Which signal indicates approach to lock under control?

• A. Flashing red
• B. Flashing amber
• C. Flashing Green
• D. Long Horn

Answer: (B)

Signals around a lock communicate status and required actions to mariners. A flashing amber light is used to show you’re approaching a lock that is under the lockmaster’s control. It’s a caution signal telling you to slow down, be ready to stop or follow the operator’s instructions, and proceed only when directed. The idea is to prompt careful, controlled movement while the lock is being operated.

Red flashing signals danger or stop, so they don’t indicate that you’re simply approaching a lock under control. A flashing green would suggest clearance to proceed, which isn’t the message when the lock is actively under the operator’s control. A long horn is an audible cue, but it doesn’t replace the specific status indication that the amber signal provides.