California Code, Vehicle Code - VEH § 565

(a) Any vehicle used primarily off the highways for construction purposes and which moves only occasionally over the highways and which because of the length, height, width, or unladen weight may not move over the public highways unladen without the permit specified in Section 35780.

(b) Any vehicle which is designed and used primarily either for grading of highways, paving of highways, earth moving, and other construction work on highways, or for construction or maintenance work on railroad rights-of-way, and which is not designed or used primarily for the transportation of persons or property and which is only incidentally operated or moved over the highway. It includes, but is not limited to, road and railroad construction and maintenance machinery so designed and used such as portable air compressors, air drills, asphalt spreaders, bituminous mixers, bucket loaders, tracktype tractors, crawler tractors, ditchers, leveling graders, finishing machines, motor graders, paving mixers, road rollers, scarifiers, earth moving scrapers and carryalls, lighting plants, welders, pumps, water wagons, power shovels and draglines, speed swings, skip loaders, weed mowers, self-propelled and tractor-drawn earth moving equipment and machinery, including dump trucks and tractor-dump trailer combinations which either (1) are in excess of 96 inches in width or (2) which, because of their length, height or unladen weight, may not be moved on a public highway without the permit specified in Section 35780 of this code and which are not operated laden except within the boundaries of the job construction site, and other similar types of construction equipment.