This is directly out of the CA drivers manual;
Don't Use Your Horn
If a driver or bicyclist is going slowly, and you want him or her to drive faster or get out of your way. The driver or bicyclist may not be able to safely go faster, due to illness, being lost, intoxication, or having mechanical problems with the vehicle.
To alert other drivers that they made a mistake. Your honking may cause them to make more mistakes or to become angry and retaliate.
Because you may be angry or upset.
To honk at pedestrians, bicyclists, or motorcyclists unless necessary to avoid a collision. Remember that your horn sounds much louder outside a vehicle.
NOTE: Honking your horn may startle other drivers. It is safer to slow down or stop instead of honking your horn.