Local authorities in their respective jurisdictions may place and maintain, or cause to be placed and maintained, speed limit, speed advisory, and mileage signs, or suitable plates affixed to or near existing signs, which indicate speeds and distances both in common standards of measures, as specified in Section 12302 of the Business and Professions Code, and in measures of the metric system authorized by Congress.
Source:
Section 21351.3, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=VEH§ionNum=21351.3..