Sec. 189. (a) "Truck-tractor" means, except as provided in subsection (b), a motor vehicle designed and used primarily for drawing other vehicles and not constructed to carry a load other than a part of the weight of the vehicle and load so drawn.
(b) "Truck-tractor", for purposes of IC 9-20-13-2, means a noncargo carrying power unit designed to operate in combination with a semitrailer or trailer and includes a dromedary deck and plate.