The Department of Transportation or local authorities, with respect to highways under their respective jurisdictions, may erect stop signs to require the traffic on a highway to stop before crossing any railroad grade crossing designated by the agency having jurisdiction of the highway as a major crossing with a demonstrated need for stop signs, except a railroad grade crossing which is controlled by automatic signals, gates, or other train-actuated control devices.
Source:
Section 21351.5, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=VEH§ionNum=21351.5..