Traffic Separation Scheme

A Traffic Separation Scheme or TSS is a traffic-management route-system ruled by the International Maritime Organization or IMO. The traffic-lanes (or clearways) indicate the general direction of the ships in that zone; ships navigating within a TSS all sail in the same direction or they cross the lane in an angle as close to 90 degrees as possible.

TSSs are used to regulate the traffic at busy, confined waterways or around capes. Within a TSS you normally see at least one traffic-lane in each main-direction, turning-points, deep-water lanes and separation zones between the main traffic lanes. In most TSS schemes you will find Inshore Traffic Zones between the traffic-lanes and the coast. The inshore traffic zone is unregulated and shouldn't be used for ongoing traffic. It is meant for local traffic, fishing and small craft.

A ship navigating in a traffic-lane should sail in the general direction of that lane. The body of water between two opposite lanes is to be avoided by vessels travelling within the TSS as far as possible except in certain circumstances such as emergencies or for fishing activities.

The TSS rules are incorporated in the International Regulations for Preventing Collisions at Sea (Under Part B, Section I, Rule 10- Traffic Separation Schemes)

As said, when sailing within a lane of a TSS that ship has to follow the general direction of the lane. Where needed there are special zones where a lane splits into two channels: one ongoing and the other to the nearby port(s).

Objectives of IMO Routeing Schemes:

If a ship wants to cross a traffic-lane it should do so at a right angle to avoid endangering ship traffic using the traffic-lanes (although traffic in the lane does not automatically have the right-of-way). To minimize the amount of time a crossing ship spend crossing the traffic-lanes, there should be a right angle between the lane direction and the keel direction - even if currents might shift the actual direction of the ship's movement to some angle other than 90 degrees.

TSSs are used in locations where there is a lot of traffic (busy shipping areas) where not regulating the traffic would lead to more accidents. In Europe, many TSS areas are found around the Southern part of the North Sea including the English Channel. Other TSS are in place off Land's End and around Ouessant (Ushant).

...
Wikipedia