Section 27001 Of Article 1. Horns, Sirens And Amplification Devices From California Vehicle Code >> Division 12. >> Chapter 5. >> Article 1.

. (a) The driver of a motor vehicle when reasonably necessary to insure safe operation shall give audible warning with his horn.
  (b) The horn shall not otherwise be used, except as a theft alarm system which operates as specified in Article 13 (commencing with Section 28085) of this chapter.