If loose materials are being transported on a flatbed trailer, the vehicle should have:

If a flatbed trailer is being used to transport a load of loose materials, such as sand, gravel, or wood chips, the driver should ensure that the materials will not spill or blow onto the roadway. To safely transport loose materials, the flatbed trailer must have boards or panels that securely enclose the entire bed and will prevent any materials from escaping.
DMV Writen Test