COLREGS Rule 10 — Traffic Separation Schemes

Rule Text

What it means on the water

• →Proceed in the correct traffic lane in the general direction of traffic flow.
• →Join or leave at the termination of the lane, or at as small an angle as practicable from the sides.
• →Avoid the separation zone and do not cross the separation line except in emergency.
• →Cross traffic lanes as nearly as possible at right angles to traffic flow.
• →Vessels under 20m and sailing vessels shall not impede power-driven vessels in a traffic lane.
• →Inshore traffic zones may be used by vessels under 20m, sailing vessels, and vessels with a destination in the inshore zone.

Common exam mistakes

• ✗Thinking you must join a TSS lane at the end only — joining from the side at a small angle is permitted.
• ✗Confusing TSS crossing angle (right angles) with narrow channel requirements.
• ✗Missing that sailing vessels and vessels under 20m have a 'shall not impede' duty in a TSS lane, just as in narrow channels.

USCG exam questions — Rule 10

These questions are drawn from the same pool used in real USCG licensing exams. Correct answers and explanations are shown.

1. 1. Under Rule 9, a vessel proceeding along a narrow channel or fairway shall:

   • A.Keep to the center of the channel at all times
   • B.Keep as near to the outer limit of the channel on her starboard side as is safe and practicable✓
   • C.Give way to all vessels overtaking from astern
   • D.Sound one short blast to signal her intention to maintain course

   Why: Rule 9(a) requires vessels in a narrow channel to keep to the starboard side of the channel — not the center. This mirrors the road rule and ensures predictable traffic flow, reducing the risk of head-on encounters in confined waterways.

2. 2. Under Rule 9, a vessel less than 20 meters in length or a sailing vessel in a narrow channel:

   • A.Has priority over power-driven vessels proceeding along the channel
   • B.Shall not impede the passage of a vessel which can safely navigate only within the channel✓
   • C.May anchor in the channel if no traffic is visible
   • D.Shall keep to the port side of the channel to leave room for larger vessels

   Why: Rule 9(b) places a duty on small vessels and sailing vessels not to impede vessels that can only safely use the channel. This is a proactive obligation — the small or sailing vessel must act to stay clear, rather than simply being low in the hierarchy when an encounter develops.

3. 3. Under Rule 9, a vessel shall not cross a narrow channel if such crossing:

   • A.Would require more than 5 minutes to complete
   • B.Impedes the passage of a vessel which can only safely navigate within that channel✓
   • C.Takes place during periods of restricted visibility
   • D.Is made at right angles to the channel

   Why: Rule 9(d) prohibits crossing a narrow channel if doing so would impede a vessel that can only safely navigate within the channel. The crossing vessel bears the obligation to yield; the channel-bound vessel has right of way in this scenario. Time or angle of crossing are not the legal standard.

4. 4. When joining or leaving a traffic lane in a TSS, Rule 10 requires a vessel to do so:

   • A.At the designated inbound or outbound waypoints shown on the chart
   • B.At as small an angle to the general direction of traffic flow as practicable✓
   • C.After broadcasting her intentions on VHF channel 16
   • D.Only at the terminal ends of the scheme

   Why: Rule 10(c) requires vessels to join or leave a traffic lane at as small an angle as practicable. A shallow joining angle minimizes the time the vessel spends cutting across traffic flow and reduces collision risk. There is no requirement to use specific waypoints or to broadcast on VHF.

5. 5. Under Rule 9(f), a vessel approaching a bend or an area of a channel where other vessels may be obscured shall:

   • A.Stop engines and navigate with extreme caution
   • B.Sound five short blasts to alert approaching traffic
   • C.Navigate with particular alertness and caution and sound the appropriate signal prescribed in Rule 34(e)✓
   • D.Reduce to bare steerageway and post a bow lookout

   Why: Rule 9(f) requires particular alertness and caution when approaching a channel bend with obscured visibility, and requires sounding the signal prescribed in Rule 34(e) — one prolonged blast. Rule 34(e) prescribes the signal and Rule 9(f) mandates its use at bends. Five short blasts is the danger/doubt signal, not the bend signal.

6. 6. Under Rule 10, a vessel using a traffic separation scheme shall normally join or leave a traffic lane at:

   • A.The termination of the lane or by entering or leaving from the side✓
   • B.Any point along the lane boundary at a right angle
   • C.Only the designated entry/exit points marked on nautical charts
   • D.The junction of two separation zones

   Why: Rule 10(b)(iii) of the 72 COLREGS states that a vessel using a traffic separation scheme shall normally join or leave a traffic lane at the termination of the lane, but when joining or leaving from either side shall do so at as small an angle to the general direction of traffic flow as practicable. (The right-angle requirement in Rule 10(c) applies only to crossing a traffic lane, not to joining or leaving it.)

7. 7. Which of the following vessels is exempted from the full requirements of Rule 10 when navigating in a traffic separation scheme?

   • A.A vessel engaged in laying or servicing a submarine cable within the scheme✓
   • B.A vessel over 300 gross tons proceeding to a nearby port
   • C.A sailing vessel making passage in favorable wind conditions
   • D.A vessel displaying not-under-command lights

   Why: Under COLREGS / 33 CFR 83.10(l), a vessel restricted in her ability to maneuver while engaged in laying, servicing, or picking up a submarine cable within a traffic separation scheme is exempted from Rule 10 to the extent necessary to carry out the operation. (The similar Rule 10(k) covers a RAM vessel engaged in maintenance of safety of navigation; size, favorable wind, or merely displaying not-under-command lights grants no Rule 10 exemption.)

8. 8. Under Rule 10, the inshore traffic zone may be used by:

   • A.A vessel of less than 20 meters in length, a sailing vessel, and a vessel engaged in fishing✓
   • B.Only vessels bound for ports within the inshore zone
   • C.Only vessels under 500 gross tons
   • D.Any vessel seeking to avoid adverse weather in the traffic lane

   Why: Rule 10(d) states that inshore traffic zones shall not normally be used by through traffic, but a vessel of less than 20 meters in length, a sailing vessel, or a vessel engaged in fishing may use the inshore traffic zone. Additionally, vessels entering or leaving nearby ports may use it.

Frequently asked questions

At what angle should you cross a traffic separation scheme?

Rule 10 requires that a vessel crossing a traffic lane shall do so on a heading as nearly as practicable at right angles to the general direction of traffic flow. This makes the vessel's course and intentions clear to other traffic and minimizes the time spent in the lane.

Can a sailing vessel use a traffic separation scheme?

Yes, but with restrictions. A sailing vessel shall not impede the safe passage of a power-driven vessel following a traffic lane. Sailing vessels and vessels under 20 meters may use the inshore traffic zone rather than the main traffic lane.

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