CA Vehicle Code 24409 (Failure to Dim Headlights) states: “(a) Whenever the driver of a vehicle approaches an oncoming vehicle within 500 feet, he shall use a distribution of light or composite beam so aimed that the glaring rays are not projected into the eyes of the oncoming driver. The lowermost distribution of light specified in this article shall be deemed to avoid glare at all times regardless of road contour. ( Whenever the driver of a vehicle follows another vehicle within 300 feet to the rear, he shall use the lowermost distribution of light specified in this article.” https://codes.findlaw.com/ca/vehicle-code/veh-sect-24409/ That means flashing your high beams at someone to warn them is illegal, but turning your low beam lights on and off is not. Sent from my iPhone using Tapatalk