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.
TSS's 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 cases you can find an "inshore traffic zone" between the traffic-lanes and the coast.
A ship navigating in a traffic-lane should sail in the general direction of that lane. The body of water between two opposite lanes are no-go areas: shipping is not allowed in these areas (compare this with the central reservation of a road), so the risks for head-on collisions is greatly reduced.
The TSS rules are incorporated in the International Regulations for Preventing Collisions at Sea (under rule 10)
As said, when sailing within a lane of a TTS 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).
In most TTS 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.
Well-known TTS locations include: The English Channel, German Bight, Singapore, and Cape Horn.
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[1]). 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.