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Right of way
Vessels at sea do not actually have any '
right of way' - they are, correctly, in the position of being 'stand on vessel' or 'give way' vessel. This means that at no time should any vessel actually navigate its way into a collision situation, and the rules are clear that no one in command of a vessel should assume a 'right of way' and should at all costs avoid a collision.
Consider two ships on courses that intersect. The rule is that the ship on the left must give way. The stand on vessel sees the green light on the
starboard (right) side of the ship on the left. The give way vessel sees the red light on the
port side of the stand on vessel. The helmsman gives way to a red light by either turning away and showing a
stern light (which is white), or by going around the stern of the stand on vessel.
This was likely the beginning of the convention for traffic lights that use red to mean stop and green to mean go.
There are other rules governing which is a stand on vessel, such as small ships giving way to big ships, powered ships giving way to sailing ships, and in some circumstances vessels under sail giving way to powered vessels that are constrained by their
draft, or restricted in their ability to maneuver. Therefore the green light does not mean an unqualified go, but rather it means proceed with caution subject to other rules applying. The earliest railway signals went red/green/white (as per the stern light) for stop/caution/go following this naval practice and were only later changed to the more familiar red/yellow/green.
The very simple application of red light and green light is that if the helmsman sees a red light, the helmsman should make sure that the other vessel can see his green light, which usually means giving way. If he sees a green light, he should stand on, but without getting into a collision situation.
The sailing rules that dictate that a sailing vessel on starboard tack (the side the wind is coming from) is the stand on vessel is as old as any other regulations. Likewise, if on the same tack, a sailing vessel that is upwind of another is the give way vessel.
