California Code, Vehicle Code - VEH § 24615

It is unlawful to operate upon a public highway any vehicle or combination of vehicles, which is designed to be and is operated at a speed of 25 miles per hour or less, unless the rearmost vehicle displays a “slow-moving vehicle emblem,” except upon vehicles used by a utility, whether publicly or privately owned, for the construction, maintenance, or repair of its own facilities or upon vehicles used by highway authorities or bridge or highway districts in highway maintenance, inspection, survey, or construction work, while such vehicle is engaged in work at the jobsite upon a highway.  Any other vehicle or combination of vehicles, when operated at a speed of 25 miles per hour or less, may display such emblem.  The emblem shall be mounted on the rear of the vehicle, base down, and at a height of not less than three nor more than five feet from ground to base.  Such emblem shall consist of a truncated equilateral triangle having a minimum height of 14 inches with a red reflective border not less than 1 3/4 inches in width and a fluorescent orange center.

This emblem shall not be displayed except as permitted or required by this section.