Aggregate material ( soil, sand, rocks, etc. ) can not be transported unless the bed or other area where it is placed is secure and free from holes or spaces where the material can escape and fall out. If material is left outside the enclosure, on a bed rail, bumper, etc, it must be removed prior to transporting. Additionally, the load may not come within 6" of the top of the enclosure and the load must be covered.
So, if you are a weekend warrior and grab a load of sand from the local supply lot, make sure to spread the load throughout the bed, bring a tarp to cover it, clean the rails and bumper, and make sure that none of the load can fall from the vehicle. Many of our Officers are commercial trained and conduct enforcement stops for size, weight, and load violations not only on large commercial trucks, but also on common pick-ups and small stake beds as well.
Refer CVC 23114 and all sub sections for specific requirements. This CVC section is long, describes each equipment requirement, and defines "aggregate material" for California drivers.