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COLREG

Rule 10: Traffic Separation Schemes

In a COLREG traffic separation scheme, follow the traffic flow and keep clear of separation, while applying its exceptions.

Rule 10 of 41

You · A
Decision 1 of 3

You are in A, a motor boat under 20 m, and must cross the traffic lane B is following. How do you cross it?

The rule in plain words

  1. How do you use a traffic lane?Proceed in the appropriate lane in the general direction of traffic flow. So far as practicable, keep clear of the separation line or zone. Join or leave at the termination; from a side, use the smallest practicable angle.
  2. How do you cross traffic lanes?Avoid them so far as practicable. If obliged, cross on a heading that is as nearly practicable at right angles to the flow.
  3. When may you use an inshore traffic zone?If you can safely use the appropriate lane, do not use the zone. Vessels under 20 metres, sailing vessels, vessels engaged in fishing, and vessels going to or from a place in the zone or avoiding immediate danger may use it.
  4. Who must not impede?Fishing vessels must not impede vessels following a lane; vessels under 20 m and sailing vessels, power-driven vessels following it. Restricted-manoeuvrability vessels working within the scheme on navigation safety or submarine cables are exempt only as necessary for that work.
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.
1 of 3

Frequent mistakes

  • Wrong: "A fishing vessel may impede traffic if it is outside the lane."

    Right: Fishing vessels must not impede any vessel following a traffic lane.

  • Wrong: "A sailing vessel may always impede a power-driven vessel."

    Right: Neither a sailing vessel nor a vessel under 20 metres may impede its safe passage.

  • Wrong: "Restricted manoeuvring ability removes every COLREG obligation."

    Right: Only RAM vessels maintaining navigation safety or laying, servicing or recovering submarine cables within the scheme qualify, as necessary.

Exam keys & interpretation

  1. If an option mixes the inshore traffic zone with the separation zone, separate the names before judging it.
  2. "Normally", "so far as practicable" and "where practicable" change the strength of the obligation.
  3. For "must not impede", identify who must keep clear and which vessel the rule protects.

How to read it

The scheme organises traffic, but no exception automatically lets you ignore the rest of Rule 10.

When an exception appears, check its exact situation and limit the manoeuvre to what that situation permits.

Key points

  • Separation zone: normal entry no · emergency or fishing → exceptions.
  • Scheme terminations → particular caution.
  • Anchoring within a TSS or near its terminations → avoid so far as practicable.
  • Not using the scheme → leave the widest practicable margin.

Traffic separation practice

Synthetic diagram: decide how to join, leave or cross without treating it as a navigation chart.

COLREG · USCG NAVCEN · 2026-09-19
Synthetic traffic-lane diagramA horizontal lane with traffic flowing right. Arrows show joining at the termination, leaving with a shallow angle, and crossing the entire lane on an approximately perpendicular heading.
Synthetic diagram, not a navigation chart; arrows do not fix a real ground track.
  1. 1. Lane termination for joining when practicable.
  2. 2. Lane termination for leaving when practicable; if side entry or exit is necessary, use the smallest practicable angle.
  3. 3. Necessary crossing on a heading as nearly as practicable at 90° to the general traffic direction.
How should a vessel join a traffic lane?

Join at the termination of the lane when practicable. If joining from the side is necessary, use the smallest practicable angle to the general direction of traffic. The diagram is synthetic and is not a navigation chart.

How should a vessel leave a traffic lane?

Leave at the termination of the lane when practicable. If leaving from the side is necessary, use the smallest practicable angle to the general direction of traffic. Do not read the illustration as fixing a real track or a fixed angle.

If crossing a traffic lane is necessary, what heading should be used and what priority follows?

Avoid crossing traffic lanes so far as practicable. If obliged to cross, use a heading as nearly as practicable at right angles to the traffic flow. Heading and ground track can differ in a current. Rule 10 does not remove any vessel's obligations under the other Rules or grant automatic priority.

Test your knowledge

3 questions to check it. Pick an answer to see why.

Question 1/3

If obliged to cross a traffic lane, which heading should a vessel use?

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Official text

IMO COLREG 1972 U.S. Coast Guard — International Navigation Rules (2024)

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.
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Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed:

Frequently asked questions about COLREG Rule 10

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.

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.

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.