How must a vessel use a traffic separation scheme?

A vessel using a scheme shall proceed in the appropriate traffic lane in the general direction of traffic flow, keep clear of the traffic separation line or separation zone so far as practicable, and normally join or leave a lane at its termination — otherwise at as small an angle to the traffic flow as practicable.

Who must not impede a vessel following a traffic lane?

Vessels under 20 metres and sailing vessels must not impede the safe passage of a power-driven vessel following a lane (Rule 10(j)). Vessels engaged in fishing must not impede any vessel following a lane (Rule 10(i)). These duties do not remove obligations under the other collision rules.

Traffic Separation Schemes

Q: At what angle should a vessel cross a traffic lane?

A vessel obliged to cross traffic lanes shall cross on a heading as nearly as practicable at right angles to the general direction of traffic flow (Rule 10(c)). Lanes should be crossed only when crossing is necessary.

Imaginary traffic-separation scheme: vessels using a traffic lane follow its general traffic direction. The inshore zone is not shown as one-way. Use of inshore zones and entry into separation zones remain subject to Rule 10.

Rule 10 applies to IMO-adopted traffic separation schemes and does not remove duties under any other rule.

Use the appropriate lane in the direction of traffic flow, keep clear of the separation line or zone so far as practicable, and normally join or leave at a lane termination. When joining or leaving from the side, use as small an angle to the traffic flow as practicable.

Avoid crossing lanes so far as practicable. If obliged to cross, use a heading as nearly as practicable at right angles to the general traffic flow; the requirement concerns heading, not track over ground.

Do not use an inshore traffic zone when you can safely use the adjacent lane. Vessels under 20 metres, sailing vessels and vessels engaged in fishing may use it. A vessel may also use it when bound to or from a port, offshore installation or structure, pilot station or another place within the zone, or to avoid immediate danger.

A vessel that is not crossing, joining or leaving a lane must not normally enter a separation zone or cross a separation line, except in an emergency to avoid immediate danger or to fish within a separation zone.

Navigate with particular caution near scheme terminations.

So far as practicable, avoid anchoring in a scheme or near its terminations.

Vessels not using the scheme should avoid it by as wide a margin as practicable.

A vessel engaged in fishing must not impede any vessel following a traffic lane.

A vessel under 20 metres or a sailing vessel must not impede the safe passage of a power-driven vessel following a lane.

–(l) RAM vessels maintaining navigational safety or laying, servicing or picking up a submarine cable within the scheme are exempt from this rule only to the extent necessary to carry out the operation.

Successive positions of one crossing vessel: its heading is perpendicular to the traffic flow, while current alters its track over ground. When obliged to cross, the heading shall be as nearly as practicable at right angles; crossing gives no priority.

One vessel joins a traffic lane from the side, aligning with its flow. Side entry should use as small an angle to the traffic direction as practicable. No fixed numerical angle or passage through the separation zone is shown.

IMO COLREG 1972Official text

Full wording of the rule, reproduced from the official source below.

(a) This Rule applies to traffic separation schemes adopted by the Organization and does not relieve any vessel of her obligation under any other rule. (b) A vessel using a traffic separation scheme shall: (i) proceed in the appropriate traffic lane in the general direction of traffic flow for that lane; (ii) so far as practicable keep clear of a traffic separation line or separation zone; (iii) normally join or leave a traffic lane at the termination of the lane, but when joining or leaving from either side shall do so at as small an angle to the general direction of traffic flow as practicable. (c) A vessel shall, so far as practicable, avoid crossing traffic lanes but if obliged to do so shall cross on a heading as nearly as practicable at right angles to the general direction of traffic flow. (d) (i) A vessel shall not use an inshore traffic zone when she can safely use the appropriate traffic lane within the adjacent traffic separation scheme. However, vessels of less than 20 meters in length, sailing vessels and vessels engaged in fishing may use the inshore traffic zone. (ii) Notwithstanding subparagraph (d)(i), a vessel may use an inshore traffic zone when en route to or from a port, offshore installation or structure, pilot station or any other place situated within the inshore traffic zone, or to avoid immediate danger. (e) A vessel other than a crossing vessel or a vessel joining or leaving a lane shall not normally enter a separation zone or cross a separation line except: (i) in cases of emergency to avoid immediate danger; (ii) to engage in fishing within a separation zone. (f) A vessel navigating in areas near the terminations of traffic separation schemes shall do so with particular caution. (g) A vessel shall so far as practicable avoid anchoring in a traffic separation scheme or in areas near its terminations. (h) A vessel not using a traffic separation scheme shall avoid it by as wide a margin as is practicable. (i) A vessel engaged in fishing shall not impede the passage of any vessel following a traffic lane. (j) A vessel of less than 20 meters in length or a sailing vessel shall not impede the safe passage of a power-driven vessel following a traffic lane. (k) A vessel restricted in her ability to maneuver when engaged in an operation for the maintenance of safety of navigation in a traffic separation scheme is exempted from complying with this Rule to the extent necessary to carry out the operation. (l) A vessel restricted in her ability to maneuver when engaged in an operation for the laying, servicing or picking up of a submarine cable, within a traffic separation scheme, is exempted from complying with this Rule to the extent necessary to carry out the operation.

Source:

U.S. Coast Guard — International Navigation Rules (2024)

STCW Bridge Watch Lens

Apply the vessel-specific duties in Rules 9(b)–(c) and 10(i)–(j).

Following a traffic lane gives no general right of way; the other collision-avoidance duties remain, including Rule 8(f).

Build the traffic picture with sight, hearing, radar/ARPA and chart context.

Do not let AIS or one isolated bearing replace systematic observation.

After manoeuvring, keep monitoring bearing, range, CPA/TCPA and passing distance until the other vessel is finally past and clear.

Exam Focus

Start every scenario by classifying the encounter: overtaking, head-on, crossing, narrow channel, traffic separation, or restricted visibility.

For vessels in sight, every overtaking vessel keeps clear until finally past and clear.

Rule 13 takes precedence over Part B Sections I and II; it does not override the other Parts of COLREG.

Questions on channels and TSS often test the difference between 'keep out of the way' and 'shall not impede'.

Read that wording carefully.

Key Takeaways

Use the lane in the direction of traffic flow and stay clear of the separation line or zone

Cross lanes only when necessary and do it as nearly as practicable at right angles

The wording 'shall not impede' is central for small craft, sailing craft and fishing vessels in a TSS

Inshore traffic zones are not a shortcut for larger ships when the main lane can be used safely

Common Mistakes

Confusing the required crossing heading with the track over ground

Treating an inshore traffic zone as an unrestricted shortcut

Assuming vessels in a traffic lane have absolute priority under every rule

Anchoring in a scheme or near its terminations when it is practicable to avoid doing so

Test Your Knowledge

Practise this topic and review the explanation for each answer.

Last reviewed: September 9, 2026