Vehicles actually engaged in the construction, removal, maintenance, or inspection of any oil or gas pipeline may display flashing amber warning lights to the front, sides, or rear when necessarily parked on a highway or when necessarily moving at a speed slower than the normal flow of traffic and only in accordance with Section 25268.
Source:
Section 25260.1, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=VEH§ionNum=25260.1..