reaction opposite of all rules.

Figure 1. Research groups participants sea experience and

ranks

Figure 2. The results of the collision avoidance action

The results of action taken for collision avoidance

show that 6 groups used vessel controlled slow down

to make safe collision avoidance, and all of them

resulted in safe and controlled situation.

Figure 3. The results of the taken collision avoidance action

All of the research groups conducted of senior

officers made the safe collision avoidance action, and

66% of them used controlled slow down. The average

vessel speed in speed reduction was 7.8 knots and the

average vessel speed in other scenarios was 11.4 knots.

Three collision situations occurred to junior officer,

where the actions were late or opposite to collision

regulations. Almost 77 % of close quarter situations

were caused by junior officers and the rest of 23 % by

student research groups. The actions in this situation

were done without slowing down and with undecided

actions to avoid collision.

Figure 4. The results of the collision avoidance action per

rank

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The average CPA in cases where the vessel speed

was reduces is 0,42 M and in other cases 0,16 M. This

result show that the speed reduction had almost three

times bigger CPA than course altering alone.

Figure 5. The results of the collision avoidance action CPA

per rank

5 DISCUSSION

The simulations conducted in this study demonstrate

that speed reduction serves as an effective collision

avoidance strategy within Traffic Separation Schemes

(TSS), yielding larger Closest Points of Approach

(CPAs) averaging 0.42 miles and safer navigational

outcomes compared to an average CPA of 0.16 miles in

scenarios without speed reduction. Notably, all six

groups employing controlled slowdowns achieved

safe and controlled passages, with senior officers—

comprising 66% of these groups—exhibiting a higher

tendency to utilize this manoeuvre, likely due to their

greater experience in assessing situational dynamics.

These findings challenge the conventional emphasis on

course alterations as the primary avoidance action

under COLREG Rule 8, suggesting that speed

reduction can be a valuable alternative or complement,

particularly in the spatially constrained, high-traffic

conditions of TSS. The literature indicates that

navigators often underutilize speed adjustments due to

operational pressures, misperceptions of efficacy

(especially for vessels like the laden AFRAMAX tanker

used here), or inadequate training, yet this study’s

results contradict such reluctance by showcasing

tangible safety benefits. However, the reliance on a

simulated environment and a small sample size of 19

groups limits the generalizability of these conclusions,

underscoring the need for further real-world

validation. Integrating speed reduction into

navigational training could enhance decision-making

flexibility, particularly for junior officers and students

who displayed higher rates of close-quarter situations

and collisions when adhering rigidly to speed

maintenance or delayed actions.

6 CONCLUSION

The quantitative analysis suggests that speed

reduction is a critical strategy for ensuring safe passing

in a TSS under heavy traffic conditions. Participants

who used controlled slowing down maintained lower

speeds, achieved greater CPAs, and had a higher

likelihood of safe and controlled maneuvers while

remaining compliant with TSS rules. Conversely, trials

without speed reduction were associated with higher

speeds, smaller CPAs, and a greater incidence of

critical situations or collisions, particularly among less

experienced ranks (e.g., Third Mate, Student). While

these results highlight the noticeably safety benefits of

speed reduction, the study’s reliance on a simulated

environment and a relatively limited sample size of 20

groups introduces constraints on the broader

applicability of the conclusions. Real-world

variables—such as unpredictable weather conditions,

and human factors like fatigue or communication

breakdowns—were not applied, suggesting that

further empirical validation in operational settings is

essential to confirm these findings. Nevertheless, the

results of research presented here challenges

prevailing navigational practice that often prioritize

course changes over speed adjustments. This research

highlights the broader effects of speed reduction in

collision avoidance, extending to training and policy

development. Integrating speed control into guidelines

could enhance navigators' decision-making, especially

for less experienced mariners, while promoting a

uniform approach to safer navigation in TSS

environments.

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