In the open ocean ships and other vessels are generally free to go wherever they wish. This is not the case in some restricted waters where compliance with certain rules becomes mandatory.
Traffic Separation Schemes (TSS) have been with us for nearly four decades. They emerged following a series of accidents in the English Channel where, over a span of just a few years, a number of ships were sunk or disabled and the lives of over 50 crew members lost. Since then TSS have been established in other localities where the geographical features bring ships together in dangerous proximity ‒ the Strait of Gibraltar being a good example. The various actions were taken under the auspices of the International Maritime Organisation (IMO) and the obligations of all mariners regarding TSS is now covered in Rule 10 of the Colregs.
Basically, a TSS divides shipping into two lanes – in the English Channel, one eastbound and the other westbound. Between the lanes is a no-go area known as a ‘separation zone’, in many ways similar to the central reservation of a dual carriageway. The inshore regions outside the TSS remain unregulated – apart, of course, from our wider obligations under the Colregs.
The illustration below shows how a smaller vessel should cross a TSS in tidal waters. It's important not to attempt to compensate for the tidal flow as this simply prolongs the time you are exposed to danger while in the lanes. Maintain a heading at 90º to the lane as if there were no tidal flow..