Crossing a traffic lane, your heading should be:

As nearly as practicable at right angles to traffic flow

Who may use an inshore traffic zone?

Vessels under 20 m, sailing vessels, and fishing vessels

Rule 10: Traffic Separation Schemes (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 10 governs traffic separation schemes. Use the correct lane in the general direction of traffic, keep clear of separation lines and zones, join or leave at the ends or at a small angle, and if you must cross, cross on a heading as nearly as practicable at right angles to the traffic flow.

The rule text

proceed in the appropriate traffic lane in the general direction of traffic flow for that lane;
International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 10

shall cross on a heading as nearly as practicable at right angles to the general direction of traffic flow.
International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 10

What it means on the water

Lanes: follow the flow; stay clear of separation lines and zones (10(b)).
Crossing: avoid it, but if necessary cross at right angles on heading (10(c)).
Inshore traffic zones: generally not for through traffic, but vessels under 20 meters, sailing vessels, and fishing vessels may use them (10(d)).
Fishing vessels must not impede vessels following a lane; vessels under 20 meters and sailing vessels must not impede power-driven vessels following a lane (10(i), 10(j)).
Avoid anchoring in or near the ends of a scheme (10(g)).

International vs Inland

Same in substance. International applies to schemes adopted by the IMO (the Organization); Inland applies to traffic separation schemes without that phrase.

Common exam traps

Crossing at a shallow angle to save time. The heading should be nearly at right angles.
Thinking a sailing vessel has priority over a ship in a lane. Rule 10(j) says don't impede.

Try 3 questions on Rule 10

Crossing a traffic lane, your heading should be:
1. Parallel to traffic
2. As nearly as practicable at right angles to traffic flow
3. 45 degrees
4. Any
Show answer
B. As nearly as practicable at right angles to traffic flow Rule 10(c).
Who may use an inshore traffic zone?
1. Only ships over 50 m
2. Vessels under 20 m, sailing vessels, and fishing vessels
3. Nobody
4. Only RAM vessels
Show answer
B. Vessels under 20 m, sailing vessels, and fishing vessels Rule 10(d)(i).
A sailing vessel near a power-driven vessel following a lane must:
1. Stand on
2. Not impede her safe passage
3. Sound one prolonged
4. Cross ahead
Show answer
B. Not impede her safe passage Rule 10(j).

Frequently asked questions

How do you cross a traffic separation scheme?

On a heading as nearly as practicable at right angles to the general direction of traffic flow (Rule 10(c)).

Can I anchor in a TSS?

Avoid it so far as practicable, including near the ends (Rule 10(g)).

Does Rule 10 override the steering rules?

No. It doesn't relieve any vessel of her obligations under other rules (Rule 10(a)).