Crossing a traffic separation scheme: what COLREGs Rule 10 requires of sailors
as of 07/10/2026 · 6 min read
In short: You cross a traffic separation scheme with your heading as close as possible to a right angle to the traffic flow, promptly, without entering the separation zone and without impeding large ships.
Under COLREGs Rule 10 you cross a traffic separation scheme (TSS) promptly, by the shortest route, heading as near as possible at right angles to the traffic flow. Enter the separation zone only to cross it; as a sailing vessel or one under 20 m, do not impede ships following the lane. Along the coast, use the inshore traffic zone.
What is a traffic separation scheme?
A traffic separation scheme organises shipping in narrows, off headlands and in harbour approaches like a motorway: two traffic lanes in opposite directions, separated by a separation zone or line. Most are adopted by the IMO, and their boundaries are shown on the chart.
The rules are set out in Rule 10 of the International Regulations for Preventing Collisions at Sea (COLREGs; in German, KVR). They apply to all vessels, so to you too, under sail or power. The other steering and sailing rules continue to apply within a TSS.
| Part | What it is | What you do |
|---|---|---|
| Traffic lane | lane with a fixed direction of traffic | follow it in the direction of traffic or cross it at right angles |
| Separation zone, separation line | separates the two traffic lanes | do not navigate in it, only cross |
| Inshore traffic zone | waters between the TSS and the coast | free to use for sailing vessels and boats under 20 m |
| Precautionary area | area where routes cross or end | navigate with particular caution |
| Deep-water route | route for ships with a deep draught | avoid, cross promptly |
How do I cross a traffic separation scheme correctly?
With your heading as close as possible to a right angle to the direction of traffic, on the shortest route through the lanes. What counts is the heading, not the course over ground: if the tidal stream sets you, your track runs at an angle, and that is allowed.
There are two reasons for the right angle. You are in the way for as short a time as possible, and on the bridge of a freighter it is immediately clear that you are crossing, not following the lane. Here is how to go about it:
- Choose the crossing point on the chart in advance, ideally where the TSS is narrow and there is no precautionary area.
- Check the wind: if it will not allow the right-angled course under sail, use the engine as well rather than tacking in the traffic lane.
- Before entering, judge the gaps in the traffic with AIS, radar and bearings. Passing astern of a ship is usually safer than crossing ahead.
- Cross promptly, keep a lookout and hold your course steady so that others can read your intentions.
- Avoid crossing in poor visibility whenever possible.
What is prohibited in the separation zone?
You only enter the separation zone to cross it, to join or leave a traffic lane, or in an emergency. You should avoid anchoring anywhere in the traffic separation scheme and near its ends.
At the ends of a TSS and in precautionary areas, traffic converges from several directions, and Rule 10 calls for particular caution there. Under Rule 10(h), vessels not using the TSS keep as far away from it as practicable.
May I use the inshore traffic zone as a sailor?
Yes. Vessels under 20 m in length, sailing vessels and fishing vessels may always use the inshore traffic zone. Through traffic that can safely use the adjacent traffic lane, on the other hand, does not belong there.
For cruising sailors it is often the better route: less big shipping, harbours nearby and no traffic lane with one freighter after another.
Who has right of way in a traffic separation scheme?
In a traffic lane, a sailing boat or a vessel under 20 m must not impede a power-driven vessel following the lane (Rule 10(j)). Do not rely on “power gives way to sail” here: act so early that no risk of collision arises in the first place.
A container ship at 20 kn covers a nautical mile in three minutes and needs room and time to alter course. Plan your crossing so that nobody has to take avoiding action because of you.
Which traffic separation schemes lie in the area?
The best known for cruising sailors are in the Strait of Dover, the German Bight, the Kadet Channel, off Skagen and in the Øresund, and in the Strait of Gibraltar. On top of that come many off the headlands of the Atlantic coast.
| Area | Location | What sailors should know |
|---|---|---|
| Strait of Dover | between Dover and Cap Gris-Nez | one of the busiest shipping lanes in the world, monitored by HM Coastguard's Channel Navigation Information Service and by CROSS Gris-Nez |
| German Bight | Terschelling-German Bight, German Bight Western Approach, Jade Approach | usually has to be crossed on the way from the East Frisian Islands to Helgoland |
| Kadet Channel | between Gedser and the Darß | narrow, deep Baltic channel with a TSS south of Gedser |
| Skagen and the Øresund | off Skagen, in the Kattegat and in the narrows between Helsingør and Helsingborg | plus dense ferry traffic between Helsingør and Helsingborg |
| Strait of Gibraltar | between Tarifa and the Moroccan coast | current, Levante and Poniente, Tarifa Traffic VTS |
| Atlantic coast | off Ushant, Cape Finisterre, Cabo da Roca and Cape St Vincent | to be crossed on the way south or passed between the TSS and the coast |
Do I have to report to the vessel traffic service?
Reporting obligations usually apply to commercial shipping above a certain size, not as a rule to pleasure craft. It is still worth listening in on the VTS channel: you hear what is moving around you, and you can be reached if someone calls you.
The channels are given on the chart and in the sailing directions. An AIS transmitter makes you visible on the bridge of large ships, a radar reflector on their radar. Even so, do not rely on being seen, and keep a lookout.
How does Sailseer help with crossings?
Sailseer checks every planned route on the map against the traffic separation schemes from OpenStreetMap and names every one the route touches: traffic lane, separation zone or precautionary area, plus the angle at which the route crosses it. If the angle is below 45 degrees, Sailseer points to COLREGs Rule 10.
- On the route from Helgoland to Norderney, for example, Sailseer names Terschelling-German Bight and Jade Approach with their crossing angles.
- When Sailseer plans tacks upwind, it puts no tack in a traffic lane or separation zone; more in the guide Sailing upwind.
- In a multi-day trip, the schemes are listed for every day on which you cross one.
- The “Traffic density” layer shows from AIS data where shipping runs, and the “Live vessels” layer shows ships with course and speed from zoom level 7.
- For crossings in the dark, the lights with their characteristics help; see Night sailing and lights.
Sailseer is not for navigation and not for collision avoidance. The official chart, the sailing directions and the Notices to Mariners apply.
Frequently asked questions
Can I sail through a traffic separation scheme in a sailing boat?
Yes. You may follow a traffic lane in the direction of traffic or cross it, in which case your heading should be as close as possible to a right angle to the direction of traffic. You must not impede ships following the lane (COLREGs Rule 10(j)).
Why do I have to cross a traffic separation scheme at right angles?
That way you spend the shortest time in the traffic lane, and large ships see immediately that you are crossing. An angled course lengthens your time in the lane and makes your intentions hard to read.
Does the heading or the course over ground count when crossing?
The heading. Rule 10(c) requires your bow to point as close as possible to a right angle to the direction of traffic. If the tidal stream sets you, your track over the ground may run at an angle.
Can I tack or anchor in a traffic separation scheme?
You should avoid anchoring in a TSS and near its ends. If you tack across, you are not holding a right-angled heading and you stay in the lane longer; use the engine as well instead.
What is an inshore traffic zone?
The waters between the traffic separation scheme and the coast. Sailing vessels, vessels under 20 m and fishing vessels may always use it; through traffic that can safely use the traffic lane may not.
Does Sailseer show traffic separation schemes along the route?
Yes. For every route Sailseer names the traffic separation schemes it touches, with their type and crossing angle, and below 45 degrees it points to COLREGs Rule 10. Sailseer never puts a tack in a traffic lane or separation zone.