The commercial driver's license (CDL) manual says you should use your horn:

Only when necessary to avoid a crash.. Section 2.4: The horn can startle others. Use it only when necessary to prevent an accident.

The commercial driver's license (CDL) manual says you should use your horn:

ANSWER BREAKDOWN
  • A. Incorrect. "To greet friends." is not correct. The horn can startle others. Use it only when necessary to prevent an accident.
  • B. Incorrect. "To signal a lane change." is not correct. The horn can startle others. Use it only when necessary to prevent an accident.
  • C. Correct. The horn can startle others. Use it only when necessary to prevent an accident.
  • D. Incorrect. "To scare deer." is not correct. The horn can startle others. Use it only when necessary to prevent an accident.