Rule 10 — Traffic Separation Schemes

What the rule requires

The two angles decide more Rule 10 questions than everything else in the rule put together, and they point in opposite directions.

A vessel shall so far as practicable avoid crossing traffic lanes, but if obliged to cross, she shall do so on a heading as nearly as practicable at right angles to the general direction of traffic flow 33 CFR §83.10. The operative word is heading. The rule regulates the direction the vessel is pointed, which is what an observer in the lane can read off her aspect and off radar, not her course over ground after set and drift. The text is identical internationally COLREGs Rule 10(c).

Joining or leaving is the other case. A vessel shall normally join or leave a traffic lane at the termination of the lane, but when joining or leaving from either side she shall do so at as small an angle to the general direction of traffic flow as practicable COLREGs Rule 10(b). Small angle to get in or out of the flow; right angles to pass through it.

Once in a lane, a vessel using the scheme shall proceed in the appropriate traffic lane in the general direction of traffic flow for that lane, and shall so far as practicable keep clear of a traffic separation line or separation zone.

You are inbound and need to leave the traffic lane on its port side to reach an anchorage. At what angle do you leave?

At as small an angle to the general direction of traffic flow as is practicable. Joining or leaving from either side takes the small-angle provision of Rule 10(b)(iii). Right angles belong to crossing the lane, Rule 10(c). Note also that the rule expects you to normally join or leave at the termination of the lane; leaving from the side is the exception the small-angle wording governs.

Three classes get a concession, and the same three owe a duty

Instructors commonly teach the small-vessel provisions of Rule 10 as the same trio that appears in the narrow channel rule: fishing, sailing, and under 20 metres. The trio holds here, and it works twice over, once as a permission and once as a restriction.

The permission is the inshore traffic zone. 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 COLREGs Rule 10(d). Beyond those three classes, any 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.

The restriction is the duty not to impede, and here the trio is lossy in a way that gets tested. The two paragraphs are not parallel:

A vessel engaged in fishing shall not impede the passage of any vessel following a traffic lane COLREGs Rule 10.
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 COLREGs Rule 10(j).

So the fishing vessel's duty runs to any vessel following the lane; the small vessel's and the sailing vessel's duty is stated against a power-driven vessel following the lane. Learn the trio for the inshore traffic zone, and learn the two paragraphs separately for the impeding duty.

Separation zones and separation lines

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, with two exceptions: in cases of emergency to avoid immediate danger, and to engage in fishing within a separation zone COLREGs Rule 10(e).

Read the carve-out at the front of that sentence carefully. The crossing vessel and the vessel joining or leaving a lane are outside the prohibition altogether, because both of them must necessarily traverse the zone to do what the rule already contemplates them doing.

The qualified duties

Three paragraphs are written with softening language, and the softening is the point.

A vessel shall, so far as practicable, avoid anchoring in a traffic separation scheme or in areas near its terminations COLREGs Rule 10(g). A vessel not using a traffic separation scheme shall avoid it by as wide a margin as is practicable. And a vessel navigating in areas near the terminations of traffic separation schemes shall do so with particular caution COLREGs Rule 10(f), which is a standard of care rather than a manoeuvre.

The two exemptions, and what they do not cover

A vessel restricted in her ability to maneuver is exempted from complying with Rule 10 in two cases only: when engaged in an operation for the maintenance of safety of navigation in a traffic separation scheme COLREGs Rule 10(k), and when engaged in an operation for the laying, servicing, or picking up of a submarine cable within a traffic separation scheme COLREGs Rule 10(l). Both exemptions run only to the extent necessary to carry out the operation.

Underpinning all of it: the rule does not relieve any vessel of her obligation under any other Rule COLREGs Rule 10(a). Nothing in Rule 10 suspends lookout, safe speed, risk-of-collision assessment or the steering and sailing rules inside a scheme.

International against Inland

The Inland codification at 33 CFR §83.10 carries the same twelve paragraphs, and the sources record Inland Rule 10 as word for word identical to the International rule. The only textual variance sits in paragraph (a): internationally the rule applies to schemes adopted by the Organization, and the Inland text applies to traffic separation schemes and preserves obligations under any other Rule in subchapter E. No duty in the rule changes. A Rule 10 question on angles, on the inshore traffic zone, or on impeding has the same answer under either regime.

Telling it apart

One criterion sorts every Rule 10 question about manoeuvring: what are you doing with respect to the lane? Four answers, four different duties, and candidates lose marks by applying the duty attached to a position they are not in.

Following the lane. Proceed in the appropriate lane in the general direction of traffic flow for that lane, and so far as practicable keep clear of the separation line or separation zone. Most often misfiled: the vessel proceeding against the flow because her destination is that way. There is no such option in the rule.
Crossing. A heading as nearly as practicable at right angles to the general direction of traffic flow. Most often misfiled: crossing on a shallow diagonal to shorten time in the lane, which is the joining-and-leaving angle applied to the wrong situation.
Joining or leaving from either side. As small an angle to the general direction of flow as practicable, the normal expectation being that you join or leave at the termination of the lane. Most often misfiled: a vessel that means to cross but describes herself as joining, then claims the small angle.
Not using the scheme. Avoid it by as wide a margin as is practicable. Most often misfiled: treating the outer edge of the lane as a boundary to be shaved rather than a scheme to be given room.

The crossing vessel and the vessel joining or leaving are also the two classes the separation-zone prohibition of Rule 10(e) does not reach.

Working a question

You have the helm of a 17-metre uninspected passenger vessel. The fishing bank you are chartered to work lies on the far side of a traffic separation scheme, and a laden tanker is standing on inbound in the near lane.

Establish whether you need to be in the scheme at all. At less than 20 metres you may use the inshore traffic zone under Rule 10(d)(i), and if the zone carried you to the bank you would have no lane problem. It does not, so you must cross.
Fix your heading, not your track. The requirement is a heading as nearly as practicable at right angles to the general direction of traffic flow. With a cross-set you will crab downstream of your intended track; you accept that rather than cocking the heading to hold the track, because the rule regulates the heading and the heading is what the tanker's bridge team is reading.
Do not confuse yourself into the small-angle provision. You are not joining the lane and you are not leaving it from the side. You are a crossing vessel, so Rule 10(b)(iii) has no application to you.
Cross the separation zone without hesitation. As a crossing vessel you are expressly outside the prohibition in Rule 10(e) on entering a separation zone or crossing a separation line.
Time the crossing around the tanker. Under 20 metres in length, you shall not impede the safe passage of a power-driven vessel following a traffic lane. Deciding to cross ahead and hold your speed up to make it is the manoeuvre that puts you in breach of Rule 10(j) whether or not it works.
Keep every other rule running. Rule 10 does not relieve you of your obligation under any other Rule. Lookout, safe speed and assessment of risk of collision apply inside the lane exactly as they do outside it.
Choose the anchorage with the scheme in mind. If the bank lies within the scheme or near a termination, you avoid anchoring there so far as practicable.

On an exam question built from this situation, the option that survives is the one that has you cross at right angles to the traffic flow. The distractors will offer a small angle, a diagonal to save time, or a claim that a vessel under 20 metres is not bound by the rule.

Where candidates lose the point

Answering "as small an angle as practicable" for a crossing. Both angles are in the same rule, one paragraph apart, and the shallow angle sounds seamanlike because it shortens exposure. Rule 10(c) says right angles for crossing; the small angle belongs only to joining or leaving from either side.

Reading the under-20-metre concession as a general exemption. The concession in Rule 10(d)(i) is narrow: those vessels may use the inshore traffic zone. It says nothing about crossing angles, nothing about direction of flow, and Rule 10(j) then loads an extra duty onto the same vessel. A 17-metre vessel crossing a lane crosses at right angles like everyone else.

Treating "shall not impede" as a right of way in reverse. A candidate who has learned that sailing vessels rank above power-driven vessels will pick the option making the sailing vessel stand-on against a ship following the lane. Rule 10(j) puts the duty the other way, and Rule 10(a) leaves her other obligations untouched.

Deciding fishing is banned inside a scheme. Rule 10(e)(ii) expressly permits engaging in fishing within a separation zone, which is one of only two grounds for a non-crossing vessel to be in that zone. The constraint on the fishing vessel is Rule 10(i): she shall not impede the passage of any vessel following a traffic lane.

Turning the qualified duties into absolute ones. Anchoring in a scheme is to be avoided so far as practicable, not prohibited. Crossing lanes is to be avoided so far as practicable, with an express provision for the vessel obliged to cross. An answer option that says "prohibited" where the rule says "shall so far as practicable avoid" is a wrong option.

Granting a restricted-in-ability-to-manoeuvre vessel a blanket exemption. She is exempted only when engaged in maintenance of safety of navigation in the scheme, or laying, servicing or picking up a submarine cable within it, and only to the extent necessary to carry out that operation. An RAM vessel transiting a scheme on passage complies with Rule 10 in full.

Check yourself

Your 40-metre power-driven vessel is obliged to cross a traffic lane. What heading do you steer?

A heading as nearly as practicable at right angles to the general direction of traffic flow for that lane, per Rule 10(c). The rule specifies the heading, so a cross-current that sets you off your intended track does not license you to cock the bow into it and present a slanted aspect to lane traffic.

You are a 15-metre power-driven vessel bound down a coast. An inshore traffic zone lies alongside the scheme and takes you where you are going. May you use it?

Yes. Rule 10(d)(i) opens the inshore traffic zone to vessels of less than 20 metres in length, sailing vessels, and vessels engaged in fishing, whether or not they could safely use the appropriate lane. A vessel outside those three classes shall not use the zone when she can safely use the appropriate traffic lane, unless she is en route to or from a port, offshore installation or structure, pilot station or other place within the zone, or is avoiding immediate danger.

Under sail in a traffic lane, you are converging with a container ship following that lane. What does Rule 10 require of you?

Not to impede her safe passage. Rule 10(j) places that duty on a sailing vessel and on any vessel of less than 20 metres in relation to a power-driven vessel following a traffic lane. Manoeuvre early and by enough that she is never obliged to alter for you. Rule 10 does not relieve you of your obligations under any other Rule either.

You are following a traffic lane and wish to cut the corner across the separation zone to reach the opposing lane, which serves your destination. Permitted?

Not on those terms. Rule 10(e) bars a vessel that is neither crossing nor joining or leaving a lane from normally entering a separation zone or crossing a separation line, the exceptions being an emergency to avoid immediate danger and engaging in fishing within a separation zone. If you genuinely need to get to the other side, you do it as a crossing vessel, at right angles to the flow.

Your vessel is restricted in her ability to manoeuvre while picking up a submarine cable inside a scheme. How far does your exemption from Rule 10 extend?

To the extent necessary to carry out the operation, and no further, under Rule 10(l). The companion exemption in Rule 10(k) covers an RAM vessel engaged in an operation for the maintenance of safety of navigation in the scheme. Neither exempts an RAM vessel simply transiting.

You intend to leave a traffic lane from its starboard side rather than run on to the lane's termination. What does the rule expect?

The rule contemplates that you normally join or leave at the termination of the lane; leaving from the side, you do so at as small an angle to the general direction of traffic flow as practicable. That is Rule 10(b)(iii), and it is the mirror image of the crossing requirement.

A question is labelled "(Inland Only)" and asks the crossing angle for a traffic lane. Does your answer change?

No. Inland Rule 10 as codified at 33 CFR §83.10 is word for word identical to the International rule, so the right-angle crossing requirement, the small-angle joining provision, the inshore traffic zone concession and the impeding duties all read the same. The only wording that differs is in paragraph (a), where the International rule refers to schemes adopted by the Organization and the Inland text preserves obligations under any other Rule in subchapter E.