Traffic Separation Schemes (TSS): lanes, zones and Rule 10 duties
A traffic separation scheme is a one-way road system for ships. It keeps opposing streams of traffic apart in busy waters such as the Dover Strait. This guide explains each part of a scheme, what COLREG Rule 10 asks of every vessel in or near one, how schemes look on a chart, and three real examples.
A traffic separation scheme (TSS) is a routeing measure that separates opposing streams of traffic by establishing traffic lanes, usually with a separation zone or line between them. Schemes adopted by IMO are governed by COLREG Rule 10: follow your lane in the general direction of traffic flow, normally join or leave at the ends, cross on a heading as nearly as practicable at right angles to the traffic flow, and keep clear of the separation zone.
COLREG Rule 10: full text and practiceSchematic only, not to scale and not for navigation. Real schemes vary: check the chart and the scheme's own provisions.
1Traffic lane: one-way traffic in the direction of the arrows.
2Separation zone: keeps the opposing lanes apart; keep clear of it unless crossing, joining or leaving, in an emergency, or fishing in it.
3Inshore traffic zone: between the scheme and the coast, for the vessels Rule 10(d) allows.
4Precautionary area: navigate with particular caution where lanes meet or end.
5Crossing: on a heading as nearly as practicable at right angles to the general direction of traffic flow.
6Joining at the termination of the lane, the normal way in.
7Joining from the side at as small an angle to the traffic flow as practicable.
The parts of a traffic separation scheme
These are the routeing terms used by IMO and in US federal regulations (33 CFR 167.5). Not every scheme has every part.
Traffic separation scheme (TSS)
A routeing measure aimed at separating opposing streams of traffic by appropriate means and by establishing traffic lanes.
Traffic lane
An area within defined limits in which one-way traffic is established. Natural obstacles, including those forming separation zones, may be a boundary.
Separation zone or line
A zone or line that separates lanes in which ships proceed in opposite or nearly opposite directions. It can also separate a lane from the adjacent sea area, or separate lanes for particular classes of ships going the same way.
Roundabout
A separation point or circular separation zone with a circular traffic lane around it, within defined limits.
Inshore traffic zone (ITZ)
A designated area between the landward boundary of a TSS and the adjacent coast. Rule 10(d) limits who may use it.
Precautionary area
An area within defined limits where ships must navigate with particular caution. The direction of traffic flow may be recommended inside it.
Deep-water route
A route within defined limits that has been accurately surveyed for clearance of the sea bottom and submerged articles, mainly for ships whose draught needs it.
Area to be avoided
An area within defined limits where navigation is particularly hazardous or where avoiding casualties is exceptionally important. All ships, or certain classes of ships, should avoid it.
Who creates a TSS and why it is binding
COLREG Rule 1(d) says traffic separation schemes may be adopted by the Organization (IMO) for the purpose of the Rules. Rule 10(a) then applies to schemes adopted by IMO and adds that it does not relieve any vessel of her obligations under any other rule.
SOLAS regulation V/10 recognizes IMO as the only international body for developing guidelines, criteria and regulations for ships' routeing systems. Governments propose a scheme; IMO adopts it. Routeing systems are recommended for all ships and may be made mandatory for all ships, certain categories of ships or ships carrying certain cargoes. A ship must use a mandatory routeing system adopted by IMO unless there are compelling reasons not to, and any such reason must be recorded in the ship's log.
Charts also show schemes adopted by national authorities rather than IMO. The UK MCA (MGN 364) advises checking IMO Ships' Routeing and the Admiralty Notices to Mariners to see whether a scheme is IMO-adopted. In US waters, 33 CFR 167.10 requires the operator of a vessel in a TSS described in that Part to comply with Rule 10.
COLREG Rule 10 in plain English
Each line quotes or closely follows the adopted wording, then says what it means at the helm. The full rule text, the official wording and practice questions are on the Rule 10 page.
10(b)(i)Proceed in the lane in the general direction of traffic flow
A vessel using a TSS shall proceed in the appropriate traffic lane in the general direction of traffic flow for that lane.
Use the lane whose arrows point the way you are going. "General direction" means you follow the flow, not a precise course line.
10(b)(ii)Keep clear of the separation line or zone
So far as practicable keep clear of a traffic separation line or separation zone.
Stay well inside your lane rather than running along its inner edge.
10(b)(iii)Join or leave at the ends, or at a small angle
Normally join or leave a traffic lane at the termination of the lane, but when joining or leaving from either side do so at as small an angle to the general direction of traffic flow as practicable.
The normal entry and exit is at the end of the lane. If you must come in or go out from the side, merge like a car on a slip road: shallow angle, never across the flow.
10(c)Avoid crossing; if you must, cross at right angles
So far as practicable avoid crossing traffic lanes, but if obliged to do so cross on a heading as nearly as practicable at right angles to the general direction of traffic flow.
The rule is about heading, not track. Point the bow at 90° to the lane even if the tidal stream sets you sideways. The MCA explains that this minimises your time in the lane whatever the stream, and gives ships in the lane a clear crossing situation.
10(d)Inshore traffic zones are for certain vessels
Do not use an inshore traffic zone when you can safely use the appropriate lane in the adjacent scheme. Vessels under 20 metres, sailing vessels and vessels engaged in fishing may use it, and any vessel may when going to or from a port, offshore installation or structure, pilot station or other place in the zone, or to avoid immediate danger.
Through traffic belongs in the lanes. The MCA's view is that heavy traffic in a lane or restricted visibility is not, by itself, a reason to use an ITZ.
10(e)Stay out of the separation zone
A vessel other than one crossing, joining or leaving a lane shall not normally enter a separation zone or cross a separation line, except in an emergency to avoid immediate danger or to engage in fishing within a separation zone.
The separation zone is a buffer, not a third lane.
10(f)–(h)Caution at the ends, no anchoring, give it a wide berth
Navigate with particular caution near the terminations of a scheme; so far as practicable avoid anchoring in a scheme or near its terminations; a vessel not using a scheme shall avoid it by as wide a margin as is practicable.
Traffic converges and turns at the ends, so slow your decision-making down there. If you are not using the scheme, stay well away from it.
10(i)–(j)Fishing vessels, sailing vessels and vessels under 20 m must not impede
A vessel engaged in fishing shall not impede the passage of any vessel following a traffic lane. A vessel of less than 20 metres or a sailing vessel shall not impede the safe passage of a power-driven vessel following a traffic lane.
Rule 8(f) defines the duty: take early action so there is enough sea room for the other vessel to pass safely. The duty still applies when risk of collision develops, and the vessel not to be impeded must still follow the steering and sailing rules.
10(k)–(l)Restricted vessels at work
A vessel restricted in her ability to manoeuvre is exempt from Rule 10 to the extent necessary while maintaining safety of navigation in the scheme, or while laying, servicing or picking up a submarine cable within it.
Buoy tenders, survey vessels and cable ships may need to work against the flow. The exemption covers only what the operation requires.
Yachts, small craft and fishing vessels
A lane gives no right of way. The MCA reminds mariners that the steering and sailing rules apply inside a scheme as anywhere else, and that vessels proceeding in a TSS do not have priority over crossing traffic. What changes things is Rule 10(i) and 10(j): fishing vessels, sailing vessels and vessels under 20 m must not impede ships following a lane.
If a sailing vessel with an auxiliary engine cannot follow the routeing procedures under sail because of light or adverse winds, the MCA says she should use her engine and show the lights, shapes and sound signals of a power-driven vessel.
Fishing vessels fishing within a scheme count as using it and must follow Rules 10(b) and (c); fishing in a separation zone they may follow any course. A vessel that needs to anchor, for example after an engine breakdown or in bad visibility, may do so in a separation zone (MGN 364).
International Code of Signals "YG" means "you appear not to be complying with the traffic separation scheme". A vessel receiving it should check her course and position and correct the situation at once.
How traffic separation schemes appear on charts
U.S. Chart No. 1 (Section M, Tracks and Routes) gives the symbols: the established (mandatory) direction of traffic flow in a lane (M10), a recommended direction of traffic flow (M11), separation lines (M12), separation zones (M13), the limit of a restricted routeing measure such as an inshore traffic zone or area to be avoided (M14), the limit of a routeing measure (M15) and precautionary areas (M16). Its examples (M20–M26) show schemes separated by a zone or by natural obstructions, a roundabout, crossings and inshore traffic zones.
Chart No. 1 explains that schemes are generally shown on charts at scales of 1:600,000 and larger, plotted to scale where possible, and that elements significant to navigation, such as regulated areas, are emphasized in magenta.
Some schemes carry special provisions for particular classes of vessels. These appear on the chart or in the Sailing Directions, so read the chart notes as well as the lines. For the Dover Strait, the MCA points to Admiralty chart 5500, the Admiralty List of Radio Signals and the Dover Strait Pilot.
Real traffic separation schemes
Dover Strait TSS (English Channel)
Between England and France. Over 400 commercial vessels use the Dover Strait every day (GOV.UK, 2024).
An IMO-adopted scheme with a south-west-bound lane and a north-east-bound lane, bordered by an English and a French inshore traffic zone. Any vessel transiting the strait must comply with Rule 10.
Channel Navigation Information Service (CNIS): the UK (Dover Coastguard) and France (CROSS Gris-Nez) monitor traffic jointly. Ships found contravening the collision regulations are reported to their flag state.
CALDOVREP mandatory reporting: vessels of 300 GT and over report to Dover for the south-west lane or to CROSS Gris-Nez for the north-east lane. Smaller vessels report when not under command or at anchor in the TSS or an ITZ, restricted in their ability to manoeuvre, or with defective navigational aids.
Cross-Channel ferries may alter course outside the lanes so that they can cross them at right angles, and the MCA warns of concentrations of fishing vessels and recreational craft.
Between Spain and Morocco, linking the Atlantic and the Mediterranean.
The reporting system GIBREP covers the area between 005°58′W and 005°15′W, which includes the traffic separation scheme "In the Strait of Gibraltar".
Two shore centres share it: Tarifa Traffic (Spain) takes reports from westbound ships and Tangier Traffic (Morocco) from eastbound ships. Both monitor the TSS by radar and AIS.
Under the amendments adopted by resolution MSC.300(87), in force since 1 December 2010, all ships of 300 GT and over take part, as do ships carrying hazardous or potentially polluting cargo, towing or pushing vessels, and smaller vessels fishing in a lane or separation zone or using it in an emergency.
33 CFR 167.150–167.155 describe the scheme: an Eastern approach off Nantucket, an Eastern approach, a South-eastern approach and a Southern approach, made up of traffic lanes and separation zones.
The approaches meet a circular precautionary area of 7 miles radius centred on 40°27.50′N, 73°49.90′W, the kind of area where Rule 10(f) asks for particular caution.
33 CFR 167.10 requires the operator of a vessel in a TSS to comply with Rule 10 of the International Regulations.
Exam questions about traffic separation schemes, answered
Does a ship in a traffic lane have right of way over a vessel crossing it?
No. Rule 10 does not relieve any vessel of her duties under the other rules, and the steering and sailing rules apply inside a scheme. A crossing vessel and a ship in the lane apply the crossing rules as anywhere else. The exception is Rule 10(i) and (j): a fishing vessel, a sailing vessel or a vessel under 20 m must not impede ships following the lane.
Why must you cross a lane on a heading at right angles, not a track?
Rule 10(c) speaks of a heading as nearly as practicable at right angles to the general direction of traffic flow. Keeping the bow at 90° shows your intention clearly to ships in the lane and keeps your time in the lane to a minimum, even when the tidal stream sets you along it.
Can a sailing yacht or a boat under 20 m use the inshore traffic zone?
Yes. Rule 10(d)(i) lets vessels of less than 20 metres, sailing vessels and vessels engaged in fishing use the inshore traffic zone. Other vessels may use it only when bound to or from a port, offshore installation or structure, pilot station or other place within the zone, or to avoid immediate danger.
Can you anchor in a traffic separation scheme?
Rule 10(g) says to avoid anchoring in a scheme or near its terminations so far as practicable. If a vessel must anchor, for example after an engine failure, the MCA's MGN 364 says she may do so in a separation zone.
When may a vessel enter the separation zone?
Under Rule 10(e), when crossing, joining or leaving a lane, in an emergency to avoid immediate danger, or to engage in fishing within the separation zone. Otherwise it should not normally enter the zone or cross a separation line.
Does Rule 10 apply in a precautionary area?
A precautionary area is a separate routeing measure. The MCA notes that it is not part of a TSS and that Rule 10 is not generally applicable there, but ships must navigate with particular caution and Rule 10(f) asks for particular caution near scheme terminations.
What does "not impede" mean for a small vessel near a lane?
Rule 8(f) says a vessel required not to impede another must, when the circumstances require, take early action to leave enough sea room for the other vessel's safe passage. It stays bound by that duty even when risk of collision develops.
Who decides where traffic separation schemes are?
Governments propose them and IMO adopts them. SOLAS regulation V/10 recognizes IMO as the only international body for ships' routeing systems, and IMO publishes the adopted schemes in its Ships' Routeing publication.
