California Vehicle Code definition of
CVC Section 655. (a) A "truck tractor" is a motor vehicle designed and used
primarily for drawing other vehicles and not so constructed as to
carry a load, other than a part of the weight of the vehicle and the
load so drawn. As used in this section, "load" does not include
items carried on the truck tractor in conjunction with the operation
of the vehicle if the load carrying space for these items does not
exceed 34 square feet.
(b) Notwithstanding subdivision (a), a truck tractor, operated by
a motor carrier whose owner is licensed by the Department of the
California Highway Patrol to transport explosives pursuant to
Division 14 (commencing with Section 31600), may be equipped with a
cargo container used exclusively for the transportation of explosives
or munitions-related security material, as specified by the United
States Department of Defense.