Effective January 1, 2005, California Vehicle Code is requiring headlights to be turned on while driving in the rain. Motorists are subject to being ticketed for violating this law.
The California Vehicle Code outlining this law:
24400. (a) During darkness and inclement weather, a motor vehicle, other than a motorcycle, shall be equipped with at least two lighted headlamps, with at least one on each side of the front of the vehicle, and, except as to vehicles registered prior to January 1, 1930, they shall be located directly above or in advance of the front axle of the vehicle. The headlamps and every light source in any headlamp unit shall be located at a height of not more than 54 inches nor less than 22 inches.